Christopher M. Stafford National Institute of Standards and Technology
Adam J. Nolte Rose-Hulman Institute of Technology
Pil J. Yoo Sungkyunkwan University
Teng Li University of Maryland
V1: Instability Theory
Monday PM, November 29, 2010
Room 205 (Hynes)
9:45 AM - **V1.1
Furrowing Instabilities on Soft Interfaces.
Evan Hohlfeld 1 , L. Mahadevan 1 Show Abstract
1 SEAS, Harvard University, Cambridge, Massachusetts, United States
Buckling and wrinkling instabilities are ubiquitous in thin films and filaments and are more a consequence of the geometry induced separation of scales rather than of material properties. A natural question that arises in systems that exhibit wrinkling instabilities is that of wavelength selection and amplitude modulation. I will discuss an extreme case of this question in the context of the formation of a furrow on a stressed, soft elastomeric interface and show that it illuminates and unifies buckling and wrinkling instabilities in a variety of systems
10:15 AM - V1.2
Diffusion-Controlled, Self-Organized Growth of Symmetric Wrinkling Patterns.
Jun Young Chung 1 , Adam Nolte 1 , Christopher Stafford 1 Show Abstract
1 Polymers Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
The formation of self-organized wrinkling patterns is a potential route for generating such tunable ordered patterns on surfaces across many length scales. Here, we demonstrate that surface wrinkling of ultraviolet/ozone (UVO) treated polymer films through osmotically driven swelling by solvent vapor sorption leads to unique and intriguing patterns, some of which have not been previously reported. The type of pattern and speed of its growth is coupled to the degree of UVO crosslinking and the rate of solvent diffusion into the film from a localized defect. This simple yet novel approach could serve as a test-bed for studying topography-driven phenomena such as wettability and adhesion and diffusion related processes, as well as facilitate a better understanding of dynamic self-assembly.
10:30 AM - **V1.3
Adhesive Wrapping of Elastic Vesicles by Cell Membranes.
Xin Yi 1 , Xinghua Shi 1 , Huajian Gao 1 Show Abstract
1 , Brown University, Providence, Rhode Island, United States
A fundamental problem in soft materials science and cell mechanics is the adhesive wrapping of nanoparticles by cell membrane that incorporates particle size, particle elasticity and particle aspect ratio, as well as receptor type and density and different specific/non-specific interaction forces on the cellular uptake of nanoparticles. The significance of this research is reflected by the urgent societal needs to understand both beneficial and hazardous effects of nanotechnology which are projected to produce and release thousands of tons of nanomaterials into the environment in the coming decades. Here, theoretical analysis and molecular dynamics simulations are carried out to investigate the adhesive wrapping of a cylindrical or a spherical elastic vesicle by cell membrane. We show that there exist three distinct wrapping phases - full wrapping, partial wrapping and no wrapping - depending on the vesicle size, the adhesion energy, the surface tension of the membrane, and the bending rigidity ratio between the vesicle and the cell membrane. In particular, the bending rigidity ratio is found to play a critical role. These results are of fundamental importance not only to understanding vesicular transport but also to modeling endocytosis/phagocytosis of elastic particles into cells.
11:00 AM - V1.4
Morphologic Instability of Graphene on Substrates.
Teng Li 1 , Zhao Zhang 1 Show Abstract
1 Department of Mechanical Engineering, University of Maryland, College Park, Maryland, United States
11:15 AM - V1:Theory
V2: Instabilities in Patterning I
Monday PM, November 29, 2010
Room 205 (Hynes)
11:30 AM - **V2.1
Mechanics of Periodic Polymeric Structures: Exploiting Instabilities to Enhance Performance.
Mary Boyce 1 Show Abstract
1 Mechanical Engineering, MIT, Cambridge, Massachusetts, United States
Periodic microstructures abound in nature and provide numerous interesting and unique mechanical, photonic, phononic and hydrophobic properties. Advances in block copolymer chemistry, lithography, controlled printing, molding, laser cutting, and other processing techniques enable synthetic production of periodic polymeric microstructures at many different lengthscales. Here, we focus on the mechanics of the finite deformation of periodic polymeric-based microstructures and how the periodic structure can give rise to interesting and novel mechanical behaviors, in many instances involving microstructural instabilities which can be used to trigger changes in either internal periodic morphology or in surface topology. The periodic structures and their switching and/or evolution with deformation can be used to either permanently or dynamically tune other attributes including, for example, phononic bandgaps and hydrophobicity. Several different examples will be presented including: (i) mechanics of periodic porous elastomers which exhibit deformation-induced pattern transformations upon reaching a critical load giving a superelastic stress-strain behavior as well as a tunability in phononic and photonic band gaps; (ii) mechanics of filled polymers where instabilities are used to provide enhanced resiliency; (iii) mechanics of lamellar block copolymers building from “single crystal” lamellar structure mechanics to “polycrystal” lamellar block copolymers; (iiv) mechanics of processing of electrospun polymeric fibers using solvent evaporation to control surface topology; and (iv) mechanics of three-dimensional periodic porous and bicontinuous structures for controlling stiffness, strength, and energy dissipation.
12:00 PM - V2.2
Swelling-induced Surface Pattern Formation and Ordering in Hydrogel Films with Depth-wise Crosslinking Gradients.
Murat Guvendiren 1 , Shu Yang 2 , Jason Burdick 1 Show Abstract
1 Bioengineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 2 Materials Science and Engineering, University of Pennsylvania, Philadlephia, Pennsylvania, United States
Hydrogels undergo extensive three-dimensional volume changes when immersed in water, the degree of which is determined by the network chemical composition and degree of crosslinking. When the hydrogel is attached to a rigid substrate, it swells preferentially perpendicular to the substrate. This anisotropic swelling generates a compressive stress, which drives the formation of surface patterns when exceeding a critical stress. In order to develop an in depth understanding of the mechanism of surface pattern formation in hydrogels, we investigated the dynamic evolution of surface patterns in photocured hydrogel films from poly(2-hydroxyethyl methacrylate) (PHEMA) crosslinked with different concentrations of ethylene glycol dimethacrylate (EGDMA, 0-3 wt%). During curing in the presence of oxygen, a modulus gradient along the film depth was generated due to oxygen inhibition of the radical polymerization near the film surface. Our results showed that the swelling-induced wrinkling pattern formation followed Fickian-type kinetics (λ~t^1/2) at early stages, which was independent of the final pattern morphology. The onset of wrinkling was found at a critical linear expansion of ~1.12, which remained constant with increasing EGDMA concentration but decreased with initial film thickness. Moreover, the equilibrium linear expansion value decreased significantly (from 2.55 to 1.20) with increasing crosslinker concentration (from 0 to 3 wt%), resulting in the transition from random patterns to highly ordered hexagonal patterns. We have found that hydrogel surface instabilities are either in the form of shallow undulations (wrinkling) or in the form of sharp folds (creasing). To understand the mechanism of wrinkling and creasing pattern formation, we investigated the dynamic evolution of surface patterns in a range of pure and mixed solvents. We found that the Hildebrand solubility parameter of the PHEMA films containing 1 wt% or 3 wt% EGDMA is between 26.5 and 29.6 MPa^1/2. The onset of the wrinkling to creasing transition was found to decrease significantly (from 2.0 to 1.3) with increasing crosslinker concentration (from 1 to 3 wt% EGDMA). Hydrogels with controlled surface patterns are useful for a range of applications, including in microdevices, sensors, responsive coatings, and adhesives. As an example, we investigated human mesenchymal stem cell interactions with uniform hydrogels and hydrogels with lamellar and hexagonal wrinkling patterns and showed that the pattern morphology was able to dictate stem cell morphology and differentiation.
12:15 PM - **V2.3
Nanoparticle Structure Formation by Confined Drying in Wrinkles.
Alexandra Schweikart 1 , Nicolas Pazos-Perez 1 , Matthias Schmidt 2 , Andrea Fortini 2 , Alexander Wittemann 3 , Luis Liz-Marzan 4 , Ramon Alvarez-Puebla 4 , Andreas Fery 1 Show Abstract
1 Physical Chemistry II, University Bayreuth, Bayreuth Germany, 2 Theoretical Physics II, University Bayreuth, Bayreuth Germany, 3 Physical Chemistry I, University Bayreuth, Bayreuth Germany, 4 Departamento de Quimica Fisica, University Vigo, Vigo Spain
The ordered deposition of nanoparticles on macro-scale surfaces is of considerable interest both for fundamental science and from an application point-of-few. If it is achieved, nanoparticle characteristics like anisotropic electronic / optical properties or stimulus sensitivity could be transferred to macroscopic objects. As well, new properties should arise from collective effects.Controlled wrinkling provides a low cost approach towards controlling nanoparticle deposition on surfaces which can be upscaled relatively simple. Wrinkling occurs when thin sheets are compressed in-plane due to mechanical buckling instability. Under suitable conditions, amplitude and wavelength of wrinkles can be controlled and adjusted between 150 nm and many 10s of micrometers. Thus substrates can be topographically structured. We report on recent experiments in which we have used wrinkled surfaces for template assisted self assembly of colloidal particles on micron  and nano-scale . As well, topographical patterns can be translated into chemical patterns using microcontact printing . Finally wrinkles can be used to produce nanochannels, in which particles and other objects can be assembled in confined geometry by a drying process. Thus complex geometries for particle aggregates can be achieved, like the pyramidal or mesh structures. We discuss perspectives of such structures for applications in Surface Enhanced Raman Scattering (SERS) .  Lu C, Möhwald H and Fery A. Soft Matter 2007; 3: 1530-1536.  Horn A, Schoberth HG, Hiltl S, Chiche A, Wang Q, Schweikart A, Fery A and Böker A. Faraday Discussions 2009; 143:  Pretzl M, Schweikart A, Hanske C, Chiche A, Zettl U, Horn A, Böker A and Fery A. Langmuir 2008; 24: 12748-12753. Pazos-Perez N, Ni W, Schweikart A, Alvarez-Puebla R, Fery A and Liz-Marzan L. Chemical Science, accepted Manuscript ID SC-EDG-01-2010-000132.R1:
12:45 PM - V2.4
Controlled Flopping of Vertical Carbon Nanotube Gratings into Multi-directional and Multi-layered Networks.
