Pierre Levitz Ecole Polytechnique
Ralf Metzler Technical University of Munich
David Reichman University of Columbia
JJ1: Anomalous Transport, Optimal Search
Monday AM, November 30, 2009
Riverway (Sheraton - 5th Floor)
9:15 AM -
9:30 AM - **JJ1.1
Single Particle Tracking: When Ergodicity and Nonergodicity Coexist.
Joseph Klafter 1 , Yasmine Meroz 1 , Igor Sokolov 2 Show Abstract
1 , Tel Aviv University, Tel Aviv Israel, 2 , Humboldt University, Berlin Germany
Single particle trajectories are investigated assuming the coexistence of two subdiffusive processes: diffusion on a fractal structure modeling spatial constraints on motion and heavy tailed continuous time random walks representing energetic or chemical traps. The particles' mean squared displacement is found to depend on the way the mean is taken:temporal averaging over single particle trajectories differs from averaging over an ensemble of particles. This is shown to stem from subordinating an ergodic anomalous process to an nonergodic one. Generalization to other subdiffusive processes is discussed.
10:00 AM - **JJ1.2
Weak Ergodicity Breaking.
Eli Barkai 1 Show Abstract
1 , Bar Ilan, Ramat gan Israel
Theory of weak ergodicity breaking will be presented, and its connectionto anomalous diffusion of single molecules (e.g. mRNA, tellomeres) in confiendspace (e.g. the cell, nucleus) will be discussed.
10:30 AM - **JJ1.3
Analytic Result for New Interacting Random Walkers Problems.
Tobias Ambjornsson 1 6 , Ludvig Lizana 2 , Michael Lomholt 3 , Eli Barkai 4 , Alessandro Taloni 5 Show Abstract
1 Dept. of Theoretical Physics, Lund University, Lund Sweden, 6 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 , Niels Bohr Institute, Copenhagen Denmark, 3 , University of Southern Denmark, Odense Denmark, 4 , Bar Ilan University, Ramat-Gan Israel, 5 , Tel Aviv University, Tel Aviv Israel
The problem of a single random walker has received a lot of attention in science community over the years. There is now an increasing amount of interest in the problem of INTERACTING random walkers (due to the strong connection of this problem to the fields of, for instance, biophysics,nanofluidics, and cell biology). In particular, much attention has been on the behavior of the non-equilibrium problem of interacting walkers in (quasi)one dimensional systems, so called single-file diffusion. The quantities of main interest in such a system is the mean square displacement (MSD) and probability density function (tPDF) of a (fluorescently) tagged particle. It has been found previously (theoretically and experimentally) that the MSD for a tagged particle in a single file system scales as t^(1/2) for long times t (in the thermodynamic limit), rather than t as for unconstrained diffusion; the tPDF is Gaussian.In the talk two new single-file results will be presented:1) The problem of hardcore interacting particles in a FINITE system (box) is solved analytically using a Bethe ansatz, see Ref . Analysis of our exact solution reveals three time regimes: (A) times shorter than the mean collision time, for which the tagged particle undergoes standard diffusion. (B) Intermediate times, i.e. times longer than the mean collision time but shorter than the equilibrium time, for which single file behavior is found. Finally, for (C) long times, the system reaches an equilibrium regime in which the tPDF is of polynomial-type, in a agreement with equilibrium statistical mechanics.2) Solving interacting random walkers problems is in general rather cumbersome mathematically. We recently introduced a procedure, which we refer as to as Harmonization, which maps the diffusive motion of any type of 1d short-range single-file system onto that of chain of harmonically coupled beads; the effective spring constant in the system is connected to the details of the potential between particles. The Harmonization procedure reproduces all known long-time results in the single-file field with some back-of-the envelope calculations and allow us to analytically solve the long-time behavior of more complicated single-file systems. For instance, the tagged particle motion in a harmonic potential, in a time-varying force and the correlation between particles are calculated. We find excellent agreement with simulations. L. Lizana and T. Ambjornsson, Single-file diffusion in a box, Phys. Rev. Lett. 100, 200601 (2008). T. Ambjornsson, L. Lizana, A. Taloni, E. Barkai and M.A. Lomholt, Foundation of fractional Langevin equations: Harmonization of a many-body problem, in preparation.
11:30 AM - **JJ1.4
Multiparticle Diffusion in One Dimension with Varying Mobilities or Long Jumps.
Michael Lomholt 1 Show Abstract
1 , University of Southern Denmark , Odense M Denmark
DNA binding proteins diffusing along DNA is an example of a diffusion process in one dimension where the diffusants can be restricted in their motion by not being able to overtake each other. For identical diffusants it has been known for almost half a century that such single-file diffusion leads to subdiffusion of an individual particle with the mean square displacement eventually growing as the square root of time. However, the DNA in a biological cell will be populated with many different proteins, motivating the study of single-files with a distribution of diffusion constants. In this talk an effective medium type approach will be presented that allows for the study of this situation. For distributions which are sufficiently heavy with respect to extremely slow particles the subdiffusive exponent is found to be smaller than the 1/2 previously found for identical particles. The analytic results are found to be in excellent agreement with simulations. Including the possibility of the proteins unbinding from the DNA and rebinding somewhere else leads to the possibility of long jumps measured along the DNA contour. A single DNA experiment is presented that demonstrates the beneficial role of these long intersegmental jumps in search processes for specific sites on the DNA.
12:00 PM - **JJ1.5
First-Passage Times and Target Search Strategies.
Olivier Benichou 1 , Raphael Voituriez 1 , Michel Moreau 1 , Joseph Klafter 2 , Sylvain Condamin 1 , Claude Loverdo 1 , Vincent Tejedor 1 Show Abstract
1 LPTMC, University Pierre et Marie Curie, Paris France, 2 , Tel Aviv University, Tel Aviv Israel
How long does it take a "searcher" to reach a "target" for the first time? This first-passage time is actually a key quantity for evaluating the spreading of diseases, the kinetics of diffusion limited reactions, or the time needed for animals to find food resources.I will first show that intermittent search strategies, which combine phases of active perception and phases of mere displacement constitute efficient strategies that allow to minimize the search time. I will rely on two examples at very different scales: the search for a specific target sequence on DNA by a protein, and the foraging strategies of animals.In a second part, I will present recent results which permit to evaluate precisely the mean first-passage time for a wide range of random searchers evolving in a confined domain. This approach reveals a general dependence of the mean first-passage time on the geometric characteristics of the problem. I will briefly show how these results apply to transport in fractal and disordered media, and to anomalous diffusion.
12:30 PM - JJ1.6
Intermittent Brownian Dynamics over a Rigid Strand: Heavily Tailed Relocation Statistics in a Simple Geometry.
Pierre Levitz 1 , Michel Zinsmeister 1 , Patrick Davidson 2 Show Abstract
1 (1) Physique de la Matière Condensée, CNRS-Ecole Polytechnique, Palaiseau France, 2 (2) Laboratoire de Physique des Solides, Univ. Paris-Sud, CNRS, Orsay France
We analyze the intermittent Brownian dynamics (a succession of adsorption and bulk relocation steps) of a test particle over a single strand. We propose an analytic expression of the relocation time distribution at all times. We show that this distribution has a non-trivial heavily tailed statistics at long time with a diverging average relocation time . In order to experimentally probe this first passage statistics, we follow the intermittent Brownian dynamics of water molecules over long and stiff imogolite mineral strands (similar in many ways to fibrillar biopolymers such as DNA or F-actin), using field cycling NMR dispersion technique [1-2]. Our analytic derivation is found to be in good agreement with experimental data on a large domain of observation. Implications for the efficiency of a search strategy on a single filament are then discussed and the importance of the confinement and/or the finite size effect is emphasized.  P. Levitz, M. Zinsmeister, P. Davidson, D. Constantin, and O. Poncelet. Phys. Rev. E 78, 030102 (R) (2008). P. Levitz, J. Phys. Condens. Matt. 17, S4059 (2005) O. G. Berg, R. B. Winter, and P. H. von Hippel, Biochem. Int. 20, 6929 (1981).
JJ2: Polymer in Confinement
Monday PM, November 30, 2009
Riverway (Sheraton - 5th Floor)
4:00 PM - **JJ2.1
Polymer Dynamics Under Quasi-uniaxial Confinement. The Case of PEO in Porous Alumina.
Karine Lagrene 1 , Jean-Marc Zanotti 1 , Mohamed Daoud 2 , Patrick Judeinstein 3 , Kay Saalwaechter 4 , Bela Farago 5 , Peter Fouquet 5 , Jacques Ollivier 5 , Marco Maccarini 5 Show Abstract
1 , Laboratoire Leon Brillouin (CEA/CNRS), Gif-sur-Yvette Cedex France, 2 , CEA/Service de Physique de l’Etat Condensé, Gif-sur-Yvette France, 3 , eRMN, ICMMO, UMR 8182, Bât 410, Université Paris-Sud, Orsay France, 4 , Institute of Physics - NMR Group, Martin-Luther-University Halle-Wittenberg, Halle Germany, 5 , Institut Laue-Langevin, 6 Rue J. Horowitz, Grenoble France
Thanks to numerous theoretical developments, it is now possible to draw a close relationship between polymer rheology in the bulk and chain dynamics at the molecular level. Nevertheless, the peculiar properties of polymers in interfacial situations or deep confinement are not completely understood. For instance, recent NMR relaxometry results suggest that the confinement of a polymer melt in a nanoscopic isotropic porous matrix leads to a chain dynamics that is dramatically different from the bulk behavior: the reptation tube diameter under confinement would be one order of magnitude smaller than in the bulk (for PEO: dconf = 0.6 nm << dbulk = 7 nm). This phenomenon has been called the “corset effect” .The aim of the present work is to extend the relaxometry study reported in the literature to the case of PEO confined in an anisotropic matrix. The confining materials are home-made Anodic Aluminum Oxide (AAO) membranes . The porosity of the AAO matrices is made of micrometers long, macroscopically highly oriented cylindrical pores, with diameters as low as 13 nm. We analyze this structure in details by a concurrent small angle scattering (SANS) and microscopy (SEM) analysis .We show how to take advantage of the macroscopic orientation of AAO pores to evidence orientational dynamical effect – namely radial or longitudinal with respect to the highly anisotropic pore geometry.At large time and spatial scales (ms and micrometers) as probed by PFG-NMR, we observe a highly anisotropic dynamics. We also probe the polymer dynamics using a technique that measures simultaneously the time and space correlations: inelastic neutron scattering (time-of-flight and neutron spin-echo). Results at different length scales (3 up to 500 Å) are reported. N. Fatkullin, E. Fischer, C. Mattea, U. Beginn and R. Kimmich, Chem. Phys. Chem., 5, 884 (2004). H. Masuda and K. Fukuda, Science, 268, 1466 (1995).  K. Lagrené and J.-M. Zanotti, Eur. Phys. J. ST, 141, 261 (2007).
4:30 PM - **JJ2.2
Dynamics of Rod-like Macromolecules in Confinement.
