Memory Effect of Surfaces: From Wetting to Electrification

The <i>“Memory Effect”</i> is often a phenomenon attributed to bulk materials, such as shape memory alloys or polymers. The resulting behaviors are often macroscopic and highly mechanical. When considering surfaces and interfaces, responses are often much more subtle in nature. Nonetheless, re-orientation of surface moieties such as oligomers, molecular groups, ions, and electrons can lead to macroscopically-detectable responses. In this work, we use water drops as the stimuli and the ambient environment (r.t.p.) for recovering the equilibrium condition.<br/><br/>In the first instance, we used a polydimethylsiloxane surface with minute amounts of uncrosslinked oligomers. Upon drop-wetting, the surface experiences increased lubricity due to the water-induced surface saturation of oligomeric moieties which decreases the contact angle hysteresis (and thus adhesion). The surface recovers its pristine behavior upon environmental equilibration. We model this contact memory behavior behind the use of first-order adaptation dynamics and demonstrate how they deviate from both traditional molecular kinetic and hydrodynamic theories (which do not account for adaptation).<br/><br/>In the second instance, we synthesized a multi-layered/composited surface. Firstly, a base hydrophobic layer (perfluoroalkylated or alkylated) is fabricated. Upon drop-wetting, it acquires a positive charge while the surface is negatively counter-charged. To dynamically flip the charge polarity (<i>i.e.</i> polarity reversal), the hydrophobic layer is then coated with a long-chained (C₁₀) multi-amine. The presence of the amine results in a redox response (ion transfer with -NH₂ as an H⁺ acceptor) during drop-wetting. The long alkyl chain hinders molecular mobility, enabling a gradual real-time observable polarity reversal (within a few seconds). Therefore, depending on the drop-contact interval, the magnitude, and polarity of charge responses are now tunable, highlighting contact-memory behaviors. The surface recovers its pristine behavior upon environmental equilibration.<br/><br/><u><b>References</b></u><br/><br/><i>Langmuir</i> <b>2020</b>, 36, 26, 7236–7245<br/><i>Langmuir</i> <b>2022</b>, 38, 19, 6224–6230