Sameh Tawfick 1 , Megan Roberts 1 , Michelle Leach 2 , Joseph Corey 2 3 4 , A. John Hart 1 Show Abstract
1 Mechanical Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Biomedical Engineering, University of Michigan, Ann Arbor, Michigan, United States, 3 Neurology, University of Michigan, Ann Arbor, Michigan, United States, 4 Geriatrics Research, Education, and Clinical Center, VAAAHC, Ann Arbor, Michigan, United States
Vertically aligned “forests” of carbon nanotubes (CNTs) and other one-dimensional nanostructures are often grown on or etched from planar substrates, thus forming networks perpendicular to the substrate. These unique materials typically have a low density along with high physical anisotropy, and can therefore be directionally infiltrated with liquids due to capillary action. We reveal how narrow micro-patterned gratings of vertical CNTs can be transformed into multi-directional and multi-layered lateral networks due to a mechanical “flopping” instability triggered by capillary induced contraction during liquid infiltration and evaporation. Importantly, liquid is introduced by condensation so each grating feature is infiltrated independently without causing capillary bridging among nearby features. The flopping direction of individual micro-gratings, and therefore the final direction of the CNTs after flopping, is controlled by patterning initiating arrows at the ends of the films. By design and patterning of the initiators, we fabricate circuits of flopped CNTs having radial, tangential, rectangular, and multi-directional arrangements of densely packed CNTs with thickness ranging from 500 nm to 5 µm. Parametric variation of the grating and arrow geometry, along with in situ observation of the flopping behavior offers insight about the process dynamics and critical length scales at which these structures form without introducing defects. These insights are also applied to fold orthogonal and curved vertical CNT films into closed 3D micro-tents and twisted micro-pillars. Finally, the attractive properties of the anisotropic CNT networks are demonstrated by a 16:1 ratio of electrical conductivity between the radial and tangential directions, and by directed neurite outgrowth from primary rat dorsal root ganglia.
V3: Metrology Applications
Monday PM, November 29, 2010
Room 205 (Hynes)
2:30 PM - **V3.1
Elastic Moduli of Thin Film Glasses Elucidated from Surface Wrinkling.
Bryan Vogt 1 , Jessica Torres 1 , Christopher Stafford 2 , Nathan Bakken 3 , Jian Li 3 Show Abstract
1 Chemical Engineering, Arizona State University, Tempe, Arizona, United States, 2 Polymers Division, NIST, Gaithersburg, Maryland, United States, 3 Materials, Arizona State University, Tempe, Arizona, United States
Surface wrinkling has been proven to be a powerful measurement tool to determine the elastic modulus of thin films. A wide range of materials have been examined to date including polymers, porous films, and organic semiconductors as examples. Additionally, the mechanical properties in very thin films determined using surface wrinkling have been shown to vary significantly from the bulk. In this presentation, the thin film moduli of polystyrene and poly(methyl methacrylate) will be discussed with a decrease in the elastic modulus for both polymers observed in ultrathin films. The influence of molecular mass on this thin film behavior is examined. Additionally, surface cross-linking and molecular additives will be explored as routes to engineer the mechanics of polymers at the nanoscale. In order to better understand why the mechanical properties of organic glass thin films are dependent upon their thickness, the moduli of a series of small molecule organic glasses with well defined absorption spectra are determined as a function of film thickness. As the absorption spectrum is related to the molecular packing, any relationship between the molecular density in thin films and their mechanical properties can be explored.
3:00 PM - V3.2
Role of Confinement on Material Flow in Thin Film Geometries for NanoImprint Lithography.
Jeremie Teisseire 1 , Amelie Revaux 2 , Maud Sarrant-Foresti 3 , Elin Sondergard 1 , Etienne Barthel 1 Show Abstract
1 , CNRS/Saint-Gobain, Aubervilliers France, 2 , LPMC / Ecole Polytechnique, Palaiseau France, 3 , Saint-Gobain Recherche, Aubervilliers France
Nano-Imprint Lithography (NIL) [1-2] is a powerful technique for surface patterning at the nanoscale. Direct embossing of the motives, usually on thin thermoplastic polymer films, has a strong potential for large area, low cost patterning [3-4]. For efficient imprinting, material flow into the mask features must be understood and models have been proposed to describe this flow [5-7]. These models have demonstrated the strong impact of the rheology of the material and its wetting properties in relation to the geometry. However NIL is often applied to thin films and the slowing down of the flow due to confinment has not always been considered in enough details. Here, we propose to quantify the effect of confinement on material flow in thin polymeric films such as resist layers in Nano Imprint Lithography (NIL). Above the glass transition temperature, surface patterns relax under the action of surface tension: from the decay rate of the patterns, the flow rate can be quantified. We have tested the impact of confinement on the flow rate on a model material, PMMA: we have monitored the viscous relaxation of nano-structures due to Laplace pressure and measured the impact of confinement on the relaxation time. For large film thicknesses, we recover the bulk regime of relaxation  while the flow rate drops dramatically for increasing confinement. In the context of newtonian fluids with small deformations, we also give an analytical expression for the flow rate as a function of confinement. We show that this simple expression matches our results fairly well. In addition, for severe confinements (i.e. very thin film) the lubrication approximation  is recovered from our expression. We show that there is a transition region where neither bulk relaxation nor the lubrication approximation hold, and in which the full expression proposed here should be used. This transition extends between 0.3 < 2πH/λ < 3 (H is the film thickness and λ the period of the features) which is a range in which many actual configuration geometries fall. Our results should allow easier qualitative understanding and prediction of imprint process parameters on systems of practical interest. S.Y.Chou, P.R.Krauss, P.J.Renstrom, Science, 272, 85 (1996) S.Y.Chou, P.R.Krauss, P.J.Renstrom, Appl.Phys.Lett., 67 (1995), 3114 C. Peroz, C. Heitz, E. Barthel, E. Sondergard, V. Goletto, J Of Vacuum Sci., 27 2007 C. Peroz, V. Chauveau, E. Barthel, E. Sondergard, Adv. Mat. 21, 555, 2009 H.D.Rowland et W.P.King, J. Micromech. Microeng., 14 (2004), 1625-32 H.D.Rowland, A.C.Sun, P.R.Schunk W.P.King, J. Micromech. Microeng., 15 (2005), 2414-25 G.L.W.Cross, J. Phys. D: Appl. Phys., 39 (2006), 363-386 M. Hamdorf, D. Johannsmann, J. of Chem .Phys., 12 (2000), 4262-70 T.Leveder, S.Landis, L.Davoust, Appl.Phys.Lett., 92 (2008), 013107
3:15 PM - V3.3
Wrinkling Delamination in Thin Polymer Coatings on Compliant Substrates.
Adam Nolte 1 2 , Jun Young Chung 2 , Christopher Stafford 2 Show Abstract
1 Department of Chemical Engineering, Rose-Hulman Institute of Technology, Terre Haute, Indiana, United States, 2 Polymers Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
When a thin, stiff coating on a compliant substrate is compressed in the plane of the film, the film may either remain adhered to the substrate and adopt a sinusoidal accordian-like morphology, or simply delaminate from the substrate surface. These phenomena are often referred to as wrinkling and buckle or blister delamination, respectively. Generally speaking, wrinkling is encouraged by factors such as good film adhesion and substrate compliance, while buckles form in systems where the substrate-film interface is weak and/or the the substrate compliance is high enough to make wrinkling energetically unfavorable. Certain systems, however, will initially wrinkle but then subsequently form blisters that act to locally relax the wrinkling instability in the film. We term this intermediate behavior "wrinkling delamination". Through the investigation of wrinkling delamination in thin films of polystyrene on rubbery silicone, we lay a basic framework for understanding the physical parameters governing this phenomenon. Furthermore, we demonstrate how the length scales of wrinkling delamination features can be utilized to measure the adhesion strength of polymer coatings and to pattern microscale features in thin polymer films.