Felix Hoefling 1 , Thomas Franosch 1 , Erwin Frey 1 Show Abstract
1 , LMU Munich, Munich Germany
The understanding of transport in heterogeneous media is fundamental for a variety of applications ranging from material sciences to biological system. Mesoporous host materials form one of the key materials in nanotechnology, because their nanometer sized channels provide room for guest species at the molecular level. In biology, dense packing of differently sized proteins, lipids and sugars in the cell cytoplasm, known as molecular crowding, leads to strongly suppressed transport whose relevance for cellular processes is presently explored. The motion of macromolecules inside such materials is strongly hindered and often displays anomalous transport properties.We have investigated the dynamics of macromoelcules in heterogeneous media, where they move in an array of randomly distributed frozen obstacles. For point-like particles we resolve a long-standing debate on the nature of the long-time anomalies and the interplay with the critical dynamics close to the percolation threshold of the Lorentz model. For rod-like particles we have employed extensive computer simulations to investigate the dynamics over a broad range of time scales, and find rich behavior of the mean-square displacement and the intermediate scattering function. The hindered motion of the rod-like macromolecules in confinement leads to strongly anisotropic dynamics manifested in a significant translation-rotation coupling which persists up to macroscopic time and length scales. We have developed a mesoscopic description of the dynamics valid from macroscopic distances down to the length scale of the interparticle distance. Our theory is based on the exact solution of the Smoluchowski-Perrin equation for the unconstrained motion. Employing the measured diffusion coefficients as input parameters we find quantitative agreement with our Brownian dynamics simulations in the dense regime. We also discuss extensions of these theories towards semiflexible polymers, like F-actin and microtubules.
5:00 PM - JJ2.3
Molecular Dynamics Simulation of Orientation Induced Homogenous Nucleation in Polymers.
Predrag Djuranovic 1 , Gregory Rutledge 2 Show Abstract
1 Materials Science, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
We report the computer simulation method to study crystal nucleation in long and short chain alkanes from orientation constrained melt. The early stages in crystallization of long chain molecules are very difficult to observe and analyze both experimentally and in simulation. In particular, due to the large free energy barrier to nucleation from the non-oriented amorphous melt state, it is difficult to observe nucleation events within reasonable simulation time. This problem is circumvented by employing the Semi-Grand Canonical Monte Carlo simulations, to create industrially relevant non-equilibrium constrained amorphous oriented states, fully characterized by their orientation distribution function, an experimentally obtainable quantity. Molecular dynamics simulation is then used to track the dynamics of the oriented systems and its evolution toward the crystalline phase. The simulation results are analyzed based on the concept of mean first-passage times, which enables us to locate the transition point (the height of nucleation barrier and the critical nucleus size) and to estimate polymer nucleation rates, solely based on analyzing molecular dynamics trajectories and with no further assumption of the underlaying mechanism of crystallization. The simulation results are then compared to the classical nucleation theory.
5:15 PM - JJ2.4
Lattice Behavior of Two Ferroelectric Polymers under Progressive Water Coverage.
Carolina Ilie 1 , Kenneth Buske 1 , Shawn Gray 1 , Jacob Chartrand 1 , Wes Laurion 1 , Peter Dowben 2 Show Abstract
1 Physics, SUNY Oswego, Oswego, New York, United States, 2 Physics and Astronomy, University of Nebraska at Lincoln, Lincoln, Nebraska, United States
We compare the interactions of water with the ferroelectric copolymer poly(vinylidene fluoride with trifluoroethylene P(VDF-TrFE) and poly(methylvinylidenecyanide) (PMVC) a strongly dipole ordered polymer. The polymers lattices swell with increase water coverage for both polymers, as studied with X rays diffraction[1,2]. This confirms one more time that both films, prepared by the Langmuir Blodgett technique from a water subphase, are crystalline, as previously determined by STM and band structure [1,2,3]. The absorption of water is believed to distort the polymer chain placement for both polymers. The water desorption from poly(methylvinylidenecyanide) is an intrinsically activated process by the strain in the polymer [1,4]. This tends to suggest that dipole rotation in the polymer substrate may play a key role. At microscopic scale, dipole interactions affect the surface chemistry at these polymer surfaces, as does lattice strain caused by water absorption, sterically hindering or enhancing desorption of adsorbed and absorbed water. 1 P. A. Dowben, Luis G. Rosa, Carolina C. Ilie, Z.Phys. Chem. 222 (2008), 755-7782 P.A. Jacobson, Luis G. Rosa, C.M. Othon, Kristin Kraemer, A.V. Sorokin, Stephen Ducharme, and P.A. Dowben, Applied Physics Letters, 84,88-90, (2004).3 Peter A. Dowben, Jie Xiao, Carolina C. Ilie, Luis G. Rosa, Journ. of Electron Spectroscopy and Related Phenomena (2009) in press4Carolina C. Ilie, P.A. Jacobson, I.N. Yakovkin, Luis G. Rosa, Matt Poulsen, D. Sahadeva Reddy, James M. Takacs, and P.A. Dowben , J. Phys. Chem. B 111, 7742-7746 (2007).
5:30 PM - JJ2.5
Phase Equilibrium and Morphology Study of Direct Isotropic/Smectic-A Polymer-dispersed Liquid Crystals.
Ezequiel Soule 1 , Nasser Abukhdeir 2 , Alejandro Rey 2 Show Abstract
1 aterials Science and Technology, University of Mar del Plata, Mar del Plata Argentina, 2 Department of Chemical Engineering, McGill University, Montreal, Quebec, Canada
An experimental study of two novel polymer-dispersed liquid crystal mixtures of PS (polystyrene) and liquid crystals that exhibit a direct isotropic/smectic-A (lamellar) mesophase transition was performed for PS/10CB (decyl-cyanobiphenyl) and PS/12CB (dodecyl-cyanobiphenyl). The study was performed using polarized optical microscopy and differential scanning calorimetry. Both phase separation (liquid/liquid demixing) and phase ordering (isotropic/smectic-A transition) were observed for different smectic PDLC compositions and partial phase diagrams determined for both materials (PS/10CB and PS/12CB). This includes isotropic/isotropic phase separation temperature (T_II) and isotropic/smectic-A phase ordering temperature (T_AI) for moderate to high concentrations of liquid crystal. PDLCs with low concentrations of liquid crystal exhibited simultaneous phase separation and ordering at temperatures below the pure liquid crystal phase transition (T_I/AI). In addition to thermodynamic observations, phase transition dynamics and morphology trends were determined depending on phase transition sequence, quench rate, and material composition (mass fraction of liquid crystal). Three stages of liquid crystal-rich domain growth morphologies were observed which result in spherical macroscale domains (“stage I”), highly anisotropic domains (“stage II”), and sub-micron spheroid domains (“stage III”). These include various complex growth morphologies typical of direct isotropic/smectic-A phase transitions.
Pierre Levitz Ecole Polytechnique
Ralf Metzler Technical University of Munich
David Reichman University of Columbia
JJ3: Dynamics of Biological Systems I
Tuesday AM, December 01, 2009
Riverway (Sheraton - 5th Floor)
9:30 AM - **JJ3.1
Exploring Nanostructured Channel Systems with Single Molecule Probes and Tracking Artificial Viruses in Living Cells.
Christoph Braeuchle 1 Show Abstract
1 , LMU Munich, Dept. Chemistry, Munich Germany
Molecular movement in confined spaces is of broad scientific and technological importance in areas ranging from molecular sieving and membrane separation to active transport along intracellular net-works.In this talk it will be shown how single dye molecules can be used as nanoscale probes to map out the structure of nanoporous silica channel systems [1-3]. In order to correlate the porous structure of the host with the diffusion dynamics of single molecules we present a unique combination of transmission electron microscopic (TEM) mapping and optical single molecule tracking (SMT) experiments. With this approach we can uncover how a single luminescent dye molecule travels through various defect structures in a thin film of nanoporous silica, how it varies its mobility in the channel structure, and how it bounces off a domain boundary. In addition, very high positioning accuracy in SMT experiments shows how a molecule can cross through defect structures into neighbouring channels and how adsorption at the walls can be observed as trapping events. These experiments reveal unprecedented details of the guest-host interactions and of the host's structure. The knowledge of these details and the use of mesoporous nanoparticles with functionalized pore walls  will lead to novel drug delivery systems.Another type of drug delivery systems are synthetic viruses. Based on our experience of investigating the infection pathway of single viruses into living cells  we have recently investigated the targeting and uptake of DNA-polyplexes as synthetic viruses in a biomimetic approach [6, 7]. Single molecule techniques can be used to improve the efficiencies of such synthetic viruses in novel gene therapy applications.
10:00 AM - **JJ3.2
If Diffusion in Cells or Cell Membranes is Anomalous, Why Is It So?
Michael Saxton 1 Show Abstract
1 Biochemistry & Molecular Medicine, University of California, Davis, California, United States
Several studies have reported anomalous subdiffusion of various species in the plasma membrane, cytoplasm, and nucleus of cells. Others have found normal diffusion for various other species by explicit tests. The vast majority of studies have simply assumed normal diffusion. The mathematical models that give anomalous subdiffusion are well-characterized, but the first key question is, what cellular structures or processes can yield those models? Percolation is not likely because the percolation threshold is extremely sensitive to the diameter of the diffusing species. Most species are below the threshold and show transient anomalous subdiffusion, or are above the threshold and immobilized. Transient binding is biologically plausible for some species, but for diffusion to be anomalous, the mobile species must be in a nonequilibrium state with respect to the binding sites, for example, a protein that has newly entered the plasma membrane or the nucleus. Two other possibilities are networks - the cytoskeleton and membrane skeleton - and crowding. Here the traps are geometric, and the nonequilibrium state is maintained over time by the traps, not the diffusing species. A final possibility is that several mechanisms operate in parallel, perhaps on different time scales. A second key question is the dependence of anomalous subdiffusion on metabolic energy. Percolation is independent of metabolic energy. Binding requires an energy input to set the traps or empty them, and to drive the insertion cycle. For networks and crowding, obstacle dynamics may be driven either by thermal fluctuations or by metabolic energy. Supported by NIH grant GM038133.
10:30 AM - **JJ3.3
The Nuclear Pore: A Thermodynamic Machine.
Ronen Kopito 1 , Michael Elbaum 1 Show Abstract
1 Dept of Materials and Interfaces, Weizmann Institute of Science, Rehovot Israel
The cells of higher living organisms, the eukaryotes, are internally divided into distinct, membrane-bound organelles embedded in the cytoplasm. The most prominent of these is the nucleus, enclosing the chromatin. Molecular exchange between the nucleus and the cytoplasm takes place through the nuclear pores. Together with soluble protein components, the nuclear pores constitute a biochemical machine capable of concentrating molecular cargo on one or the other side, against a gradient in chemical potential. The phenomenon is typically known as nuclear import (or export). Specific protein receptors, the importins, mediate the interaction between molecular cargo and the nuclear pore. This interaction is in turn regulated by a small GTPase, Ran. The transport can be established in vitro using nuclei reconstituted from Xenopus egg extract. This cell-free system provides ideal conditions to reveal the underlying thermodynamic mechanism.A combination of classical kinetic measurements by confocal fluorescence microscopy and correlation spectroscopy show an unexpectedly simple behavior. Accumulation of a model cargo in the nucleus follows first-order kinetics, and its rate displays a Michaelis-Menten dependence on concentration. This is a hallmark of receptor-mediated transport. Saturation, on the other hand, represents a stable coexistence between differing nuclear and cytoplasmic concentrations. In contrast to implication of the nomenclature, net accumulation coexists with a balanced, bidirectional flux of cargo-receptor complexes through the nuclear pores. This has profound implications for communication and signalling in the cell. A useful paradigm considers the transport receptors as enzymes that lower the free energy barrier for passage through the pore. The receptors, as enzymes, have multiple substrates, i.e., transport cargoes and Ran in its GTP state. The measured behavior is completely consistent with equilibration of the receptor-cargo complexes across the pore. A competing interaction with RanGTP in the nucleus establishes the cargo concentration gradient between the nucleoplasm and the cytoplasm. The transport system thus defines a chemical pump with molecular specificity but no moving parts or force-generating elements.References:1.R.B. Kopito & M. Elbaum (2007) Reversibility in nucleocytoplasmic transport. Proc Nat Acad Sci USA 104: 12743-8.2.R.B. Kopito & M. Elbaum (2009) Nucleocytoplasmic transport: a thermodynamic mechanism. HFSP J 3: 130.