3:30 PM - V3.4
Nanopatterned Polymer Thin Films Through Wrinkling.
Joseph Peterson 1 , Sarav Jhaveri 1 , Kenneth Carter 1 Show Abstract
1 Polymer Science & Engineering, University of Massachusetts-Amherst, Amherst, Massachusetts, United States
Patterning via spontaneous or self-assembly processes have steadily gained attention as interest in smaller, more complex or hierarchical patterns has increased. These patterns, which may be difficult to obtain through traditional patterning techniques, can be created through the proper manipulation of spontaneous events such as block copolymer phase separations, breath figures, etc. Buckling or wrinkling based techniques have been harnessed to create well define and complex surface patterns with many systems. Wrinkling based patterning has the potential to create large areas of small features and has applications in adhesion, optics, and microelectronics. Spontaneous surface patterning of poly(2-hydroxyethylmethacrylate) was achieved through reactive silane infusion induced wrinkling. The wrinkle patterns can cover large areas and have small primary feature sizes approaching 100nm. The surface features can also display a variety of functional groups determined by the choice of silane crosslinker. In combination with simple lithography techniques this process gives control of the surfaces' micro-, nano-, and chemical atributes.
3:45 PM - V3.5
Elastic Modulus of Vapor Deposited Organic Electronic Materials.
Jessica Torres 1 , Nathan Bakken 2 , Christopher Stafford 3 , Jian Li 2 , Bryan Vogt 1 Show Abstract
1 Chemical Engineering, Arizona State University, Tempe, Arizona, United States, 2 Material Science Engineering, Arizona State University, Tempe, Arizona, United States, 3 Polymers Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Flexible devices based upon active organic components represent a potential paradigm shift for electronics away from traditional rigid silicon electronics. Due to the device flexibility, how the active organic materials respond to strain could be a critical issue in the long term reliability of bendable devices. However, little is known regarding the mechanical properties of organic electronic materials due to challenges involved with limited material quantities, which requires these materials be examined as thin films, and their intrinsic soft properties. Of particular interest are vacuum deposited small molecules due to their prevalence in OLED and TFT technologies. In this presentation, the elastic modulus of vacuum deposited tri-(8-hydroxyquinoline) aluminum (Alq3), N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4″-diamine (NPD), and 4,4'-N,N'-dicarbazole-biphenyl (CBP) films is determined using a wrinkling-based metrology. Ellipsometric measurements reveal that direct vacuum deposition results in diffusion of the OLED material into the elastomeric substrate (PDMS) required in the wrinkling metrology. Wrinkling of Alq3 directly deposited on PDMS can yield an apparent modulus that is an order of magnitude low. We find that a thin polystyrene barrier film enables a temporally stable film. Interestingly, the film modulus is thickness independent unless less than 20 nm. For these thinner films, the modulus can either increase (Alq3 and NPD) or decrease (CBP) in comparison to the ‘bulk’-like modulus obtained for thicker films. For example, the plain strain modulus increases from approximately 1.1 GPa to 1.75 GPa and 1.1 GPa to 2.1 GPa as the film thickness is decreased from 20nm to 10nm for Alq3 and NPD, respectively. In contrast, the plain strain modulus of the more planar CBP exhibits an order of magnitude decrease from 2.5 GPa to 0.9 GPa as the film thickness is reduced from 20nm to 10nm. The optical properties determined by spectroscopic ellipsometry appears to thickness dependent as well, which suggests changes in thin film morphology.. Surface wrinkling is found to be an efficient route to measure the elastic properties of vapor deposited glassy materials for application in organic electronics.
4:00 PM - V3: Metrology
V4: Instabilities in Gels
Monday PM, November 29, 2010
Room 205 (Hynes)
4:15 PM - **V4.1
Large Deformation and Instability in Swelling Gels.
Zhigang Suo 1 Show Abstract
1 , Harvard University, Cambridge, Massachusetts, United States
Flexible, long-chained polymers can crosslink into a three-dimensional network. The network can imbibe a large quantity of a solvent and swell, resulting in an aggregate known as a polymeric gel. Gels are used in diverse applications, including medical devices, actuators in microfluidics, and packers in oil wells. The process of swelling can be markedly influenced by a mechanical load. When the network, the solvent, and the mechanical load equilibrate, the deformation in the gel is usually inhomogeneous. This talk describes a nonlinear field theory of gels. The theory is illustrated with examples of swelling induced large deformation, contact, and bifurcation.
4:45 PM - V4.2
A Finite Element Method for Transient Analysis of Instability in Gels.
Hanqing Jiang 1 , Jiaping Zhang 1 , Nic Fang 2 Show Abstract
1 , Arizona State University, Tempe, Arizona, United States, 2 , University of Illinois, Urbana, Illinois, United States
Long-chain polymers may crosslink by strong chemical bonds into a three-dimensional network. The resulting material, an elastomer, is capable of large and reversible deformation. The elastomer may imbibe a large quantity of solvents, aggregating into a gel. The solvent molecules in the gel interact by weak physical bonds and can migrate. The dual attributes of a solid and a liquid make the gel a material of choice in nature and in engineering, such as tissue engineering, drug delivery and soft MEMS.Many processes in gels involve concurrent deformation and migration. For example, a drug loaded in a gel can migrate out in response to a change in the physiological conditions (i.e., the temperature, the level of pH, or the concentration of an enzyme). The rate of the release may be modulated by the deformation of the gel. As another example, patterns of crease often appear on the surface of a swelling gel, along with many other forms of buckling. Furthermore, swelling may induce stress localization in gels, which leads to cavitation and delamination. Hydrogels with sub-millimeter size have been extensively used as valves in microfluidics due to the short swelling time and large deformation. This paper studies the concurrent deformation and migration in the gel by a finite element method. We combine the kinematics of large deformation, the conservation of the solvent molecules, the conditions of local equilibrium, and the kinetics of migration to evolve simultaneously two fields: the displacement of the network and the chemical potential of the solvent. The finite element method is demonstrated by analyzing several phenomena, such as swelling, draining and buckling. This work builds a platform to study diverse phenomena in gels with spatial and temporal complexity.
5:00 PM - **V4.3
Swell Induced Surface Instability of Confined Hydrogel Layers on Substrates.
Rui Huang 1 , Min Kang 1 Show Abstract
1 , University of Texas at Austin, Austin, Texas, United States
In response to external stimuli, polymeric hydrogels can change volume and shape dramatically. Experimental studies have observed a variety of instability patterns of hydrogels, due to swelling or shrinking, many of which have not been well understood. The present paper considers swellinduced surface instability of a hydrogel layer on a rigid substrate. Based on a recently developed theoretical framework for neutral polymeric gels, a linear perturbation analysis is performed to predict the critical condition for onset of the surface instability. Using a nonlinear finite element method, numerical simulations are presented to show the swelling process, with evolution of initial surface perturbations followed by formation of crease-like surface patterns. In contrast to previously suggested critical conditions for surface creasing, the present study suggests a material specific condition that predicts a range of critical swelling ratios from about 2.5 to 3.4 and quantitatively relates the critical condition to material properties of the hydrogel system. A stability diagram is constructed with two distinct regions for stable and unstable hydrogels depending on two dimensionless material parameters. By including the effect of surface tension, an intermediate wavelength is selected at a critical swelling ratio for the onset of surface instability. Both thecritical swelling ratio and the characteristic wavelength depend on the initial thickness of the hydrogel layer. It is found that the hydrogel layer becomes increasingly stable as the initial layer thickness decreases. A critical thickness is predicted, below which the hydrogel layer swells homogeneously and remains stable at the equilibrium state.
5:30 PM - V4.4
Gear Formation via Buckling Multilayered Film-hydrogel Structures.
Xiao-Xin Zhang 1 , Tian-Fu Guo 2 , Yong-Wei Zhang 2 Show Abstract
1 Department of Materials Science and Engineering, National University of Singapore, Singapore Singapore, 2 , Institute of High Performance Computing, Singapore Singapore
Spontaneous formation of patterns via buckling multilayered structures provides a fascinating route for the generation of nano- to micro-structured functional devices. For soft hydrogels coated with thin elastic films, residual compressive stresses are generated during shrinking/swelling of hydrogels, triggering the buckling of thin films. A variety of morphologies and spatial shapes may occur due to the difference in stress states. Previous studies primarily focused on flat substrate surfaces. Recently the curved substrates have attracted great attention due to the sensitivity of the buckling conditions of thin shells to substrate curvature. In the present work, we investigate the buckling instability of a multilayered hydrogel tubular structure under shrinking using both analytical method and computer simulation. We demonstrate that spontaneous formation of patterns via buckling multi-layered hydrogel-film structure is able to provide a convenient way to fabricate components for gear systems. Hydrogel gears with inner teeth, outer teeth and double teeth can be obtained. We also show that the analytical model is able to capture the tendency of the finite element results, and thus can be used to approximately predict for teeth formation in hydrogel gears. There are a few of important features about the proposed hydrogel gear systems. First, the teeth spatial shape and teeth number can be controlled via material and geometry properties, giving rise to a great flexibility in controlling gear components; second, hydrogels are capable of undergoing large volume change in response to external stimuli like temperature and pH value, this may provide an easy control for switching on and off the gear transfer system, and finally through adjusting these stimuli fields via the transition axis or surrounding fluid, the gear teeth height can be controlled, providing high flexibility for adjusting teeth height and interlock tolerance even after fabrication. Consequently, the proposed hydrogel gear systems may have important applications in hydrogel-based MEMS systems for transferring minute amount of moment.
5:45 PM - V4.5
Osmotic Collapse of a Void in an Elastomer: Breathing, Buckling and Creasing.
Shengqiang Cai 1 , Katia Bertoldi 1 , Huiming Wang 1 , Zhigang Suo 1 Show Abstract
1 , Harvard, Cambridge, Massachusetts, United States
This paper studies the collapse of a void in an elastomer caused by osmosis. The void is filled with liquid water, while the elastomer is surrounded by unsaturated air. The difference in humidity motivates water molecules to permeate through the elastomer, from inside the void to outside the elastomer, leaving the liquid water inside the void in tension. When the tension is low, the void reduces size but retains the shape, a mode of deformation which we call breathing. When the tension is high, the void changes shape, possibly by two types of instability: buckling and creasing. The critical conditions for both types of instability are calculated. A tubular elastomer collapses by buckling if the wall is thin, but by creasing if the wall is thick. As the tension increases, a thin-walled tube undergoes a buckle-to-crease transition.
V5: Poster Session
Tuesday AM, November 30, 2010
Exhibition Hall D (Hynes)
9:00 PM - V5.2
Modeling Pattern Formation in Heterogeneous Three-dimensional Gel Membranes.
Olga Kuksenok 1 , Anna Balazs 1 Show Abstract
1 Chemical Engineering Dep, University of Pittsburgh, Pittsburgh, Pennsylvania, United States
Gel membranes are used in a variety of applications, from controlled release of chemicals in drug delivery systems to flow regulation within microfluidic devices. The degree of swelling within the gel membrane regulates its permeability and hence, controls transport of different species through such membranes. One of the effective ways to control the degree of swelling of gels is by varying the temperature of the system; another is by varying the nature of its confinement. Here, we develop a computational model that allows us to simulate both, the effects of varying temperature and varying the confinements on the dynamics of three-dimensional gel membranes. We use this model to investigate the structural evolution of heterogeneous gel membranes that contain embedded metal filaments and focus on two examples of different arrangements of these filaments. We show that dynamics of pattern formation strongly depends on filaments’ arrangements and on the confinement of the membranes. We demonstrate that by varying the temperature, one can effectively control the shape and the permeability of such heterogeneous membranes.
9:00 PM - V5.4
Curvature-driven Instability and Wrinkling in Elastic Thin Films.
Amin Ajdari 1 , Ashkan Vaziri 1 , Babak Jahromi 1 Show Abstract
1 Department of Mechanical and Industrial Engineering, Northeastern University, Boston, Massachusetts, United States
Despite the significance, many phenomenological aspects of the behavior of naturally-curved shells are still ambiguous and pose fundamental challenges for applications of mechanics in new areas such as nanostructures and biology. The highly nonlinear behavior of shells is mainly governed by inextensible or almost inextensible deformations, which are energetically preferred by the shell. In large deformations, this leads to appearance of structural features such as dimensionless developable cone and curvature condensates and almost inextensible one-dimensional ridges. The key challenge is the intricate interplay of physics and geometry, which leads to a mechanical response which is much different from the response of solid objects. The quest to understand the underlying phenomena has spawned theoretical and experimental studies, which have helped in understanding the underlying mechanisms of deformation and response of shells. In this work, several simple, but insightful assays, on the mechanics of spherical shells were studied to unravel the mechanisms of instability in shells. This includes probing the behavior of ellipsoidal shells as they get in contact with a rigid surface. The ellipsoidal shells showed a progressive directional delamination and wrinkling from rigid surfaces – a deformation mechanisms that has not been previously reported and is governed by the non-equivalent curvature of the shell in two fundamental directions.
9:00 PM - V5.5
Switchable Transparency and Wetting Properties of Elastomeric Smart Windows Based on Surface Wrinkling.
Seung Goo Lee 1 , Dong Yun Lee 1 , Ho Sun Lim 1 , Kilwon Cho 1 Show Abstract
1 Department of Chemical Engineering, Polymer Research Institute, Pohang University of Science and Technology, Pohang Korea (the Republic of)
We report a facile and efficient method for fabricating elastomeric smart windows with switchable optical transparency and wetting properties by combining replica molding with surface wrinkling. Reversible switching between transparent/water-pinned and translucent/superhydrophobic properties was achieved by controlling the surface topography via mechanical strain. The resulting smart windows, which featured controlled nanopillars and surface wrinkling, exhibited not only significant changes in optical transmittance and wetting properties, but also displayed additional functionalities such as durability, rapid response, self-cleaning, and antireflection. This approach provides a low-expertise route to fabricating multifunctional smart windows, and the methodology may be applied in a wide range of applications requiring external stimuli-responsive surfaces.Acknowledgement. This work was supported by a grant (Code No. 2010K000284) from the Center for Nanostructured Materials Technology under the 21st Century Frontier R&D Programs of the Ministry of Education, Science and Technology, Korea.
9:00 PM - V5.6
High Aspect Ratio Wrinkles Created by Carbon Deposition on Pre-patterned Soft Polymers.
Sk Faruque Ahmed 1 , Geon-Ho Noh 1 , Kwang-Ryeol Lee 1 , Ashkan Vaziri 2 , Myoung-Woon Moon 1 Show Abstract
1 , Korea Inst. Sci. Tech., Seoul Korea (the Republic of), 2 , Northeastern University, Boston, Massachusetts, United States
Instability of a thin film attached to a compliant substrate often leads to emergence of exquisite wrinkle patterns with length scales that depend on the system geometry and applied stresses. These patterns have potential applications in many areas including tissue engineering, flexible electronics and semiconductor industry. However, the patterns that are created using the current techniques in polymer surface engineering, generally have low aspect ratio of undulation amplitude to wavelength, thus, limiting their application. Here, we present a novel and effective method that enables us to create wrinkles with a desired wavelength and high aspect ratio of amplitude over wavelength as large as to 2.5:1. First, we create buckle patterns with high aspect ratio of amplitude to wavelength by deposition of an amorphous carbon film on a surface of a soft polymer poly(dimethylsiloxane) (PDMS). Amorphous carbon films are used as a protective layer in structural systems and biomedical components, due to their low friction coefficient, strong wear resistance against, and high elastic modulus and hardness. The deposited carbon layer is generally under high residual compressive stresses (~ 1GPa), making it susceptible to buckle delamination on a hard substrate (e.g. silicon or glass) and to wrinkle on a flexible or soft substrate. Then, we employ glancing angle deposition (GLAD) for deposition of a amorphous carbon film on a PDMS surface. GLAD is a physical vapour deposition method used to fabricate functional thin films with columnar morphology.Using this method, pattern amplitudes of several nm to submicron size can be achieved by varying the carbon deposition time, allowing us to harness patterned polymers substrates for variety of application. Specifically, we demonstrate a potential application of the high aspect wrinkles for changing the surface optical band gap, resulting in reduction in optical band gap, with the measured range of 3.27 to 2.79 eV with respect to wrinkle wavelength.
9:00 PM - V5.7
Understanding Ar Ion Induced Structural Change of Polymers Surface and Its Dependency on Ion Energy and Polymer Backbones (PE and PDMS): Molecular Dynamics Simulation and Comparison with Experiments.
Chansoo Kim 1 , Sk. Faruque Ahmed 1 , Myoung-Woon Moon 1 , Kwang-Ryeol Lee 1 Show Abstract
1 Computational Science Center, Korea Institute of Science and Technology (KIST), Seoul Korea (the Republic of)
This research mainly focuses on explaining the process of wrinkles (buckles) structure generation on polymer surface that arise during ion beam injection. Ion beam bombardment is generally known to form nanosize patterns such as ripples, dots or wrinkles on the surface of polymers in ambient temperature and pressure. Many support the ion beam can alter the polymer surface that induces skins stiffer or the density higher by higher compressive stress or strain energies associated with chain scissions and crosslinks of the polymer. Atomic scale structure evolution in polymers is essential to understand a stress generation mechanism during the ion beam bombardment, which governs the nanoscale surface structure evolution.Molecular Dynamics (MD) simulations are employed to characterize the phenomenon occurred in bombardment between the ion beam and polymers that forms nanosize patterns. We investigate the structure evolution of Low Density Polyethylene (LDPE) at 300 K as the polymer is bombarded with Argon ions having kinetic energies ranging from 100 to 950 eV with 50 eV intervals having the fluence of 1.45 × 1014 #/cm2. These simulations use the Reactive Force Field (ReaxFF), which can mimic chemical covalent bonds and includes van der Waals interactions.The results show the details of the structural evolution of LDPE by the low energy Ar ion bombardment. Analyses through kinetic and potential energy, number of crosslinks and chain scissions, local densification and motions of atoms support that the residual strain energies on the surface is strongly associated with the number of crosslinks or scissored chains. Also, we could find an optimal Ar ion beam energy to make crosslinks.Young’s modulus of each Ar ion treated case is obtained. Conditions for wrinkle formation are directly compared with experiments through wavelengths of wrinkles. This possibly drives us to conclude that Ar ion bombardment can modify the polymer surface through a similar process of wrinkle formation.Moreover, we investigate material dependency of Ar ion treatment on the surface of polymer substrates. Using PE and PolyDiMethylSiloxane (PDMS) samples with the moderate energy ranges of 500 eV and 700 eV, it is shown how Ar ions affect different polymers, which have different chemical structure but similar physical backbones.