11:30 AM - JJ3.4
Measuring Anomalous Dynamics in Biological Systems.
Ralf Metzler 1 Show Abstract
1 Physics Department, Technical University of Munich, Munich Germany
Single particle tracking offers unique possibilities to gather information on the passive motion of larger biomolecules in living cells. In numerous cases the measured random motion was found to be subdiffusive. To interpret such anomalous motion physically, several complementary quantities will be analysed, in particular base on mean maximal excursions. These quantities allow us to more precisely pin down the stochastic mechanism underlying the motion. A discussion of different sets of experimental data may suggest that none of the standard subdiffusion models may explain the observations.
11:45 AM - **JJ3.5
Active Transport on Disordered Microtubule Networks.
Rony Granek 1 4 , Aviv Kahana 2 1 , Mario Feingold 2 4 , Michael Elbaum 3 Show Abstract
1 Dept. of Biotechnology Engineering, Ben-Gurion University of The Negev, Beer-Sheva Israel, 4 The Ilse Katz Center for Meso and Nanoscale Science and Technology, Ben-Gurion University of The Negev, Beer-Sheva Israel, 2 Department of Physics, Ben-Gurion University of The Negev, Beer-Sheva Israel, 3 Department Materials and Interfaces, The Weizmann Institute of Science, Rehovot Israel
The motion of small cargo particles on microtubules by means of motor proteins in disordered microtubule networks is investigated theoretically using both analytical tools and computer simulations . This study differs from our previous studies concerning cargo particles that are much larger than the network mesh size for which a superdiffusion behavior ~t3/2 has been observed and theoretically explained . Different network topologies in two and three dimensions are considered, one of which has been recently studied experimentally in Ref. . A generalization of the random velocity model is used to derive the mean square displacement of the cargo particle. We find that all cases belong to the class of anomalous super-diffusion, that is sensitive mainly to the dimensionality of the network and only marginally to its topology. Yet, in three dimensions the motion is very close to simple diffusion, with sub-logarithmic corrections that depend on the network topology. When details of the thermal diffusion in the bulk solution are included, no significant change to the asymptotic time behavior is found. However, a small asymmetry in the mean microtubule polarity affects the corresponding long time behavior. We also study a 3-dimensional model of the microtubule network in living animal cells. Three first passage time problems of intracellular transport are simulated and analyzed for different motor processivities: (i) cargo that originatesnear the nucleus and has to reach the membrane, (ii) cargothat originates from the membrane and has to reach the nucleus, and (iii) cargo (e.g., mRNA) that leaves the nucleus and has to reach a specific target in the cytoplasm (e.g., the ribosome). We conclude that while a higher motor processivity increases the transport efficiency in cases (i) and (ii), in case (iii) it has theopposite effect. We conjecture that the balance between thedifferent network tasks, as manifested in cases (i) and (ii) vs case (iii), may be the reason for the evolutionary choice of a finite motor processivity.References: A. Kahana et al., Phys. Rev. E, vol. 78, 051912 (2008). A. Caspi et al., Phys. Rev. E, vol. 66, 011916 (2002). H. Salman et al., Biophys. J., vol 89, 2134 (2005).
12:15 PM - JJ3.6
Structural Regularities, Tessellations, and Collectivity of Folded Proteins.
Ibrahim Inanc 1 , Ali Atilgan 1 , Canan Atilgan 1 Show Abstract
1 , Sabanci University, Istanbul Turkey
Coarse-grained network models of proteins successfully predict equilibrium properties related to collective modes of motion. In this study, the network construction strategies and their systematic application to proteins are used to explain the role of network models in defining the collective properties of the system. The analysis is based on the radial distribution function, a newly defined angular distribution function and the spectral dimensions of a large set of globular proteins. Our analysis shows that after reaching a certain threshold for cut-off distance, network construction has negligible effect on the collective motions and the fluctuation patterns of the residues. The results demonstrate that the slow modes are immune to the details of network construction once the essential contacts in the first few coordination shells are included. Thus, the properties that depend on the most collective modes may be studied independent of this choice. Depth dependent analysis, on the other hand, shows that the densely packed core region of the protein has a different local structure built around it compared to its surface. In the core of the protein, the second neighbors have a non-random distribution that is more pronounced than the first neighbors. In the surface residues, the reverse is observed. Specifically, networks constructed by using Voronoi tessellations fail to correctly define the local interactions while they successfully incorporate the long-range pair-wise interactions.
JJ4: Dynamics of Confined Fluids in Porous Materials
Tuesday PM, December 01, 2009
Riverway (Sheraton - 5th Floor)
2:30 PM - **JJ4.1
Survival Probability in Heterogeneous Media.
Denis Grebenkov 1 Show Abstract
1 PMC, CNRS - Ecole Polytechnique, Palaiseau France
We consider a diffusive process in a bounded domain with relaxing sinks which are heterogeneously distributed either inside the domain (bulk relaxation), or on its boundary (surface relaxation). This is a mathematical model for many biological and industrial systems, e.g.1) chemical reactors with inhomogeneous spatial istributions of catalytic germs;2) biological cells with specific arrangements of organelles;3) mineral porous media with relaxing agents in NMR experiments.The survival probability, or the concentration of survived particles that diffuse in such a medium, exhibits an exponential decay at long times. Classically, the decay constant (i.e., the "lifetime" of a diffusing particle) is expected to be proportional to the total amount, or "strength", of relaxing sinks, irrespectively of their spatial distribution. We revisit this classical approximation and illustrateits limitations. For this purpose, we represent the survival probability in the form of a spectral decomposition which is based on the Laplace operator eigenfunctions. Using spectral tools, we calculate the survival probability for particles that diffuseinside the unit disk with a heterogeneous spatial distribution of relaxing sinks (either in the bulk, or on the surface). We show that the spatial heterogeneity may drastically change the expected long-time behavior. This result is potentially important for optimizing chemical reactors, understanding biological processes and interpreting relaxed signals in NMR.
3:00 PM - JJ4.2
Colloidal Transport Through Micro Porous Media.
Yujie Li 1 2 , Clemens Bechinger 1 2 Show Abstract
1 , Max-Planck-Institut für Metallforschung, Stuttgart Germany, 2 , 2. Physikalisches Institut, Universität Stuttgart, Stuttgart Germany
We present an experimental study of the transport properties of colloidal particles in disordered micro porous structures. Quasi two-dimensional porous structures were designed via modified point processes and produced by the soft lithography. As particles we used colloidal spheres with diameters between 1 μm and 6 μm whose flow was driven by a pressure drop along the sample. Individual particle trajectories were recorded by a fast CMOS camera with a temporal resolution of 2 ms and spatial resolution of 0.3 μm. By precisely controlling the flow rate, we were able to tune the Reynolds number (Re) and Péclet number (Pe) by up to three orders of magnitude. In agreement with theoretical predictions, we find the mean square displacement (MSD) to increase with time t according to a power law 〈R2〉∼tα, where α varies for different regimes. From the crossover between ballistic (α=2) and super diffusive (α>1) transport, we can estimate the characteristic length scale of the structures. Within the super diffusive regime the exponent α depends on the applied pressure due to the change in the dynamic fractal dimensions of the system. When plotting the longitudinal dispersion coefficients in ordered and disordered structures as a function of the Pe number, we find clear evidence for different dispersion mechanisms. Our results suggest that such systems can serve as powerful model systems to understand transport mechanisms through porous materials on a single particle level and thus may help to understand how the microscopic and macroscopic properties are related in such systems.
3:15 PM - JJ4.3
Transport of Ions, Polymers, Biomolecules and Gold Nanoparticles through Hierarchical Biogenic Silica Nanostructures.
Kai-Chun Lin 1 , B. Ramakrishna 1 , Nipun Chaplot 2 , Michael Goryll 2 , Srivatsa Aithal 2 , Shalini Prasad 2 , Huang-Chiao Huang 3 , Kaushal Rege 3 Show Abstract
1 School of Materials, Arizona State University, Tempe, Arizona, United States, 2 Electrical Engineering Department, Arizona State University, Tempe, Arizona, United States, 3 Chemical Engineering Department, Arizona State University, Tempe, Arizona, United States
Biogenic silica nanostructures, derived from diatoms, possess highly ordered porous hierarchical nanostructures and afford flexibility in design in large part due to the availability of a great variety of shapes, sizes, and symmetries from the over 100,000 known species. These advantages have been exploited for study of transport phenomena of ions and molecules towards the goal of developing ultrasensitive and selective filters and biosensors. Controlled etching of the silica structure has been employed to tune both manipulate the dimensions of the nanopores as well as the hierarchy. These structures across a wide length scale regime have been characterized by scanning electron microscopy and atomic force microscopy. Results from the study of the transport phenomena based on size and chemical considerations will be presented for ions, such as potassium and sodium, present in physiologically relevant isotonic buffers, polystyrene nanobeads and gold nanoparticles. Thermally responsive bio-inspired polypeptides and photothermally responsive gold nanorod-polypeptide nanoassemblies have been investigated with the goal of achieving reversible modulation of the transport across the hierarchical multiscale pore structure of the diatoms.The effect of hierarchical pore structure on particle transport through the biogenic silica membrane has been modeled using COMSOL multi-physics modeling platform. We expect to leverage the understanding of the transport phenomena through well-characterized hierarchical pore structures towards designing hybrid devices for applications in separation and sensor technologies.
3:30 PM - JJ4.4
Dynamical Surface Affinity of Diphasic Liquids as a Probe of Wettability of Multimodal Porous Media.
Jean-Pierre Korb 1 , Gabriel Freiman 1 , Benjamin Nicot 2 , Patrice Ligneul 2 Show Abstract
1 Laboratoire PMC, Ecole Polytechnique, Palaiseau France, 2 Dhahran center, Schlumberger SDCR, Al-Khobar Saudi Arabia
We introduce a new method for estimating the wettability of rock/oil/brine systems using non-invasive in situ nuclear magnetic relaxation dispersion (NMRD). This technique scans over a large range of applied magnetic fields and yields unique information about the extent to which a fluid is dynamically correlated with a solid rock surface. Unlike conventional transverse relaxation studies, this approach is a direct probe of the dynamical surface affinity of fluids. To quantify these features we introduce a microscopic dynamical surface affinity index which measures the dynamical correlation (i.e. the microscopic wettability) between the diffusive fluid and the fixed paramagnetic relaxation sources at the pore surfaces. We apply this method to carbonate reservoir rocks which are known to hold about two thirds of the world’s oil reserves. Although this non-destructive method concerns here an application to rocks, it could be generalized as an in situ liquid/surface affinity indicator for any multimodal porous medium, including porous biological media.
3:45 PM - JJ4.5
Humidity Transport in a Material of Interconnected Meso- and Nanopores: X-ray Studies of a Model Clay.
Lars Alme 1 , Henrik Hemmen 1 , Jon Otto Fossum 1 , Yves Meheust 2 Show Abstract
1 Physics, Norwegian University of Science and Technology, Trondheim Norway, 2 Geosciences Rennes, Université Rennes 1, Rennes France
We study the transport of water vapor through a weakly hydrated, loosely-aggregated powder of the synthetic clay Na-fluorohectorite (NaFh). Our NaFh crystallites consist of a stack of ~100 individual 1 nm thick platelets, with a layer charge that allows for stepwise swelling of the interlayer space. This is a well known property of smectites, and has previously been characterized for this clay [1-3]. The dynamics of water molecules through the clay powder's interparticle mesoporosity is a complex phenomenon, since (i) part of the humidity can get trapped in the clay particles, and (ii) the particles swell as a result of water intercalation, which leads to a change in the geometry of the mesopores. By imposing a humidity gradient along samples contained in 1 mm diameter cylinders kept at fixed temperatures, we have studied quasi one-dimensional water transport in these systems [4,5]. X-ray scattering from the nano-stacks, resolved in time and in space, provides two types of information: firstly, the amount of water intercalated in the nanopores; secondly the associated relative humidity levels in the surrounding mesopores. The latter is obtained through direct XRD observations of minute changes (~ 0.1 nm) in the interlayer repetition distance. We are thus able to follow in situ the dynamics of two fronts: an intercalation front of water entering the nanopores, as well as a front characteristic of mesoporous humidity levels. Both fronts obey a near-normal diffusive process, evident from the fact that they rescale onto a universal curve when plotted as function of the scaling variable x/t1/2 [5,6].  da Silva G.J., Fossum J.O., DiMasi E., Maloy K. J., Lutnaes S.B. (2002), Synchrotron x-ray scattering studies of water intercalation in a layered synthetic silicate. Phys. Rev. E 66, 011303. da Silva G.J., Fossum J.O., DiMasi E., Maloy K.J. (2003), Hydration transitions in a nanolayered synthetic silicate: A synchrotron x-ray scattering study. Phys. Rev. B 67, 094114. R. P. Tenório, L. Ramstad Alme, M. Engelsberg, Jon Otto. Fossum, F. Hallwass (2008) Geometry and Dynamics of Intercalated Water in Na-Fluorhectorite Clay Hydrates. J. Phys. Chem. C 112, 575-580 Løvoll G., Sandnes B., Meheust Y., Maloy K.J., Fossum J.O., da Silva G.J., Mundim M.S.P., Droppa R. Jr., Fonseca D. M. (2005). Dynamics of water intercalation fronts in a nano-layered synthetic silicate: A synchrotron X-ray scattering study. Physica B: Condensed Matter 370, 90-98. Lars Ramstad Alme (2007) Water transport in selected nanoporous media. Master thesis – Department of Physics, Norwegian University of Science and Technology  Eduardo N. de Azevedo, Lars. R. Alme, Mario. Engelsberg, Jon Otto Fossum, Paul Dommersnes (2008), Fluid imbibition in paper fibers: Precursor front. Phys.Rev.E 78, 066317
4:30 PM - JJ4.6
Drug-Model Molecules Confined in Raw and Organically Modified Mesoporous Silica: Characterization Through Hyperpolarized 129Xe and Multinuclear Solid State MAS NMR Spectroscopy.
Thierry Azaies 1 , Daniela Aiello 1 , Guillaume Laurent 1 , Andrei Nossov 2 , Flavien Guenneau 2 , Florence Babonneau 1 Show Abstract
1 Chimie de la Matière Condensée, Université Paris 6, Paris France, 2 Systèmes Interfaciaux à l’Echelle Nanométrique, Université Paris 6, Paris France
Nowadays, there is a growing interest for researchers in the design of materials for drug storage and controlled release in order to increase efficiently the solubility of hydrophobic drugs in biological fluids. In particular, drug molecules confined in mesoporous silica are good candidates for that purpose as they exhibit a fast in vitro release that can be modulated by the modification of the silica surface. On a pharmaceutical point of view, it is of high importance to characterize the physical state of the encapsulated entities, as unexpected physical state for the entrapped molecules was already observed in these materials at ambient temperature due to the existence of confinement effects [1-2].In this contribution, we present the study of drug-model molecules such as carboxylic acids (benzoic acid ΦCOOH, lauric acid C11H22COOH and ibuprofen, a nonsteroidal anti-inflammatory drug) as well as phenylphosphonic acid ΦPO(OH)2 confined in MCM-41 or SBA-15 mesoporous silica materials. We demonstrate using T2(1H) and T2(13C) measurements from -60°c to ambient that organic acids are submitted to confinement effects as their phase transitions are found to be lower once they are encapsulated. Nevertheless, the depression depends on the chemical nature of the entrapped specie. For instance in MCM-41, confined ibuprofen exhibit a liquid-to-solid transition at -55°C (vs. +77°C for the bulk) whereas for confined ΦPO(OH)2 the transition is found at +125°C (vs. +162°C for the bulk). Curiously, the phase transition in our systems do not follow the Gibbs-Thomson equation as similar liquid-to-solid transition temperatures are found for pore sizes ranging from 30 to 100 Å. Furthermore, we studied the molecules repartition in the mesoporous matrix by hyperpolarized 129Xe spectroscopy. A homogenous pore filling corresponding to a monolayer is deduced from low temperature hyperpolarized 129Xe spectroscopy combined to nitrogen adsorption-desorption experiments. Nevertheless, in wide pore samples (100 Å), some patches are found that probably lead to the crystallization of the molecules at low temperature whereas a glass transition is found in small pore sample (30 Å).Finally, cross polarization CP MAS NMR spectroscopy allowed us to probe finely the organic / inorganic interface in these systems. Indeed, 1H-X (where X = 13C, 29Si or 31P) CP MAS experiments revealed the specific interaction of the carboxylic or phosphonic acid moieties with the silica walls. The proton spectral resolution improvement will be illustrated through high field (700 MHz) and high spinning speed (up to 65 kHz) experiments T. Azaïs, C. Tourné-Péteilh, F. Aussenac, N. Baccile, C. Coelho, J.-M. Devoisselle, F.Babonneau Chem. Mater. 18 (2006) 6382-90. T. Azaïs, G. Hartmeyer, S. Quignard, G. Laurent, C. Tourné-Péteilh, J.-M. Devoisselle, F. Babonneau, Pure Appl. Chem., ASAP Article, doi:10.1351/PAC-CON-08-11-10.
4:45 PM - JJ4.7
Percolation and Water Transport in the Nanopores of Nafion Membrane.
Ram Devanathan 1 , Michel Dupuis 1 Show Abstract
1 Chemical & Materials Sciences Division, Pacific Northwest National Laboratory, Richland, Washington, United States
Proton exchange membrane fuel cells (PEMFC) offer the promise of conversion of chemical energy of fuel to electrical energy with high efficiency and minimal pollution. The heart of the PEMFC is a polymer membrane that separates the reactants and conducts protons. Proton transport in polymer membranes, such as the widely-used Nafion, requires the presence of water. At the same time, electroosmotic drag of water across the membrane creates water management challenges. We have used molecular-level computer simulation to systematically study the connectivity and dynamics of water molecules confined in nanopores of Nafion membrane. At low hydration levels, the protons are bound to the sulfonate groups and water molecules are isolated in the cavities of the membrane. As the hydration level increases, water molecules form increasingly larger clusters and the diffusion coefficient of water increases. At a critical threshold hydration level of about 7 water molecules per sulfonate group, the water network percolates through the membrane. The formation of such a network is essential for proton hopping to occur. Our results shed light on experimental observations of the dynamics of water molecules in fuel cell membranes.
5:00 PM - JJ4.8
Nanocomposite Protonic Electrolytes: Multi-scale Structural and Dynamical Studies.
Patrick Judeinstein 1 , Joao Teles 1 , Cristina Iojoiu 2 , Jean-Yves Sanchez 2 , Ksenia Maver 3 , Urska Lavrencic-Stangar 3 , jean-Marc Zanotti 4 , Jacques Ollivier 5 Show Abstract
1 ICMMO, UMR 8182, CNRS, Univ. Paris-Sud, Orsay France, 2 LEPMI, UMR 5631, CNRS-INPG-UJF, Saint Martin d'Hères France, 3 Laboratory for Environmental Sciences, University of Nova-Gorica, Nova Gorica Slovenia, 4 Laboratoire Léon Brillouin (CEA-CNRS), CEA Saclay, Gif-Sur-Yvette France, 5 , Institut Laue-Langevin, Grenoble France
Proton Exchange Membrane Fuel Cells (PEMFCs) are expected to fulfill many promising applications as power source. Electrical generation result from concerted redox reactions of hydrogen and oxygen gas. The heart of such systems are the membranes which should provide sufficient protonic conduction as well as adequate mechanical properties in adequation with the operating condition of the fuel cell. Perfluorinated ionomers such as Nafion membranes are considered as promising candidates. However their protonic conductivity involves water molecules entrapped inside the membranes and then the main drawback is the upper limit temperature which is around 80°C. Nanocomposite materials provide challenging opportunities to design new protonic conducting materials and prevent such disadvantages. Charge carrier mobility is provided from a liquid-like phase which is entrapped inside a solid-like matrix. Controlling the confinement of the mobile phase is a key point to tune the final properties of such polyphasic materials. This paper reports on studies of two families of proton conducting membranes. In the first one, hydrated heteropolycid clusters are dispersed inside organic-inorganic membranes, and the choice of the matrix (hydrophilic/hydrophobic balance) allows to control the segregation of the different components. In the second one, protic ionic liquids obtained by reacting alkylamines with various acids are used to swell modified Nafion® films. The conductivity and stability of these membranes are strongly related to the intimate structure of the ionic liquid component, ionic dissociation and their interactions with the polymer host-structure. Combination of various nuclear magnetic resonance (NMR) experiments and Quasi-Elastic Neutron Scattering (QENS) allow to decipher structure and dynamics in these complex structures in the pm-μm space and pico- to second scales.- NMR scalar correlations and intermolecular Overhauser spectroscopies (13C, 1H and 15N, 1H) experiments probe the nature of protonic species and ion-pairing inside pure ionic liquids;- QENS yields elementary mobility steps at the molecular level while pulsed field gradient NMR measure the long range (μm) translational self diffusion coefficient of all the mobile components as well as concerted translational motions. - NMR cryporometry and PFG-NMR allows to obtain the mixing degree of the different phases, their molecular interactions and the geometry of the confined phase. Iojoiu C., Judeinstein P., Sanchez J.-Y., Electrochimica Acta, 53, (2007), 1395-1403 Iojoiu C., Martinez M., Hanna M., Molmeret Y., Cointeaux L., Leprêtre J.-C., El Kissi N., Guindet J., Judeinstein P., Sanchez J.-Y., Pol. Adv. Techn., 19, (2008), 1406-1414.  Judeinstein P., Iojoiu C., Sanchez J.-Y., Ancian B. , J. Phys. Chem. B, 112, (2008), 3680-3683.  K. Maver, U. Lavrencic-Stangar, P. Judeinstein, J.M. Zanotti, J. Non-Cryst. Solids, 354, (2008), 680-687.
5:15 PM - JJ4.9
Molecular Dynamics in Bulk and Confined Supercooled Glycerol: Results From 13C NMR Spectroscopy.
Pragati Jain 1 , Sabyasachi Sen 1 , Andrey Levchenko 2 , Olga Trofymluk 2 , Alexandra Navrotsky 2 Show Abstract
1 Chemical Engineering & Materials Science, University of California, Davis, Davis, California, United States, 2 Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California, Davis, Davis, California, United States
13C NMR spectra of bulk glycerol and glycerol confined in ~ 2 nm pores in mesoporous silica are collected over the entire temperature range of supercooling: Tg (185 K) ≤ T ≤ Tm (293 K). The temperature dependent evolution of the 13C NMR line shapes indicates dynamical averaging of the chemical shift anisotropy, indicating isotropic rotational jump of the constituent molecules in both the bulk and the confined supercooled liquids. In bulk glycerol, the temperature dependence of the rotational timescale τNMR is non-Arrhenius and it corresponds well with the α-relaxation timescales. In the confined case, however, τNMR shows an Arrhenius behavior characteristic of a β-process with a crossover, being slower at higher temperatures (T > 230K) and faster at lower temperatures than in the bulk. The α-relaxation process of the confined liquid couples with τNMR at T ≥ 273 K implying a fragile-to-strong transition in glycerol upon nano-confinement.
Pierre Levitz Ecole Polytechnique
Ralf Metzler Technical University of Munich
David Reichman University of Columbia
JJ5: Dynamics of Biological Systems II
Wednesday AM, December 02, 2009
Riverway (Sheraton - 5th Floor)
10:00 AM - **JJ5.2
Nucleic Acid Interaction Kinetics Modulate the Chaperone Activity of Retroviral Nucleocapsid Proteins.