Christopher M. Stafford National Institute of Standards and Technology
Adam J. Nolte Rose-Hulman Institute of Technology
Pil J. Yoo Sungkyunkwan University
Teng Li University of Maryland
V6: Adhesion & Wetting
Tuesday AM, November 30, 2010
Room 205 (Hynes)
9:30 AM - **V6.1
Microstructured Elastomeric Surfaces with Reversible Adhesion and an Example of Their Use in Deterministic Assembly by Transfer Printing.
Seok Kim 2 , Yonggang Huang 1 , John Rogers 2 Show Abstract
2 Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 1 Civil&Environmental Engr and Mech Engr., Northwestern University, Evanston, Illinois, United States
Reversible control of adhesion is an important feature of many desired, existing and potential systems, including climbing robots, medical tapes, and stamps for transfer printing. We present experimental and theoretical studies of pressure modulated adhesion between flat objects and elastomeric surfaces with sharp features of surface relief in optimized geometries. Here, the strength of non-specific adhesion can be switched by more than three orders of magnitude, from strong to weak, in a reversible fashion. Implementing these concepts in advanced stamps for transfer printing enables versatile modes for deterministic assembly of solid materials in micro/nanostructured forms. Demonstrations in printed two and three dimensional collections of silicon platelets and membranes illustrate some capabilities. An unusual type of transistor that incorporates a printed gate electrode, an air gap dielectric, and an aligned array of single walled carbon nanotubes provides a device example.
10:00 AM - V6.2
Dynamic Nanoflakes: A Supramolecular Candidate for Strong Energy Absorption.
Chichao Yu 1 , Li Tan 1 Show Abstract
1 , University of Nebraska, Lincoln, Nebraska, United States
Energy absorption or dissipation from thin films is increasingly demanded by civil and military applications. And aluminum is probably the most well-known material that can absorb significant amount of mechanic energy as thin films. Staircase-like loading and sharp unloading curves were revealed by nanoindentation, which were attributed to nucleation and glide of large number of dislocations. Certainly, when great amount of these nanometer scale dislocations are made available in organic thin films, they offer unmatched advantages including unlimited variability and processing ease. While few studies were steered toward molecularly engineering an aluminum-like material, a simple mixing of fibrous-(1D) or laminar-like (2D) fillers within a bulk material could deliver a composite with significant energy absorption. To name a few, these include carbon fiber/PEEK, carbon fiber/epoxy, and glass cloth/epoxy. Central to the success of these polymer-based composites is their much increased flexibility. Under external impact, the composites dissipate energies via mechanisms, such as structure buckling, interface cracking, delamination, or even laminar fragmentation. Unfortunately, when such a macroscopic structure is condensed into a thin film, the absence of bulk deformation in a confined space cannot generate enough response. Hence, it is desirable to have a solid, lightweight counterpart to aluminum thin film and engineer dislocations therein for superior energy absorption. In this presentation, we address our solution to this need by varying interfaces in supramolecules. Over the past years, extensive studies on these well-ordered nanomaterials focused on modifying building blocks via synthetic organic chemistry. Other major efforts rely predominantly on optoelectronic device properties, rarely has attention been paid by utilizing easy-to-configure interfaces inside supramolecules for energy absorption. We show performance of resulting nanomaterials (dubbed dynamic nanoflakes) after such reconfiguration is outstanding. The specific energy absorption (energy dissipated per unit mass) approached 275 J/g, while the state-of-the-art property for thin films of nylon and Al(100) is 60 and 140 J/g, respectively. In addition, our material showed one unique feature by decoupling the mechanical strength with the capability of energy absorption. As a consequence, our material has a rather high modulus (12.5 GPa). Since supramolecules enjoy a large interface-to-mass ratio and a one-pot synthesis pathway, we expect them great impact on many important fields needing structure or performance protection with minimal added weight or volume.
10:15 AM - **V6.3
Wrinkles and Cracks: Versatile Tools for Gecko-inspired Dry Adhesion and Microfluidics.
Hong-Nam Kim 1 , Sung-Hoon Lee 1 , Hoon-Eui Jeong 1 , Kahp Suh 1 2 Show Abstract
1 School of Mechanical and Aerospace Engineering, Seoul National University, Seoul Korea (the Republic of), 2 World Class University Program on Multiscale Mechanical Design, Seoul National University, Seoul Korea (the Republic of)
The analysis and control of wrinkles and cracks has been extensively studied both experimentally and theoretically, to fabricate various micro- and nanostructures in a simple and low-expertise fashion. In this invited talk, I will present use of wrinkles and cracks as a means for stretchable, adhesion controllable polydimethyl siloxane (PDMS) micropillars or PDMS microchannels with specific size (width and depth) and space gradients. In the first part, a stretchable, adhesion-tunable dry adhesive is presented by combining replica molding and surface wrinkling. By utilizing a thin, wrinkled PDMS sheet with a thickness of 1 mm with built-in micropillars, active, dynamic control of normal and shear adhesion was achieved. It was found that relatively strong normal (10.8 N/cm2) and shear adhesion (14.7 N/cm2) forces could be obtained for a fully extended (strained) PDMS sheet (prestrain of 3%), whereas the forces could be rapidly reduced to nearly zero once the prestrain was released (prestrain of 0.5%). In the second part, a simple method is presented to generate cracks of varying dimensions in terms of size and space by directionally stretching a surface-oxidized PDMS slab on a cylindrical support. The control of size and space gradients was achieved by independently modulating the thickness of PDMS slab and distance from UV source. Furthermore, multiple, sequential generation of cracks was demonstrated by applying multiple strains with an orientation angle.
10:45 AM - V6.4
Large Contact Angle Hysteresis for Wetting of Wrinkled Surfaces.
Colton Bukowsky 2 , Jessica Torres 1 , Bryan Vogt 1 Show Abstract
2 Materials , Arizona State University, Tempe, Arizona, United States, 1 Chemical Engineering, Arizona State University, Tempe, Arizona, United States
Wetting on a corrugated surface that is formed via wrinkling of a hard skin layer formed by UV oxidation (UVO) of a poly(dimethylsiloxane) (PDMS) slab is studied using advancing and receding water contact angle measurements. The amplitude of the wrinkled pattern can be tuned through the pre-strain of the PDMS prior to surface oxidation. These valleys and peaks in the surface topography lead to anisotropic wetting by water droplets. As the droplet advances, the fluid is free to move along the direction parallel to the wrinkles, but the droplet moving orthogonal to the wrinkles encounters energy barriers due to the topography and slip-stick behavior is observed. As the wrinkle amplitude increases, anisotropy in the sessile droplet increases between parallel and perpendicular directions. For the drops receding perpendicular to the wrinkles formed at high strains, the contact angle tends to decrease steadily towards zero as the drop volume decreases, which can result in apparent hysteresis in the contact angle of over 100 degrees. The wrinkled surfaces can exhibit high sessile and advancing contact angles (>115 degrees), but the receding angle in these cases is generally vanishing as the drop is removed. This effect results in micrometer sized drops remaining in the grooves for these highly wrinkled surfaces, while the flat analogous UVO-treated PDMS shows complete removal of all macroscopic water drops under similar conditions.
11:00 AM - V6:Adhesion
V7: Applications in Electronics
Tuesday PM, November 30, 2010
Room 205 (Hynes)
11:30 AM - **V7.1
Applications of Buckling in Electronics and Optoelectronics.
John Rogers 1 Show Abstract
1 , University of Illinois, Urbana, Illinois, United States
Recent work shows that buckling instabilities in hard materials on soft substrates can be exploited in diverse applications, from precision metrology to stretchable electronics. This talk describes results from systematic experimental and theoretical studies of geometry and adhesion controlled buckling in composite structures of diamond, carbon nanotubes, graphene, lead zirconate titanate, silicon and gallium arsenide on elastomer substrates of poly(dimethylsiloxane). Various examples of system-level use in stretchable silicon and gallium arsenide electronics and optoelectronics will be presented.
12:00 PM - V7.2
Coupling of Localized Buckling Instabilities and Optical Properties in 2D and 3D Soft Photonic Crystal Materials.
Harley Johnson 1 , Dwarak Krishnan 1 Show Abstract
1 Mechanical Science and Engineering, University of Illinois, Urbana, Illinois, United States
Soft material photonic crystals (PCs) are novel periodic dielectric materials that can be used as optomechanically sensitive, highly flexible structures actuated by strain, pH, light, heat, or other stimuli. In this work we report on interesting pattern transformations mediated by localized buckling inside 2D or 3D soft material PCs due to applied stress or incident light. These effects change the optical behavior as a consequence of changes in symmetry, periodicity, and filling fraction of the PC. First, we analyze the shrinkage of a 3D PC structure patterned in SU-8 photoresist by means of holographic lithography. Our model accurately predicts localized buckling and twisting of ligament like interconnects that induces a macroscopic structural collapse. Optical transmittance simulations on the deformed structure correctly model the primary and secondary reflectance spectrum peaks as observed experimentally. Second, we analyze a swelling-induced pattern transformation observed in experiments on a hydrogel inverse FCC opal PC and we develop a poroelasticty-based FEA model to capture the deformation mode. We also carry out optical transmittance calculations and are able to predict with good accuracy the shift in peak reflectance wavelength and values. Third, we introduce simulation evidence for a fully optomechanically coupled soft material PC: a novel azobenzene liquid crystal elastomer (LCE) based 2D PC that can be reversibly optically actuated. Incident light at a particular wavelength induces a trans- to cis- isomerization inside the material in regions experiencing localization of the incident energy. This causes a macroscopic contraction of the structure, a shift in periodicity, and an associated buckling-like symmetry transformation. The deformation modulates the optical transmittance via changes in the photonic bandstructure, and the new structure ultimately returns to its initial configuration via the slow kinetics associated with stress relaxation. We demonstrate that through this mechanism, the material can be made to undergo a fully optomechanically coupled cyclic deformation under steady illumination.
12:15 PM - V7.3
Designing Micro-patterned Ti Films that Survive up to 10% Applied Tensile Strain.
Noble Woo 1 , Kunigunde Cherenack 2 , Gerhard Troester 2 , Ralph Spolenak 1 Show Abstract
1 Department of Materials, ETH Zürich, Zürich, Zürich, Switzerland, 2 Institute for Electronics, ETH Zürich, Zürich, Zürich, Switzerland
Reducing the strain in brittle device layers is critical in fabrication of flexible electronic devices. In this study, the cracking behavior of micro-patterned 500-nm-thick Ti films was investigated via uniaxial tensile testing by in situ SEM and 4-point probe measurements. Visual observations by SEM and 4-pt resistance measurements showed that strategically patterned oval holes, off-set and rotated by 45°, had a significant effect on limiting the extent of cracking, specifically, in preventing cracks from converging. Failure with regard to electrical conduction was delayed from less than 2% to more than 10% strain by the presence of percolation path around the approaching parallel cracks. Hence, understanding and controlling crack propagation by strategic patterning of thin films may enable designing of robust flexible electronic devices.
12:30 PM - V7.4
Field Emission Displays on Flexible Substrates from Crumples and Capillary Bridges.
Sanjiv Sambandan 1 Show Abstract
1 , Indian Institute of Science, Bangalore India
We aim to develop field emission tips on flexible substrates such as plastics sheets using low temperature self assembly methods. The advantage of this is the possibility of low cost field emission displays on flexible substrates, low power micro electro-magnetic actuators, low cost microscopy etc. We consider two approaches to achieve the low temperature self assembly of field emission tips. The first is by taking advantage of the sharp features formed of crumples of low work function metal films. The ridges and tips seen in a crumple has an elemental geometric entity – the developable cone. We self assemble d-cones using two dimensional buckling of thin metal films deposited on flexible substrates. Measurements so far have show field emission with a maximum current of 1e-4A/cmsq at ~1e7V/m field measured in 1e-6 bar pressure.The second approach is to use low melting point metals (such as Tin/Lead) and create a capillary bridge between the molten metal ‘droplet’ and a foreign micro-tip. The metal is frozen as the bridge is drawn. If the bridge is thin enough, we are left with the formation of a sharp tip. The molten metal droplets can be deposited on treated flexible substrates. Measurements from single tip have shown field emission currents of the order of 1e-7A at ~1e7V/m field have been measured at 1e-6 bar pressure.
12:45 PM - V7.5
Mechanical Instability of Single-crystal Semiconductor Nanomembranes due to Isotropic Volume Expansion of Elastomeric Substrates in Solvents.
Francesca Cavallo 1 , Kevin Turner 1 2 , Max Lagally 1 Show Abstract
1 Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States, 2 Mechanical Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States
Thin films deposited on a soft material are often compressively strained, and they can form into wrinkles under appropriate conditions. Buckling and wrinkling of thin films on a soft material have been regarded for a long time as a nuisance: only recently they have proved to be a resource in the application space of flexible and stretchable electronics. Furthermore periodic wrinkles occurring in thin films of conventionally hard materials on a soft substrate have potential applications as tunable diffraction gratings, sensors for advanced metrology methods, guiding structures to achieve alignment of particles and cells, channels for fluidic devices integrating optical and electrical functionality. We present a novel approach for the fabrication of matrices of semiconductor wrinkled structures on insulating elastomeric substrates. We demonstrate long-range ordering with a periodicity of the order of a few tens of micrometers for single crystalline nanomembranes (NMs) with thicknesses in the range of tens to hundreds of nanometers. For this purpose we use group IV based NMs originally bonded to a SiO2 buffer layer on a Si substrate. The semiconductors templates are patterned in a near checker-board fashion with squared islands separated by narrow interconnect regions at the corners. After release, NMs are transferred to a polydimethylsiloxane (PDMS) substrate in a solvent. Isotropic volume expansion is induced in the elastomer upon swelling in solvents. Next the sample is exposed to air at room temperature, resulting in evaporation of the solvent and contraction of the substrate leading to compressive strain in the NM bound on the elastomer substrate. The poor adhesion of the interconnect regions to PDMS and their low bending stiffness compared to the large NM squares result in out-of-plane buckling at the interconnects. We investigate buckling of NMs due to different values of compressive strain using various solvents. We use a combination of analytical and finite-element mechanics models to understand the mechanics of the release process, and establish design principles. Furthermore analytical and finite element techniques are employed to estimate the strain distribution in the wrinkled film. The modeling is supported by micro-Raman spectroscopy. Strain values are extracted from spectra acquired at the buckled and flat region of the NMs. Furthermore an estimation of strain variation in the out-of-plane direction in the buckled NM is obtained by Raman spectroscopy performed at different penetration depths.Supported by AFOSR and DOE
V8: Instabilities in Patterning II
Tuesday PM, November 30, 2010
Room 205 (Hynes)
2:30 PM - **V8.1
Plasma-polymer Interactions For Nanoscale Patterning Of Materials: Mechanistic Origins of Surface Roughness.
Gottlieb Oehrlein 1 Show Abstract
1 Materials Science and Engineering Department and Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland, United States
Photolithographic patterning of organic materials and plasma-based transfer of photoresist patterns into other materials have been remarkably successful in enabling the production of nanometer scale devices in various industries. These processes involve exposure of highly sensitive polymeric nanostructures to energetic particle fluxes that can greatly alter surface and near-surface properties of polymers. For manufacturing of below 30-nm scale devices, overcoming the problem of uncontrolled introduction of surface and line edge roughness in organic mask features has become a key challenge. We will review results of plasma-polymer interaction studies for selected model polymers and advanced photoresist materials. The observed polymer modifications include the formation of a thin (~1 nm) dense graphitic layer at the polymer surfaces due to ion bombardment, and deeper-lying modifications by vacuum UV irradiation, that depend strongly on initial polymer structure. The formation of surface roughness is also highly polymer specific. The picture of a possible fundamental mechanism of plasma-induced polymer surface roughness formation that emerges from this work is one where polymer material-dependent near-surface modifications introduced at various depths by different plasma fluxes synergistically interact and produce surface roughness. In particular, a model aimed at explaining measured surface roughness parameters using elastic buckling theory, with the measured properties of the modified surface layer as inputs, will be discussed. * Based on collaborations with R. L. Bruce, F. Weilnboeck, S. Engelmann, T. Kwon, T.C. Lin, R. Phaneuf, X. Hua, M. Sumiya, D. Graves, D. Nest, J. Vegh, Ting-Ying Chung, Y. C. Bae, C. Andes, M. Li, E. A. Hudson, B. Long, and G. WillsonWe gratefully acknowledge financial support of this work by the National Science Foundation under awards Nos. DMR-0406120, DMR-0705953 and NIRT CTS-0506988.
3:00 PM - V8.2
Self-wrinkling of UV-cured Polymer Films via Oxygen Inhibition.