Fei Wang 1 , Kristen Stewart-Maynard 2 , Dominic Qualley 4 , Robert Gorelick 5 , Ioulia Rouzina 3 , Karin Musier-Forsyth 4 , Mark Williams 1 Show Abstract
1 Department of Physics, Northeastern University, Boston, Massachusetts, United States, 2 Department of Chemistry and Institute for Molecular Virology, University of Minnesota, Minneapolis, Minnesota, United States, 4 Departments of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio, United States, 5 AIDS Vaccine Program, Basic Research Program, SAIC-Frederick, Inc., NCI-Frederick, Frederick, Maryland, United States, 3 of, Biochemistry, Molecular Biology, and Biophysics, University of Minnesota, Minneapolis, Minnesota, United States
Retroviral nucleocapsid (NC) proteins are essential for several viral replication processes including specific genomic RNA packaging and reverse transcription. The nucleic acid chaperone activity of NC facilitates the latter process. In this study, we use bulk and single molecule methods to quantify the chaperone activity of NC proteins from human immunodeficiency virus type 1 (HIV-1), Moloney murine leukemia virus, Rous sarcoma virus, and human T-cell leukemia virus type one (HTLV-1). We find that the nucleic acid interaction properties of these proteins vary significantly depending on the virus, with HIV-1 NC showing rapid protein binding kinetics, significant duplex destabilization, and strong DNA aggregation, all properties that are believed to be critical components of nucleic acid chaperone activity. In contrast, HTLV-1 NC exhibits significant destabilization activity but extremely slow DNA interaction kinetics and poor aggregating capability. This result explains why HTLV-1 NC is a poor nucleic acid chaperone. However, removal of HTLV-1 NC’s anionic C-terminal domain (CTD) results in a protein with chaperone activity comparable to that of other retroviral NCs. Removal of the CTD also dramatically increases the protein-DNA interaction kinetics. These results suggest that HTLV-1 NC’s anionic CTD interacts with its cationic N-terminal domain (NTD), either intra- or intermolecularly, which in turn slows down the protein’s nucleic acid binding kinetics. This electrostatic attraction between bound molecules leads to polymerization of HTLV-1 NC on the nucleic acid, which inhibits nucleic acid aggregation, as well as rapid protein dissociation from single-stranded DNA. These results may also help to explain the mechanism by which the CTD of HTLV-1 NC prevents packaging of human APOBEC3G. This work was funded in part by Federal Funds from NCI, NIH under contract N01-CO-12400 (RJG).
10:30 AM - **JJ5.3
Nonequilibrium Force Fluctuations on Magnetic Probes inside Living Cells.
D. Robert 1 , F. Gallet 1 , C. Wilhelm 1 Show Abstract
1 Laboratoire Matière et Systèmes Complexes (MSC), UMR 7057 CNRS & University Paris Diderot, Paris France
The disposal of magnetic vesicles (endosomes or phagosomes) inside living cells opens a new window to explore the deviation from equilibrium and to evaluate the forces generated by molecular motors.The magnetic vesicles act both as probes that can be manipulated with external magnetic fields to infer the viscoelastic modulus of their surrounding microenvironment, and as biological vehicles that are trafficked along the microtubule network by means of forces generated by molecular motors. - The intracellular viscoelastic modulus exhibits a power law dependence with frequency, which is microtubule and actin-dependent. This demonstrates that intracellular microrheology is not tied to a particular relaxation time but is governed by multiscale processes involving a distribution of relaxation times. - The mean square displacements of the probes do not follow the predictions of the fluctuation-dissipation theorem, with movements largely superdiffusive.The active forces could then be deduced from the combination of active and passive measurements, through the use of a generalized Langevin equation. The results reveal an actin-dependent reinforcement of the force with time. Microtubule disruption brings the intracellular medium closer to thermal equilibrium: the generation of active forces applied to endosomes depends on microtubule-associated motors. The modeling of the experimental active forces power spectrum demonstrates that the probes are pulled by simultaneously at least three molecular motors, developing a force of 7 pN each.
11:30 AM - **JJ5.4
Anomalous and Non-linear DNA Dynamics.
Roland Netz Netz 1 , Michael Hinczewski 1 , Hirofumi Wada 1 Show Abstract
1 , Technical University Munich, Garching Germany
Theoretical approaches for DNA dynamics rely on a combination of simulations, continuum modeling and scaling approaches. This is explained with a few recent examples:- The local dynamics of DNA is scale dependent and exhibits elastic effects, entropic effects and center-of-mass dynamics as one goes from smaller to larger scales, in accordance with recent fluorescence-correlation spectroscopy data and with measurable consequences for the binding rate of peptides to DNA.- In modern single-molecule studies of protein unfolding DNA functions as force-transducer. Extracting protein folding landscapes and transition rates requires deconvolution of the experimental data and thus knowledge of the dynamic response of DNA chains.- A DNA polymer that is continuously rotated at one end exhibits a critical rotational frequency, at which the dominant hydrodynamic dissipation mode changes from speedometer-cable-like axial spinning to a loop creation/diffusion mode. This is relevant for in-vivo DNA replication and transcription processes.
12:00 PM - **JJ5.5
Water at Biological Interfaces.
Robert Bryant 1 , Jean-Pierre Korb 2 Show Abstract
1 Chemistry, University of Virginia, Charlottesville, Virginia, United States, 2 Laboratoire de Physique de la Matiere Condensee, Ecole Polytechnique, CNRS 91128, Palaiseau France
Biological surfaces pose unique problems because of topological, chemical, and electrostatic heterogeneity. In addition, the water exploration of the interface may include penetration or binding events of whole water molecules as well as chemical exchange reactions between water and proton labile functional groups such as amines, alcohols, and oxygen acids. Nuclear spin-lattice relaxation is not spontaneous, but is driven by the magnetic noise created by molecular motions. Therefore, nuclear spin-lattice relaxation measurements as a function of magnetic field strength (Magnetic Relaxation Dispersion) provide a fundamental report of the molecular dynamics in the system. In the case that water protons sample an interface, the relaxation is usually much more efficient at the interface compared with the bulk liquid. Although there is exchange of water molecules between the interface and the bulk, the relaxation rate constant is very largely determined by the interfacial dynamics. Depending on the system, the MRD profile may reflect the diffusive anisotropy, the local translational diffusion constant, and the interfacial water lifetimes. When the surface is sticky or penetrable, the water spin-lattice relaxation may reflect the structural dynamics of the interface, the water residence times, and their distribution or the spin dynamics of the underlying solid. Proteins are the major factor in many functional biological systems. In protein solutions, the water interactions report the rotational mobility of the protein, the number of long-lived water molecules, and the nature of the diffusive motions at the interface. When the rotational motions of the protein is quenched as in cross-linked gels or semisolid systems such as tissues or hydrated solid systems, the spin relaxation is strongly influenced by the inter internal dynamics of the protein matrix as well as local motions of long-lived water molecules that are entrapped in the interface or the folded structure of the protein. The balance between these contributions is a function of the particular protein structure, the temperature, and the isotope composition of the protein and the solvent. Water residence times range from tens of microseconds to tens of picoseconds. The vast majority of interfacial correlation times are short, within a factor of 5 of the bulk values and the dimensionality of the diffusive exploration may be modified by the presence of a macromolecule that is significantly larger than the diffusing solvent molecules.
12:30 PM - JJ5.6
Active and Passive Biomechanics of Single Cells and Cell Aggregates.
Mareike Zink 1 , Anatol Fritsch 1 , Karla Mueller 1 , Franziska Wetzel 1 , Claudia Brunner 1 , Michael Goegler 1 , Josef Kaes 1 Show Abstract
1 Faculty of Physics and Geological Sciences, EXP I/ Soft Matter Physics, University of Leipzig, Leipzig Germany
Probing active and passive mechanical properties of cells in multicellular spheroids has become an evolving field in biophysics since it opens the possibility for a deeper understanding of cell-cell interactions in tissues. The two major methods used here to probe different scales and physical properties of cells are the Micro-fluidic Optical Stretcher, a divergent two beam optical laser trap enabling contact-free, whole cell elasticity measurements, and the atomic force microscope (AFM) which is used to measure the force generation in motile cells as well as the local elasticity of single cells and cells in aggregates. Malignant tumours are not only agglomerates of homogeneous cells, but rather complex structures containing diverse normal and pathological cells in different stages of aggressiveness. Recent investigations show that the passive biomechanical properties of benign cells differ from those of cancerous and metastatic cells which can result in a much more precise staging of tumours for diagnosis. Consequentially, ongoing clinical breast and lung cancer trials show great promise. Optical deformability is not solely a promising cell marker in diagnosis. Moreover, it allows the selection of mesenchymal stem cells that are particularly potent in cartilage formation. The passive cell mechanics is also the basis of a cell’s active behaviour. Cell motility as active behaviour plays a major role in embryogenesis and in tumour metastasis. An essential step in cell motility is the advancement of the cell’s leading edge which is believed to be driven by actin polymerization against the plasma membrane of the cell. Measurements of the force generation of cells show that actin polymerization at the very leading edge of fish keratocytes is indeed the mechanism driving lamellipodial protrusion. Our studies show that the active and passive biomechanics of cells are an essential element that defines cell type and function. This will not only open new routes in cell-based diagnosis. Moreover, it may point out new strategies to promote nerve regeneration and to inhibit cancer metastasis.
JJ6: New Methods to Image Confinment at Different Scales
Wednesday PM, December 02, 2009
Riverway (Sheraton - 5th Floor)
3:00 PM - **JJ6.1
Synchrotron-based Tomographic Microscopy: Fast, High-Sensitive Micron- and Nanoscale Imaging.
Marco Stampanoni 1 2 , Federica Marone 1 , Rajmund Mokso 1 Show Abstract
1 Swiss Light Source, Paul Scherrer Institut, Villigen Switzerland, 2 Institute for Biomedical Engineering, University and ETH Zürich, Zürich Switzerland
Synchrotron-based tomographic microscopy is a powerful technique for fast non-destructive, high resolution quantitative volumetric investigations on samples of diverse nature. At the Swiss Light Source of the Paul Scherrer Institut, a beamline for TOmographic Microscopy and Coherent rAdiology experiments (TOMCAT) has been recently put into operation. This beamline gets synchrotron light from a 2.9 T superbend and the main optical component, a Double Crystal Multilayer Monochromator, covers an energy range between 8 and 45 keV. The standard TOMCAT detector offers field of views ranging from 0.75x0.75 mm2 up to 15x15 mm2 with a theoretical pixel size of 0.37 μm and 7.4 μm, respectively. In addition to routine measurements which exploit the absorption contrast, the high coherence of the source also enables phase contrast tomography, implemented with two complementary techniques based on a modification of the Transport of Intensity method and grating interferometry. Typical acquisition times for a tomogram are in the order of few minutes, ensuring high throughput and allowing for semi-dynamical investigations. Raw data are automatically post-processed online and full reconstructed volumes are available shortly after a scan with minimal user intervention. Thanks to the brilliance of modern third generation sources, voxel sizes in the micrometer range are routinely achieved by the major synchrotron based microtomography devices around the world, while the isotropic 100 nm barrier is reached and trespassed only by few instruments. Recently, TOMCAT has been equipped with a full field, hard X-rays microscope with a theoretical pixel size down to 30 nm and a field of view of 50 microns. The nanoscope performs well at 10 keV in both absorption and phase contrast regime, which is realized with an ad-hoc implementation of the well known Zernike’s approach.This talk gives an overview of the TOMCAT beamline introducing different imaging approaches (in-line imaging, full-field microscopy, phase contrast and dark-field imaging) and discusses a selection of applications of this versatile technique in the biology and geosciences. Examples range from the visualization of cellular structures to the quantification of vascular micro-architecture in the brain. Other applications go from the high-resolution, non-destructive investigations of the internal structure of invaluable and unique fossilized specimens to the quantitative analysis of pore networks in diverse rock types, for instance for improving oil recovery, understanding element mobilization by hydrothermal fluids, studying dynamics of volcanic eruptions or refining current contaminant diffusion models. In-situ (e.g. cryotomography) and semi-dynamical (e.g. compression, crack) experiments will also be discussed.