Dinesh Chandra 1 , Alfred Crosby 1 Show Abstract
1 Polymer Science and Engineering, University of Massachusetts, Amherst, Amherst, Massachusetts, United States
We demonstrate a novel mechanism of self-wrinkling in UV-cured acrylate polymer films resulting in a single-step fabrication of wrinkles with well-controlled wavelength and amplitude. During free-radical UV-curing of polymer films in the presence of oxygen, a thin liquid layer of non-polymerized monomers is always present on top of the underlying cured film due to quenching of free radicals by oxygen. Here, we take advantage of this non-cured liquid layer to generate swelling induced surface wrinkle patterns with well-controlled dimensions in a single fabrication step. The characteristic wavelength of the wrinkled patterns is proportional to the thickness of the cured films. The wrinkle amplitude is controlled by the amount of oxygen available to the curing film. Finally, by controlling the spatial distribution of oxygen in contact with the curing film through a patterned barrier to oxygen diffusion, we demonstrate microscale spatial patterning of the wrinkles.
3:15 PM - V8.3
Modulated Interface Lithography: A Theoretical Proposal for 3D Nanostructure Sculpting Based on Bénard Flux Flow.
Sandra Troian 1 , Mathias Dietzel 1 Show Abstract
1 Applied Physics, California Institute of Technology, Pasadena, California, United States
Structures with dimensions in the micron to nanometer range manifest exceedingly large surface to volume ratios. Liquid structures with at least one free boundary are therefore particularly susceptible to flow modulation by surface forces. In the past, this feature has been used to maneuver the motion of small liquid elements along the exterior surface of glass or silicon substrates for micro- and optofluidic applications . Normal and tangential stresses along the free surface of liquid films has been used to steer, mix and dispense films or droplets on demand . Flow at low Reynolds number which also exhibits small aspect ratios will respond instantaneously to variations in interfacial stress due to negligibly small inertial and phase lag effects. In this talk, we discuss special characteristics of thermocapillary modulation of liquifiable nanofilms with the potential for direct-write capability of ultrasmooth 3D nanostructures. This unusual patterning technique relies on the directed flow of liquid toward patterns and designs whose surfaces are cooler than the local fluid temperature. The system acts much like a cooled pen able to draw fluid into 3D shapes without actual contact. For polymer films of PS or PMMA with low glass transition temperatures, the resulting 3D structures will solidify in place once the thermal gradient is removed and manifest ultrasmooth surfaces without further annealing.Our model is based on imposed thermal distributions which generate periodic protrusions in all viscous films no matter how small the applied thermal gradient [3,4]. The linear stability analysis corresponds to an extreme limit of Benard-Marangoni flow in which hydrostatic forces are absent and deformation amplitudes are small in comparison to the pillar spacing. Finite element simulations of the full nonlinear equation provide estimates of the array pitch and structure growth rates beyond the linear regime. Simulations of the Lyapunov free energy confirm that in contrast to typical cellular instabilities in macroscopically thick films, pillar-like elongations are energetically preferred in nanofilms. Provided there occurs no dewetting during film sculpting, fluid elongations will continue to grow until contact with the cooler substrate is achieved. Structures of specified height can also be fabricated by controlling the time of exposure to the external temperature distribution. Our findings indicate that this flow mechanism can provide a realistic technique for fabrication of extended arrays for nanoscale optical, photonic and biological applications. A. A. Darhuber, J. P. Valentino, J. M. Davis, S. M. Troian and S. Wagner, Appl. Phys. Lett. 82, 657 (2003) A. A. Darhuber and S. M. Troian, Annu. Rev. Fluid Mech. 37, 425 (2005)  M. Dietzel and S.M. Troian, Phys. Rev. Lett. 103, 074501 (2009) M. Dietzel and S. M. Troian, to be published in J. Appl. Phys. (2010)
3:30 PM - V8.4
Guided Wrinkling from Swelling of SU-8 Photoresist Films with Depth Gradient Crosslinking Density.
Chi-Mon Chen 1 , Jason Reed 2 , Shu Yang 1 Show Abstract
1 Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 2 Materials Science and Engineering, Cornell University, Ithaca, New York, United States
Harnessing the mechanical instability in thin film is an attractive way of generating various complex patterns. The mechanical instability can be initiated by thermal or mechanical load. Here, we report a new method of generating self-organizing wrinkling patterns by swelling SU-8 photoresist thin films with depth-wise modulus gradient. To create the modulus gradient, a rhodamine dye was added into SU-8 resist formulation to absorb the ultraviolet light (UV) during photoexposure, After post-exposure bake to crosslink the film, a depth-wise gradient in crosslinking density was generated with the outer surface more crosslinked and rigid. Subsequently, the film was exposed to the vapor of a good solvent of SU-8, tetrahydrofuran or propylene glycol methyl ether acetate. The inhomogeneously swelling created an anisotropic compressive stress, where a hard thin layer on the outer surface buckled on a softer film underneath, much like the wrinkling in oxide-on-PDMS bilayer film under strain. By controlling the solvent quality, swelling time and film thickness, a library of patterns including hexagon, peanut and lamellar structures were produced with wavelength ranging from few tens of micron to submicron scale. Using photoresist as the target materials makes it possible to guide the wrinkling patterns into photopatterned region, which allows for formation of more complex patterns. Possible applications of this method include tunable wettibility, microfluidics and sensors. Detail temporal evolution and propagation of the wrinkling patterns will also be presented.
3:45 PM - V8.5
Observation of the Deformation of Microfabricated Surface Roughness by Confocal Microscopy.
Demet Tatar 1 , Jiangdong Deng 2 , Michael Aziz 1 , Frans Spaepen 1 Show Abstract
1 Harvard School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States, 2 Center for Imaging and Mesoscale Structures, Harvard University, Cambridge, Massachusetts, United States
When two rough surfaces are in contact, knowledge of the surface roughness is essential for modifying them for specific applications. There are different models and approaches in the literature for predicting the interaction between two rough surfaces in contact at the size scale of their asperities. In order to deduce the friction, wear and contact fatigue properties, it is necessary to determine the contact area, pressure distribution and the stress field change in time of a material under shear. The aim of this project is to observe the formation, deformation, and breaking of contacts with a confocal optical microscope, which has a shear-cell attachment with a micromanipulator. The instrument is to permit such imaging during the controlled sliding of two surfaces in direct contact and measure the contact area change simultaneously. To demonstrate and mimic the aspect of atomic and molecular systems in smaller scales, we propose a model surface with geometrically well-defined asperities at the micron scales made of a material with known elastic-plastic properties. As the two rough surfaces slide upon each other, the sliding force is determined, and the imaging capability of the microscope is used to evaluate the distribution of pressure and stress simultaneously. Contact between two- and three dimensional sinusoidal elastic-plastic surfaces in-contact with flat and alike surfaces of the same material are investigated under the confocal optical microscope. In order to fabricate a well-defined sinusoidal surface, photolithography was used. Since the parameters defining friction and wear properties are related to the topography of the asperities, a variety of sample with different geometrical asperities are studied, in order to vary the contact regime of the interacting surfaces.
4:00 PM - V8:Patterning II
V9: Membranes, Pillars, & Fibers
Tuesday PM, November 30, 2010
Room 205 (Hynes)
4:15 PM - V9.1
Tilting Polymeric Micropillars by Ion Beam.
Myoung-Woon Moon 1 , Tae-gon Cha 1 2 , Kwang-Ryeol Lee 1 , Ashkan Vaziri 3 , Ho-Young Kim 2 Show Abstract
1 , Korea Inst. Sci. Tech., Seoul Korea (the Republic of), 2 , Seoul Natl Univ, Seoul Korea (the Republic of), 3 , Northeastern University, Boston, Massachusetts, United States
Asymmetric adhesion is used by many insects and gecko lizards, allowing them to move on nearly any surface – horizontal, tilted or vertical. The foot of many of these creatures is covered by intricate fibrillar structures that are responsible for their superb maneuvering ability. Among these creatures, gecko lizards have one of the most efficient and interesting adhesion devices consisting of finely angled arrays of branched fibers (setae). Here, we developed a method to create tilted Janus (two-face) micropillars on the surface of an elastomeric polymer to mimic the geometry of gecko’s footpad. The method combines soft lithography to create straight micropillars and ion beam irradiation to tilt the straight micropillars in a controlled fashion. First, we fabricated straight micropillars on the surface of the polydimethylsiloxane (PDMS) using soft lithography. After fabrication of the straight pillars, Ar+ broad ion beam irradiation was used to tilt the micropillars having diameter 9.3μm, height 30μm and spacing 10μm, where spacing is defined as the distance between the edges of the adjacent micropillars. The Ar ion beam irradiation generates wrinkles on the polymer surface, as well as the side of micropillars that is exposed to ion beam. The ion beam irradiation causes surface modification of the PDMS and induces a stiff skin, which is 70-100 times stiffer than PDMS. Ion beam irradiation also causes shrinkage of the surface, resulting in a strain mismatch between the induced stiff skin and soft polymer and thus, instability of the surface skin in the form of wrinkles. Thus after the ion beam exposure, the micropillars have two completely different surface topologies and form an array of ‘uniformly tilted Janus pillars’. The effect of ion beam incident angle on the tilting angle of the micropillars and various treatment durations of ion beam was explored. A set of experiments were performed to measure the adhesion and friction characteristics of the fabricated tilted micropillars. Our experiments showed that the friction force along the tilting direction is approximately three times higher than the friction force associated with the sliding against the tilting direction of tilted micropillars due to the difference in the contact area during sliding of a glass ball. Potential applications of the created structures are vast and range from non-wetting painting and smart adhesives to bioinspired machines such as nano- and micro- robots with climbing abilities.