3:30 PM - JJ6.2
Liquid TEM for Soft Matter Imaging.
Haimei Zheng 1 2 3 , Ulrich Dahmen 1 2 , Paul Alivisatos 2 3 Show Abstract
1 National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Chemistry, University of California, Berkeley, California, United States
Imaging through liquids with nanometer resolution has extraordinary potential for addressing many fundamental questions in materials science, chemistry, physics and biology. This capability was enabled by using a self-contained liquid cell operating in a transmission electron microscope (TEM), which allows imaging through a thin film of liquid confined between two silicon nitride membranes. We have employed this capability to study colloidal nanocrystal growth kinetics. Single colloidal platinum nanocrystal growth trajectories in a liquid solution have been observed in real time. We have also been able to image and track the real time diffusion of both spherical (radii from 5 to 15 nm) and rod-shaped (5x10 nm) colloidal gold nanocrystals in a water-15%glycerol solution with nanometer resolution. This is opening opportunities for liquid TEM in soft matter and possibly in biology. Prospects for the future will be discussed.
3:45 PM - JJ6.3
Intracellular Confinement of Magnetic Nanoparticles by Living Cells: Impact for Imaging and Therapeutic Applications.
Michael Levy 1 , Claire Wilhelm 1 , Martin Devaud 1 , Pierre Levitz 2 , Florence Gazeau 1 Show Abstract
1 , Laboratoire Matière et Systèmes Complexes, Paris France, 2 , Laboratoire de Physique de la Matière Condensée (LPMC), Paris France
Anionic magnetic nanoparticles are spontaneously internalized by living cells. Their internalization obeys the following pathway. Nanoparticles are first adsorbed electrostatically on the cell membrane, triggering endocytosis mechanism. Invagination of plasma membrane leads to the formation of intracellular vesicles confining the nanoparticles. By successive fusion of vesicles, nanoparticles occupy different compartments of the cytoplasm with increasing concentration.Such magnetic labeling of living cells enables numerous biomedical applications, among them cell manipulation, magnetically-induced hyperthermia and MRI cell tracking. The ability of iron oxide nanoparticles to create MRI contrast or to generate heat under alternating field is governed by the dynamical behavior of their magnetic moment. The dynamics results both from Brownian motion of the particles and from internal fluctuations of the magnetic moment within the nanocristal (Néel dynamics). Intrinsic properties of nanoparticles, such as their size, their magnetization and their magnetic anisotropy, are the relevant parameters governing these dynamics. However environmental characteristics such as the local viscosity, the temperature and the way nanoparticles are organized within intracellular compartments, also affect their behavior.Here we study the influence of cellular environment on nanoparticle properties, stressing the role of interparticles magnetic interactions created by intracellular confinement. Impact of cell internalization was investigated using several techniques, including SQUID magnetic measurements, Ferromagnetic Resonance (FMR), Nuclear Magnetic Resonance Dispersion (NMRD) and calorimetry measurements under high frequency magnetic field (hyperthermia). Intracellular confinement affects the Néel dynamics via dipole-dipole interactions and the Brownian dynamics via a reduction of local mobility. Both effects are shown to influence the MRI contrast properties of nanoparticles as well as their efficiency as heating mediators.
4:30 PM - JJ6.4
Direct Imaging of Biphasic Colloidal Mixtures Under Flow through Square Microchannels.
Summer Rhodes 1 2 , Robert Lambeth 1 3 , Jacinta Conrad 1 , Jeffrey Moore 1 , Jennifer Lewis 1 Show Abstract
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 Materials Science and Engineering, University of Illinois Urbana-Champaign, Urbana, Illinois, United States, 3 , U.S. Army Research Laboratory, Aberdeen Proving Grounds, Maryland, United States
Model biphasic colloidal mixtures have been created for direct imaging of colloidal structure and dynamics under both quiescent conditions and pressure-induced flow through square microchannels. These biphasic mixtures are composed of both attractive and repulsive silica microspheres, whose surface chemistries are specifically tailored to induce the desired difference in stability in a nearly index-matched DMSO-water solution. Each particle population is dyed with a different fluorophore to facilitate their independent imaging via confocal microscopy. In the quiescent state, the presence of the repulsive microspheres disrupted the structure of the attractive microsphere network, leading to a greater spatial homogeneity. From their mean square displacements, the attractive microspheres were found to be nearly arrested, while the repulsive microspheres remained highly mobile. Under pressure-driven flow, the velocity and intensity profiles of each microsphere population were independently determined as a function of their position within the square microchannels. For biphasic mixtures possessing an 1:1 attractive:repulsive microsphere ratio or higher, the attractive microspheres are depleted in regions near the microchannel walls. Additionally, the flow profiles of the attractive species are more plug-like than their repulsive counterparts. Our findings have important implications on the design of colloidal suspensions for myriad applications including ceramic components novel coatings, inks for direct-write assembly, and drug delivery.
4:45 PM - JJ6.5
In situ Observation of Spreading and Solidification Behavior of Zirconia Molten Drop Impinging on Solid Surface.
Kentaro Shinoda 1 , Kosuke Nagashio 2 , Hideyuki Murakami 1 , Seiji Kuroda 1 , Kazuhiko Kuribayashi 3 Show Abstract
1 Hybrid Materials Center, National Institute for Materials Science (NIMS), Tsukuba, Ibaraki, Japan, 2 Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo, Japan, 3 Division of Materials Science and Engineering, Graduate School of Engineering, Shibaura Institute of Technology, Koto-ku, Tokyo, Japan
Impact phenomena of a high-temperature molten drop on a solid surface involve not only kinetic spreading but also rapid solidification process via the heat transfer through the solid surface. In addition, it is very sensitive to the surface state such as surface adsorbates and roughness. These factors can cause the transition of spread phenomenon from stable steady spreading to splashing. Among the high-temperature melt, yttria-stabilized zirconia (YSZ) is widely used for plasma spraying of thermal barrier coatings. However, because of its high melting point close to 3000 K, it was difficult to observe the impact phenomena of the YSZ molten drop. In this study, we have challenged to observe the impact phenomena of YSZ molten drop in millimeter size utilizing a novel aerodynamic levitator (ADL). This ADL nozzle was designed to split into two parts by an electromagnetic coil so that the levitated sample could be dropped downward through the nozzle. YSZ spheres approximately 2 mm in diameter were made from commercially available 8 wt. % YSZ powder. The YSZ sphere was levitated in the ADL nozzle with oxygen gas, and melted with a carbon dioxide gas laser. After the sample temperature reached to a target temperature, the ADL nozzle was split and the sample was dropped onto a stainless steel substrate located 15 cm below the nozzle. The drop impacted on the substrate at the speed of 1.7 m/s. The spreading and solidification behavior of the YSZ molten drop was observed with a high-speed video camera. The Reynolds number Re and the Weber number We at impact were 680 and 58, respectively. The splash parameter K defined by K = Re1/4We1/2 was 39, which indicates the drop will not splash at impact. The degree of undercooling of YSZ melt at a container-less state was up to 500 K, and the solidification speed was on the order of 1 m/s. When a drop impacted at a super heated state, the solidification occurred after the spreading of the drop completed. Meanwhile, when a drop impacted at the undercooled state, the solidification occurred just after the impact and suppressed the spreading. Interesting observation was when a drop impacted on a substrate with an acetone thin film. The drop was splashing over the surface, followed by recoiling and rebounding. This suggests that the volatile acetone film caused the slipping of the drop over the substrate surface and at the same time suppressed the solidification, which gave time to recoil and rebound.
JJ7: Poster Session
Wednesday PM, December 02, 2009
Exhibit Hall D (Hynes)
9:00 PM - JJ7.2
Order, Dynamics and Diffusion Processes of Mesogenic Molecules Confined in a Polymeric Matrix.
Frederick Roussel 1 , Patrick Judeinstein 2 Show Abstract
1 LDSMM, CNRS 8024, Univ. Lille 1, Lille France, 2 ICMMO, CNRS 8182, Univ Paris Sud, Orsay France
Liquid crystals (LC) in complex geometries have attracted considerable attention because of several interesting physical phenomena resulting from strong surface effects between the LC and the host . Numerous materials like porous filters, membranes, or polymer matrices have been used to confine LC phases. In the case of dispersions of LC in polymers, the sample morphology, the concentration of LC, or the surface anchoring conditions for the LC at the polymer interface, can be tuned to investigate the order and the dynamics of the confined LC phase. NMR spectroscopy has proved to be a useful technique to probe the effect of confinement on the molecular orientational ordering and the surface-induced order of mesogenic molecules. First, the orientational ordering of 4-n-pentyl-4’-cyanobiphenyl (5CB) droplets confined in a thermoplastic polyethylhexylacrylate matrix was investigated by 13C-NMR. It is also shown that the 2-ethyl hexyl fragment of the polymer chain is partially ordered at the polymer/LC interface due to interdigitation with 5CB molecules. Above the nematic-isotropic transition temperature, a weak pretransitional behavior is shown by the polymer. In the case of cross-linked polymer networks, one can tailor the anchoring energy by making small changes in the side group of polyacrylate matrices without significantly changing the sample morphology. By this means, one expects to exercise significant control over the cavity surface, which, in turn, is a strong determining factor of the director-field configuration. Again, 13C-NMR spectroscopy was used to probe the orientational ordering of micrometre-sized nematic LC droplets exhibiting planar or homeotropic anchoring.Finally, diffusion processes in dynamically asymmetric binary fluid mixtures made of monodisperse polystyrene (PS) and a rodlike nematogen molecule (5CB) are studied by pulsed-field gradient spin echo NMR in the vicinity of the phase-separation/phase-dissolution temperature. Below the instability point of the mixture, two self-diffusion coefficients, named Dfast and Dslow, are observed and assigned to mobile molecules i) dissolved in the polymeric matrix and ii) phase-separated in isolated or interconnected domains, respectively. The temperature dependence of Dfast exhibits an Arrhenius-like behaviour when the mixture is submitted to a slow cooling rate whereas a Vogel–Fulcher–Tamman–Hesse law is obeyed for deep quenches. These results show the dynamic heterogeneities existing in the PS/5CB system below the UCST. Characteristic length scales are estimated from modelling echo attenuation curves exhibiting diffraction-like patterns.References1- F. Roussel, C. Canlet, B. M. Fung, Phys. Rev. E, 65, 021701 (2002).2- F. Roussel, B. M. Fung, Phys. Rev. E, 67, 041709 (2003).3- F. Roussel, P. Judeinstein, Soft Matter, 4, 888 (2008).
9:00 PM - JJ7.3
A Single Degree of Freedom ``Lollipop” Model for Carbon Nanotube Bundle Formation.
Steven Cranford 1 , Haimin Yao 2 , Markus Buehler 1 2 Show Abstract
1 Department of Civil and Environmental Engineeirng, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Center for Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
A common phenomenon in nanotube arrays is inter-tube adhesion due to weak interactions, resulting in the formation of irregular clusters and bundles. Such behavior complicates theoretical investigations of the mechanical properties and dynamics of such arrays. Using atomistic methods to determine material parameters, individual carbon nanotubes (CNTs) are represented by a simple single degree of freedom (SDOF) “lollipop” model to investigate the formation of CNT bundles due to weak interactions (van der Waals forces). Such a model has a twofold purpose: first, to provide a predictive means of bundle formation of synthesized nanotube arrays at a large scale, and; second, to efficiently investigate the fundamental mechanics of such behavior. The assertion of energy conservation between full atomistic and SDOF model is enforced to arrive at model parameters, implemented using modified molecular dynamic techniques based on a theoretical continuum nanomechanical model where the interaction of adjacent nanotubes is assumed a priori. By using a simplified SDOF model, the self-organization of arrays consisting of hundreds of thousands of CNTs are investigated with relatively efficient simulation methods. This technique is used here to efficiently investigate the effects of nanotube parameters such as length (aspect ratio) and surface energy on bundle size and formation, as well as the intentional manipulation of bundle pattern formation. Such a hierarchical framework can also be applied to other systems as general approach to investigate the nanomechanics of intramolecular adhesion of similar nanotube arrays. The present model is implemented in both single wall carbon nanotubes (SWCNTs) and double wall carbon nanotubes (DWCNTs) of varying aspect ratios (i.e. varying height) to illustrate bundle formation and possible manipulation of patterns of large nanotube arrays.
9:00 PM - JJ7.6
Mesoscale Modeling of Mechanics of Hierarchical Metal-matrix Composites.
Dipanjan Sen 1 , Markus Buehler 2 Show Abstract
1 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
The nanostructural makeup and hierarchical assembly of natural composite materials such as bone or nacre are crucial for their superior mechanical properties over their constituent phases, providing high strength and toughness at high stiffness. However, the transfer of similar mechanical properties to functional metal-matrix composites remains challenging. Here we propose the design of a hierarchical biomimetic metal-matrix nanocomposite inspired by the structural motif found in biological materials. The response to tensile loading, of the nanocomposite with two levels of structural hierarchy, is probed through atomistically-informed mesoscale particle simulations. In particular, general design strategies to maximize material toughness through assembly at two levels of hierarchy are explored. We observe that geometric confinement at each structural level, that mimics the nanostructural arrangement of bone, maximizes strength and toughness of the material. We also present theoretical analyses based on a hierarchical transition state theory to predict the performance of hierarchical materials. Through a simple deformation-rate-temperature model, we show that structures with hierarchical assembly of deformation mechanisms show higher toughness than structures with all mechanisms at the same length scale and hierarchy. The discussion concludes with an illustration of how hierarchical designs can be used to optimize the material behavior at different levels in a material’s organization, leading to superior performance.
9:00 PM - JJ7.7
Phonon Confinement in Porous Silicon.
Rodolfo Cisceros 1 , Monserrat Bizarro 1 , Miguel Cruz-Irisson 2 , Chumin Wang 1 Show Abstract
1 Instituto de Investigaciones en Materiales, Universidad Nacional Autonoma de Mexico, Mexico D.F. Mexico, 2 ESIME-Culhuacan, Instituto Politécnico Nacional, Mexico D.F. Mexico
Nanostructured materials have exceptional physical properties, such as improved hardness, increased ductility, superior magnetic and novel optoelectronic properties . The origin of these peculiar behaviors is mainly related to the large percentage of atoms at the surface and the quantum confinement of elementary excitations. In this work, we study the phonon confinement in porous silicon (PSi) by using the Raman scattering technique, which is an inelastic light dispersion broadly used to investigate vibrational properties of semiconductors and it can be modeled by means of a local bond-polarization model based on the displacement-displacement Green’s function within the linear response theory, where the Born potential is used, including central and non-central interatomic forces . Using the supercell method, the ordered pores are produced by removing columns of Si atoms from a crystalline silicon (c-Si) structure . On the other hand, the samples of PSi are obtained by anodizing p-type (001)-oriented c-Si wafers in a hydrofluoric acid bath . The morphology and the thickness of skeleton walls in PSi samples at nanometric scale are measured by the atomic force microscopy. Both theoretical and experimental results show a notable shift of the main Raman peak towards lower energies, in comparison to the bulk c-Si signal. This shift is quantitatively analyzed within the quantum confinement framework and the morphological effects are also discussed.  C.P. Poole and F.J. Owens, Introduction to Nanotechnology, (John Wiley & Sons Inc., New York, 2003). R.J. Elliott, J.A. Krumhansl, and P.L. Leath, Rev. Mod. Phys. 46, 465 (1974). P. Alfaro, M. Cruz, and C. Wang, Nanoscale Res. Lett. 3, 55(2008). R. Cisneros. C. Ramirez, and C.Wang, J. Phys.: Condens. Matter 19, 395010 (2007).
Pierre Levitz Ecole Polytechnique
Ralf Metzler Technical University of Munich
David Reichman University of Columbia
JJ8: Numerical Simulations of Transport and Adsorption
Thursday AM, December 03, 2009
Riverway (Sheraton - 5th Floor)
9:30 AM - **JJ8.1
Characterization and Modeling of Multiscale Porous Media: Application of Multiresolution Wavelet Transformations.
Muhammad Sahimi 1 Show Abstract
1 , University of Southern California, Los Angeles, California, United States
Natural and even man-made porous media often represent multiscale systems in that, they contain heterogeneities at distinct, disparate, and identifiable length scales. Characterization of such porous media and modeling of fluid flow and transport in them is difficult, as one must take into account the effect of the heterogeneities at all the relevant scales, which entails using a computational grid with millions of grid points that render the computations almost impossible. A systematic procedure is described for achieving the goal of efficient modeling and simulation of such porous media based on the multiresolution wavelet transformations. Applications of the method to two types of porous media are described: Large (field-)-scale porous media, and hierarchical porous materials.
10:00 AM - **JJ8.2
Adsorption and Translocation of Chain Molecules in Nanopores.
Alexander Neimark 1 , Shuang Yang 1 , Yang Kan 1 , Aleksey Vishnyakov 1 Show Abstract
1 Chenmical and Biochemical Engineering, Rutgers University, Piscataway, New Jersey, United States
We study the equilibrium adsorption and dynamics of polymer translocation through and into nanopores with two different coarse-grained modeling techniques. The self-consistent field theory (SCFT) represents the polymer chain as a random walk trajectory in an external field fulfilling the Edwards equation, which treats excluded volume effects in a mean field fashion; the process of translocation is described by the Fokker-Plank equation. The dissipative particle dynamics (DPD) represents the polymer chain as a sequence of soft repelling beads linked together by virtual springs in an explicit solvent; the chain movement is directly simulated with Newton equations, which account for random thermal motion, external field, inter-bead forces and friction with explicit solvent. The goal of our work is to understand the interplay of entropic (confinement size) and external field (adsorption potential) effects. Different model set-ups are considered: translocation through a hole in an impermeable membrane under purely diffusive and forced flow conditions, adsorption into a spherical pore from the bulk, and translocation between the pores of different sizes. Special attention is paid to modeling chromatographic separation of polymers at the conditions of critical adsorption.
10:30 AM - JJ8.3
Multiscale Study of the Structure and Dynamics of Ions in Hydrated Clays.
Marie Jardat 1 , Jean-Francois Dufreche 1 , Virginie Marry 1 , Benjamin Rotenberg 1 , Pierre Turq 1 Show Abstract
1 Laboratory PECSA, University Pierre et Marie Curie (Paris 6), Paris France
We report a multiscale study of hydrated clays in presence of added salt by numerical simulations. We develop a coarse-graining procedure to study the salt exclusion in montmorillonite in equilibrium with a reservoir of electrolyte solution. This is an example of the so-called Donnan equilibrium in a charged porous medium. We start from an atomic description of this system to derive a reliable coarse-grained description. Brownian dynamics simulations are used to compute the insertion rate of ions in the porous medium as a function of the concentration in the reservoir and of the size of the pores. The simulation results are compared to analytical calculations (Poisson-Boltzmann approximation). We show that the calibration of the mesoscopic model is a key point and has a strong influence on the result. We observe that the salt exclusion increases when kL decreases (where k is the inverse of the Debye length and L the size of the pore) and that this effect is modulated by the correlations between ions. Two different regimes are revealed. At low concentrations in the reservoir, we observe a regime controlled by electrostatics: the Coulomb attraction between ions increases the amount of salt in the interlayer space. On the opposite, at high concentrations in the reservoir, the excluded volume effect dominates. Finally, the diffusion coefficients of ions inside and outside the porous medium are computed at the atomic and coarse-grained levels and compared. The diffusion of all species along the clay surface is slowed down compared to the bulk case.
10:45 AM - JJ8.4
Aqueous Solutions on Silica Surfaces: Structure and Dynamics from Simulations.
Dimitrios Argyris 1 , Alberto Striolo 1 Show Abstract
1 School of Chemical Biological and Materials Engineering, University of Oklahoma, Norman, Oklahoma, United States
We are interested in understanding the properties of aqueous electrolyte solutions at interfaces. The fundamental questions we seek to answer include: (A) how does a solid structure perturb interfacial water? (B) How far from the solid does this perturbation persist? (C) What is the rate of water reorientation and exchange in the perturbed layer? (D) What happens in the presence of simple electrolytes? To address such topics we implemented atomistic molecular dynamics simulations. Recent results for water and simple electrolytes near silicon dioxide surfaces of various degrees of hydroxylation will be presented. The simulations are analyzed in terms of density profiles away from the solid surfaces, residence times for water and/or ions at contact with the surface, and structure and dynamics of the hydrogen bond network at the interface. The data suggest the formation of a layered aqueous structure near the interface. The layered structure of interfacial water, dictated by a complex hydrogen-bonding network, is responsible for anisotropic dynamics at the solid-liquid interface and for multiple types of translational diffusions as a function of the distance from the solid support.Further, the density profile of interfacial water seems to dictate the density profiles of aqueous solutions containing NaCl, CaCl2, CsCl, and SrCl2 near the solid surfaces. These results suggest that ion-ion and ion-water correlations are extremely important factors that should be considered when it is desired to predict the distribution of electrolytes near a charged surface. Our results will benefit a number of practical applications including water desalination, exploitation of the oil shale in the Green River Basin, nuclear waste sites remediation, and design of nanofluidic devices.
11:30 AM - **JJ8.5
A Unifying Approach to Capillary Condensation and Evaporation in Nanopores.
Roland Pellenq 1 2 , Benoit Coasne 3 Show Abstract
1 Centre Interdisciplinaire des Nanosciences de Marseille, CNRS, Marseille cedex 09 France, 2 CEE, MIT, Cambridge, Massachusetts, United States, 3 Institut Charles Gerhardt Montpellier, CNRS Université de Montpellier, Montpellier France
This paper reports a theoretical and simulation study on the temperature dependence of adsorption hysteresis in porous media having different morphologies and topologies. We aim at gaining some insights on the concept of critical hysteresis temperature, Tcc, defined as the temperature at which the adsorption/desorption isotherm becomes reversible. The off-lattice Grand Canonical Monte Carlo simulation technique is used to model adsorption of fluids (rare gases, nitrogen, water) in disordered porous materials and in materials with simple pore geometry. Our theoretical calculations and molecular simulations of the temperature dependence of capillary condensation hysteresis can be summarized as follows. Tcc obtained from GCMC simulations for unconnected cylindrical, ellipsoidal, and constricted pores follow the experimental scaling law (Tc3D-Tcc)/ Tc3D =2σ/R0 that is observed for MCM-41 silica materials with R0 (R0 being the size of the largest cavity in the system, i.e. the locus of the condensation and evaporation phenomena). As a result, the determination of Tcc for a given adsorbate in a given porous medium appears to be reliably related to the mean pore size, morphology, and topology of the material. We also found that the genuine mean-field nature of the DFT gives a rather poor description of Tcc for systems such as Ar and N2 in cylindrical pores. It is also shown that, although it allows one considering very large systems, the CGLT cannot be used to relate Tcc to the porous material features, as results for Tcc strongly depend on the lattice spacing used in the calculations. These results suggest that Tcc is a critical temperature that is reminiscent of the bulk critical temperature TC3D affected by the pore size R0 through a correction factor 1/R0 in good agreement with the theoretical predictions of Chalyy [Low Temp. Phys. 30, 686 (2004)]: even a simple fluid confined in a pore with a diameter as large as 6 nm has not recovered its bulk behavior.