4:30 PM - V9.2
Experimental Determination of Driving Mechanism for Pillar Formation in Nanofilms Exposed to a Thermal Gradient.
Euan McLeod 1 , Yu Liu 1 , Sandra Troian 1 Show Abstract
1 Applied Physics, California Institute of Technology, Pasadena, California, United States
The manufacture of nanoscale devices is nowadays based on optical lithography techniques in which a geometric pattern defined by a photomask is transferred onto a photosensitive resist layer by exposure to UV light. While this technique generates feature sizes below 100 nm, it is both costly and time-consuming. Numerous steps involving film deposition, exposure and photoresist removal are necessary for constructing 3D components layer by layer. Inhomogeneities in photoresist thickness, composition, exposure dose or developer concentration also causes significant surface roughness which diminishes performance of optical or electronic devices. In order to fabricate ultrasmooth, inexpensive 3D nanostructures in a single step, researchers are exploring alternative, lower resolution patterning techniques using common polymeric materials. Our current focus involves nanofilm patterning by thermally triggered fluid instabilities, which spontaneously establish periodic arrays with a characteristic pitch and feature size. Several groups have reported formation of pillar arrays in molten polymer nanofilms confined to closely spaced and parallel substrates maintained at different temperatures. The driving mechanism is not well understand although it is generally agreed that the structures elongate in the direction of the cooler substrate. Models based on linearly unstable flow induced by variation in surface charge density at the air/polymer interface [1,2], variation in acoustic phonon radiation pressure within the film , and variation in thermocapillary stress along the air/polymer interface  have been proposed as possible driving mechanisms. To our knowledge, however, all experimental measurements of the most unstable wavelength have been conducted in the solidified state and long after the original molten structures had contacted the cooler substrate. In contrast to previous studies, we present experimental results based on direct observation of nanopillar array growth. Measurements are obtained at sufficiently early times during which predictions of linear stability analysis are valid. By investigating the dependence of the fastest growing wavelength on the substrate separation distance, temperature difference and initial film thickness, we find closest agreement with predictions of the thermocapillary model. The experiments also indicate that significant rearrangement of pillars occurs after contact with the cooler substrate. Film dewetting or capillary migration can also alter the measured wavelength. These findings will now allow optimization of the formation process to reduce both feature size and formation times, both critical to commercial implementation.  L. Zhuang, Ph.D. thesis, Princeton University, Princeton, NJ (2002) L. F. Pease and W. B. Russel, J. Chem. Phys. 125, 184716 (2006) E. Schaffer et al., Macromol. 36, 1645 (2003) M. Dietzel and S.M. Troian, Phys. Rev. Lett. 103, 074501 (2009)
4:45 PM - V9.3
Capillarity Induced Pattern Transformation in Water-swellable Polymer Membranes: Breathing to Buckling and Recovery.
Xuelian Zhu 1 , Rong Dong 1 , Ji Feng 1 , Chi-Mon Chen 1 , Shu Yang 1 Show Abstract
1 Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States
Control over the mechanical instabilities in periodic microstructures holds potential applications for tuning their photonic, phononic, transport and adhesive properties. Here, we present a systematic study of pattern transformation and its recovery in poly(2-hydroxyethyl methacrylate) (PHEMA) membranes with a square lattice of micron-sized cylindrical holes. When exposed to deionized (DI) water, the void reduced size but retained the shape, in so called breathing state. However, during the water drying process, each cylindrical hole collapsed into a slit, and the square lattice bifurcated into a diamond plate pattern with neighboring slit perpendicular to each other. Meanwhile, it was found that the diamond plate pattern was not a single crystal but had many frustration boundaries. The boundary morphology (either random or aligned) is characteristic of the form of capillary flow, which is dependent on the pinning of the contact line of the evaporating water. Upon re-exposure to DI-water, the transformed pattern could recover back to the original circular holes. This whole process could be repeated many times. Further, we utilized the dynamic Monte Carlo method to simulate the kinetic process of pattern transformation and recovery, which qualitatively matched well with the experiments. Finally, we suggest a new way of shaping the frustration boundary by modulating the nucleation sites in the pristine membrane, which will shed light on creating a single crystal of deformed patterns.
5:00 PM - V9.4
Stability of Viscoelastic Thin Films during Ion Irradiation.
Scott Norris 2 1 , Kenneth Kamrin 3 1 , Michael Brenner 1 , Michael Aziz 1 Show Abstract
2 Mathematics, Southern Methodist University, Dallas, Texas, United States, 1 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States, 3 Mechanical Engineering, Massachussets Institute of Technology, Cambridge, Massachusetts, United States
An ion beam bombarding a solid surface has been long been known to produce an instability leading to a modulated surface (with ripples or dots); though the basic mechanisms for this instability remain under considerable debate. During our investigation of this problem, we have been led to a basic problem in thin film continuum mechanics: the instability of a viscoelastic thin film that is under compressive stress. This applies to the ion bombarded problem because there is evidence that the ion beam fluidizes a thin viscoelastic layer, and that this layer is then stressed by the ion beam. By varying the ratio of the shear modulus to the viscosity, we analyze this problem and connect the known limits of a stressed elastic solid film or a surface-tension driven lubrication flow. In particular, we identify the presence or absence of a surface instability as a function of these two parameters.
5:15 PM - V9.5
Long Range Correlations and Instabilities in the Mechanics of Random Fiber Networks Under Large Deformations.
Ali Shahsavari 1 , Catalin Picu 1 Show Abstract
1 , Rensselaer Polytechnic Institute, Troy, New York, United States
Semi-flexible random fiber networks are present in biological and non-biological systems such as the cytoskeleton, tissue scaffolds and cellulose structures. In this work we show that long-range correlations exist in structural (e.g. density) and mechanical (e.g. elastic moduli) parameters of the network. All these quantities have large fluctuations with position and the magnitude of the fluctuations exhibits power law scaling when the global, system scale deformation is small. Under large deformations local instabilities set in. The interaction of these instabilities controls the global mechanical response of the network. We study the effect of the network geometry, the ratio between the axial and bending stiffness of fibers and their density on spatial correlations of instabilities and the dynamics of the network. The effect of strain (large strain) on the correlation length and therefore on the scale of homogeneity is also studied.
5:30 PM - V9.6
Symmetry Selection via a Reversible Ionic Buckling of Elastic Membranes.
Rastko Sknepnek 1 , Graziano Vernizzi 1 , Monica Olvera de la Cruz 1 Show Abstract
1 Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States
We demonstrate that an elastic shell with oppositely charged components on its surface undergoes reversible change of shape. The shape and symmetry is controlled externally by modifying the ionic conditions. We consider ionic tethered membranes with charge stoichimonetric ratios z:1, where z=1, 2,...n. At zero or low charge densities, buckling into an icosahedron results upon increasing the elastic energy or decreasing the bending rigidity of the shell. With further increasing the charge density, the shells is distorted and the symmetries are lost.
5:45 PM - V9.7
Instability Growth and Capillary Break-up during Jetting of Weakly Viscoelastic Fluids.
Vivek Sharma 1 , Arezoo Ardekani 1 , James Serdy 3 , Phil Threlfall-Holmes 2 , Gareth McKinley 1 Show Abstract
1 Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Laboratory for Manufacturing and Productivity, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 , AkzoNobel, Redcar United Kingdom
The use or processing of many multicomponent, microstructured complex fluids like paints, inks, insecticides and pesticides, cosmetics, food, etc involves surface tension driven break-up of cylindrical fluid elements into droplets during spraying, jetting and coating. These industrial fluids are typically formulated using dilute polymer solutions, and are exposed to a wide range of shear and extension rates. Since the polymer solutions and the resulting dispersions have low viscosity and short relaxation times, their non-Newtonian behavior is not apparent in the conventional rheometric measurements. However, the presence of even a dilute amount of polymer alters the character of instability growth and capillary break-up during jetting. The interplay of capillary, inertial, elastic and viscous effects on small length and time scales typically leads to complex dynamics in a necking fluid thread and in some cases, the extensional stresses generated in the neck lead to formation of very thin and stable filaments between drops, or to ‘beads-on-a-string’ structures. We use experiments and simulations to study the influence of both elasticity and extensibility on the growth of instability and capillary break-up of harmonically perturbed jets of the viscoelastic fluids. We show how and when capillary thinning analysis can be applied to capillary break-up during jetting to measure rheological response of fluids. While bead formation and extension rates are self-determined in a Capillary Breakup Extensional Rheometer (CABER) experiment, we show that it is possible to influence the dynamics of the capillary break-up during jetting by controlling the amplitude and frequency of the imposed disturbances. Finally we also touch upon the effects of adding particles to the dilute (associative) polymer solutions, which leads to formation of physical associations in solution and marked changes in the shear and extensional rheology, as well as in the instability growth and break-up of the multicomponent jet.