12:00 PM - JJ8.6
Effect of Surface Structural Periodicity on Water Transport in Flat Graphene Nano-channels.
Wei Xiong 2 , Zhe Liu 1 , Quanshui Zheng 2 Show Abstract
2 Department of Engineering Mechanics, Tsinghua University, Beijing China, 1 Department of Mechanical and Aerospace Engineering, Monash University, Clayton, Victoria, Australia
The extremely fast transport of water in carbon nanotubes in recent experiments has been attributed to the hydrophobic nature and the atomic smoothness of graphene surface. In this paper, molecular dynamic simulations are carried out to investigate the effect of the structural periodicity of the graphene solid walls on the water fluid dynamics in the flat nano-channels with height from 1nm-6nm. Two ways are employed to alter the smoothness of the potential energy landscape from the solid wall: applying various elastic strains on the solid wall and adding more stacking graphene layers at the interface. Influence on the flow rate enhancement is investigated. The water structures in the interfacial depletion layer, e.g., molecule orientations and hydrogen bonding network, are studied. The linkage to the flow rate enhancement is established.
12:15 PM - JJ8.7
Level Set Modeling of Lipid Bilayer Vesicles.
David Salac 1 , Michael Miksis 1 Show Abstract
1 Engineering Sciences and Applied Mathematics, Northwestern University, Evanston, Illinois, United States
A level set based model of lipid bilayer vesicles in external fluid flow is presented. The model takes into account the bending rigidity of the interface and enforces lipid conservation. This model is implemented on an adaptive, non-graded Cartesian grid. A fractional-step Navier-Stokes solver is utilized to obtain the multiphase fluid flow. A mixed implicit-parametric scheme has been developed to calculate required quantities on the interface in a stable manner. Advancement of the lipid interface is achieved using a localized semi-implicit level set scheme. The dynamic behavior of the vesicle will be given, showing good agreement with both experimental and analytic solutions.
JJ9: Friction and Nanotribology
Thursday PM, December 03, 2009
Riverway (Sheraton - 5th Floor)
2:30 PM - **JJ9.1
Optical and Micromechanical Probes of Multiscale Dynamics of Polymer Chains Confined at Solid Interfaces.
Liliane Leger 1 , Frederic Restagno 1 , Celine Cohen 1 Show Abstract
1 Physique des Solides, Université Paris Sud-11, Orsay France
We shall present and discuss two different experimental approaches specifically developed to probe the local dynamics of polymer chains confined at a solid surface. The first technique is an optical one, based on Near Field illumination and fluorescence recovery after photobleaching. We shall describe the technique, and discuss two different applications: Near Field Laser Velocimetry, which allows one to monitor the interfacial velocity of a fluid submitted to shear and Near Field Fluorescence Recovery after Photobleaching, which allows one to monitor the diffusion coefficient parallel to the solid surface for polymer chains confined at nanometric distances from a solid wall. We shall show results of these two techniques in the case of model systems made of a polymer melt in contact with various surfaces decorated with end grafted chains having the same chemical nature as the melt. The degree of confinement of the chains attached to the surface can be varied and controlled by adjusting the grafting density and the molecular weight of the grafted chains. This degree of confinement affects the ability of bulk chains to interpenetrate into the grafted layer, and directly reflects in their own dynamics. We shall show how this local dynamics of bulk chains immediately in contact with the grafted layer can be precisely investigated by the two optical near field illumination techniques, and interpreted in terms of molecular models. We shall show how the confinement affects the local dynamics and leads to specific non linear friction phenomenon. In a second part of the talk we shall compare the results deduced from the optical experiments presented above to those of a micromechanical test that we have named “JKR friction”, in which a micro-lens of crosslinked elastomer is put into contact and slide at a chosen velocity along a surface decorated with grafted chains of the same chemical nature as those in the elastomer. Instrumentation of the test in terms of friction force measurements allows one to directly measure the friction force between the crosslinked elastomer and the polymer brush, for typical contact diameters in the 100 µm range. We shall show how the measured friction can be related to the friction force due to the extraction of one grafted chain from the elastomer in the low grafting density regime, while, for large grafting densities, the friction force can be related to the collective response to shear of the whole grafted layer confined between the solid surface and the elastomer. We shall discuss how these quasi-macroscopic friction force measurements can give access to the local individual or collective dynamics of the grafted chains.
3:00 PM - JJ9.2
Shear Rate Threshold for the Boundary Slip in Dense Polymer Films.
Nikolai Priezjev 1 Show Abstract
1 Mechanical Engineering, Michigan State University, East Lansing, Michigan, United States
The rheology of complex fluids in thin films is important for theoretical and experimental studies of such common phenomena as friction, lubrication and wear. Experimental measurements of the flow profiles and shear stresses on submicron scales might be subject to errors due to the possibility of liquid slip at the solid wall. An accurate prediction of flow, therefore, requires specification of a proper boundary condition. In this study, the shear rate dependence of the slip length in thin polymer films confined between atomically flat surfaces is investigated by molecular dynamics simulations. The unentangled polymer melt is described by the bead-spring model of linear flexible chains.We found that at low shear rates the velocity profiles acquire a pronounced curvature near the wall and the absolute value of the negative slip length is approximately equal to thickness of the viscous interfacial layer. In this regime the slip length is nearly rate-independent. At higher shear rates, the velocity profiles become linear and the slip length increases rapidly as a function of shear rate. The gradual transition from no-slip to steady-state slip flow is associated with shear-melting of the interfacial layer. The relaxation dynamics of polymer chains in shear flow was analyzed by evaluating the decay of time autocorrelation function of the first normal mode in the vorticity direction. We found that the rate behavior of the slip length correlates well with the inverse relaxation time of the polymer chains in the interfacial layer.The rate-dependent slip boundary conditions were also reformulated in terms of the friction coefficient at the polymer/wall interface and slip velocity of the first fluid layer. In agreement with the results of the previous study [Niavarani and Priezjev, Phys. Rev. E 77, 041606 (2008)], we found that the friction coefficient at lower melt densities undergoes a transition from a constant value to the power law decay as a function of the slip velocity. At higher melt densities the friction coefficient decays as a power law function in a wide range of slip velocities. When the magnitude of the surface induced peak in the fluid structure factor is below a certain value, the friction coefficient is determined by a combination of parameters (structure factor, temperature, and contact density) of the first fluid layer near the solid wall.
3:15 PM - JJ9.3
Effect of Spatial Confinement on Large Strain Mechanics of Entangled Polymer Glasses and Melts.
Graham Cross 1 , Roseanne Reilly 1 Show Abstract
1 Physics, Trinity College, Dublin Ireland
Large strain mechanical forming of polymer materials at dimensions similar or smaller than the macromolecular pervaded volume is important for new patterning nanotechnologies such as nanoimprint and block copolymer lithography. We present results of rigorous, variable temperature nanomechanical compression testing1 of thin film polystyrene (PS) homopolymers and copolymer diblocks to large strain. In confined conditions where films are prepared to a thickness less than the size of the bulk macromolecule, resistance to deformation is dramatically reduced for both solid glass forging and liquid melt molding. For melt flow, we further observe a complete inversion of conventional polymer viscosity scaling with molecular weight2. In polystyrene-polymethyl methacrylate (PS-b-PMMA) copolymer film systems we demonstrate a room temperature solid like response in the mixed state, while microphase separation into 20 nm domains leads to a vanishing shear strength suggestive of a liquid-like state. These results show that anomalous large strain deformation occurs at small scales by an unexpected influence of confining dimension in polymer materials.(1) Cross, Graham L.W. et al., Variable Temperature Thin Film Indentation with a Flat Punch. Review of Scientific Instruments 79, 013904 (2008).(2) Rowland, Harry D., King, William P., Pethica, John B., and Cross, Graham L. W., Molecular Confinement Accelerates Deformation of Entangled Polymers During Squeeze Flow. Science 322, 720 (2008).
3:30 PM - **JJ9.4
Boundary Flow on a Bubble Mattress.
Elisabeth Charlaix 1 , Audrey Steinberger 1 , Cottin-Bizonne Cecile 1 Show Abstract
1 , Universite Lyon 1, Villeurbanne France
Reducing the friction of liquid flows on solid surfaces has become an important issue with the development of microfluidics systems, and more generally for the manipulation of fluids at small scales. The use of gas as a lubricant - such as microbubbles trapped in superhydrophobic surfaces - has been suggested as a good way to achieve high and controlled slippage of liquids at walls.We present an experimental study of the boundary flow on a surface with calibrated microbubbles. We use a dynamic surface force apparatus to measure the force response to an oscillating drainage flow between a sphere and the surface. Firstly, we show that the force response allows to measure the surface elasticity conferred by the entrapped bubbles without contact, using the liquid film as a probe. The elasticity of the bubble mattress is dominated by the menisci stiffness, and its determination enables us to probe the shape of these meniscii. Secondly, we show that, contrary to the common belief, gas trapped at a solid surface can act as an anti-lubricant and promote high friction. The liquid-gas menisci have a dramatic influence on the boundary condition, and can turn it from slippery to sticky. The control of the meniscus shape is thus a key point to control the slippage and should be integrated in fluidic microsystems designed to reduce wall friction.
4:30 PM - **JJ9.5
Temperature Dependence of Nanoscale Friction.
Michael Urbakh 1 Show Abstract
1 School of Chemistry, Tel Aviv University, Tel Aviv Israel
A fundamental understanding of the mechanisms responsible for frictional energy losses is critical to the management of thermal equilibrium, mitigation of potential wear, and the design of interfaces in which such losses are minimized. Substantial progress in understanding the leading factors that determine the mechanisms of energy dissipation during sliding has opened new possibilities to modify and control motion at the nanoscale. Up to now, most frictional measurements have been performed at room temperature, and the temperature dependence of the nanoscale friction has rarely been addressed in experimental works. However, a few interesting experimental results have been published recently that strongly disagree with the previous theoretical predictions. We discuss two approaches to model a temperature dependence of friction at the nanoscale. First of them is based on a generalized Prandtl-Tomlinson model which includes the contribution of thermal fluctuations, while the second one describes a frictional response in terms of rupture and formation of the microscopic bonds (junctions) between an AFM tip and underlying surface. We demonstrate that these models predict a rich temperature dependence of friction, including a peak and/or plateau in friction force as a function of T, and a sharp increase or decrease of friction with T. A comparison of theoretical results with recent experimental data on the temperature dependence of friction allowed extracting important microscopic characteristics of nanoscale contacts.
5:00 PM - JJ9.6
Viscosity Dependent Slip Behavior at Molecularly Smooth Hydrophobic Surfaces.
Sean McBride 1 , Bruce Law 1 Show Abstract
1 Physics Department, Kansas State University, Manhattan, Kansas, United States
In this work, we study the slip behavior of eighteen Newtonian liquids from two homologous series, the n-alcohols and n-alkanes. These experiments were conducted against molecularly smooth n-hexadecyltrichlorosilane (HTS) coated surfaces using colloidal probe atomic force microscopy (AFM). The primary result of this work is that the slip length is dependent upon the bulk viscosity with the functional form b ~ η^x where x ~ 0.33. The same surfaces were used for each liquid, which allows any relative trends in slip behavior to be attributed to the properties of the liquid. The slip length was also shown to be independent of the shear rate permitting the use of Vinogradova’s slip theory.