Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session S58: Soft Interface Mechanics IIFocus
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Sponsoring Units: GSOFT DPOLY GSNP DBIO Chair: Ryan McGorty, Univ of San Diego Room: BCEC 257A |
Thursday, March 7, 2019 11:15AM - 11:27AM |
S58.00001: Phospholipid bilayer interleaflet friction from coarse-grained numerical simulations Othmène Benazieb, Fabrice Thalmann
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Thursday, March 7, 2019 11:27AM - 11:39AM |
S58.00002: Modifying lubrication in micro-patterned and polymer-grafted soft interfaces Lilian Hsiao, Yunhu Peng, Christopher Serfass Soft contacts are encountered in a variety of natural and technological applications. Two common strategies to modify interfacial friction include the deliberate micropatterning of surfaces and the grafting of charged polymer brushes onto the surfaces. Nevertheless, the connection between macroscopic friction and surface energies on textured substrates is not well understood. We synthesize model poly(dimethylsiloxane) (PDMS) substrates with microtextures and polyzwitterionic brushes, and use them as tribopairs to investigate the effect on elastohydrodynamic (EHL) and boundary lubrication (BL). Our results indicate that patterned surfaces exhibit micro-EHL to EHL transitions in which the critical friction coefficient and Sommerfeld numbers vary as a function of the surface geometry. The experimental data show excellent agreement with a simple scaling theory we developed from Reynold's equations for 1D lubrication flows. Furthermore, we investigate the BL changes in PDMS tribopairs in which betainized poly(2-dimethylamino ethyl methacrylate) (PDMAEMA) brushes are covalently grafted onto the surface. Our preliminary results suggest that the polymer relaxation physics may play an important role in the lubricating efficiency of the PDMAEMA brushes. |
Thursday, March 7, 2019 11:39AM - 11:51AM |
S58.00003: Ink Transfer in Ultrathin Flexographic Printing Using Nanoporous Stamps Dhanushkodi Mariappan, Kim Sanha, Michael S. H. Boutilier, Junjie Zhao, Hangbo Zhao, Justin Beroz, Ulrich Muecke, Hossein Sojoudi, Karen Gleason, PT Brun, John Hart Printing of ultrathin layers of liquid and colloidal inks is critical to manufacturing of low-cost electronics on non-conventional substrates such as polymer films and flexible glass. A recent invention from our research group, engineered nanoporous stamps made from polymer-coated carbon nanotube (CNT) forests, are highly porous (>90%) and can retain colloidal nanoparticle inks within their volume. Using these stamps, we have achieved printing of micron-scale features with highly uniform sub-100 nm thickness. In this work, we use high-speed imaging of the contact line motion during printing on transparent substrates to observe the dynamics of liquid spreading during contact, and the evolution of a capillary liquid bridge in the stamp-substrate gap. At high approach speeds, spreading speed increases with approach speed whereas at low approach speeds, flow from the porous medium defined by the fluid and stamp properties determine the spreading speed. After bridge rupture at a critical stamp-substrate gap, liquid respreads to fill the area defined by a precursor film matching the stamp geometry with high precision, and the respreading dynamics follow Tanner’s law. The transferred liquid volume decreases with retraction speed enabling speed based process control of layer thickness. |
Thursday, March 7, 2019 11:51AM - 12:03PM |
S58.00004: Effects of Bidisperse Wettability on Interfacial Viscoelasticity of Particle Laden Interfaces Syed Ehsanur Rahman, Gordon Christopher Increasing use of Pickering Emulsions has led to increasing study of interfacial particles. It is known that particle contact angle determines emulsion type and modifies interfacial viscoelasticity, effecting emulsion stability and viscosity. Most studies of particle interfaces examine a mono-population of colloids. However, in applications, there can be significant distribution in contact angle. What role such distribution plays on interfacial mechanical properties is unclear. |
Thursday, March 7, 2019 12:03PM - 12:15PM |
S58.00005: Particle assembly on an evolving interface Benjamin Druecke, Xiang Cheng, Sungyon Lee The behavior of particles on fluid-fluid interfaces has a history extending back to Pickering. Much of the work on these particle rafts treats the fluid as a quasi-static interface and investigates the behavior of the particles on this passive interface. In this work, we investigate the behavior of particle rafts on dynamically evolving interfaces such that there is a strong coupling between the behavior of particles on the interface and the underlying hydrodynamics of the evolving interface. As a prototypical system, we consider a water-oil interface inside a funnel, such that the interface deforms as the water level is varied. We examine the behavior of a variety of particles on such an interface to elucidate the coupling between bulk and interfacial phenomena. |
Thursday, March 7, 2019 12:15PM - 12:27PM |
S58.00006: A Computational Method to Study Packing on Deformable Shells Sanjay Dharmavaram, Luigi E Perotti Many problems in softmatter and membrane biophysics, such as studying equilibrium configurations of protein clusters on cell-membranes, highly defect ridden structures of Gag polyproteins in immature HIV capsids, and the unusual fluid-like state of Archaeal viruses [1] can all be analyzed as systems of interacting particles (typically representing proteins or protein capsomers) on deformable fluid surfaces. The coupled interactions between the particles and the underlying elastic medium to which they are constrained pose significant computational challenges. Existing methods often employ expensive constraints to anchor particles to the surface or artificially restrict their movement yielding spurious equilibrium states. In this talk, we present a new approach that circumvents existing challenges to obtain reliable equilibria. We apply the method to study the generalized Thomson problem of packing interacting particles on a deformable shell. In this context, we investigate particle symmetries, surface tessellation, and surface shape as the fluid membrane is progressively covered by interacting particles. |
Thursday, March 7, 2019 12:27PM - 12:39PM |
S58.00007: Rigid Bubbles: Novel Instabilities in Colloidal Film Rupture Phalguni Shah, Srishti Arora, Michelle R Driscoll When a soap bubble pops, a rupture opens up and grows on the timescale of milliseconds. Culick (1960) showed that this rupture grows at a constant rate. Recently, Petit et al. (2015) studied films that were rigidified due to their high surfactant concentration. They observed that these films developed crack-like instabilities during rupture, and their rupture velocity was slower than that predicted by Culick. We investigate whether soap films rigidified by adding colloidal spheres show similar instabilities. We rupture a flat film containing surfactant and colloidal spheres using a needle and record it with a high-speed camera at 75,000 frames per second. We control film rigidity by varying particle concentration and find that this dramatically alters rupture dynamics. The rupture opens at a rate that is non-constant and an order of magnitude slower than the Culick velocity. Additionally, we observe a wide variety of instabilities in these rupturing colloidal films. We systematically study film rupture dynamics as a function of colloid concentration and film thickness. |
Thursday, March 7, 2019 12:39PM - 12:51PM |
S58.00008: Thermally assisted buckling of colloidal assemblies Simon Stuij, Jan-Maarten van Doorn, Tom Kodger, Joris Sprakel, Corentin Coulais, Peter Schall We investigate the effect of thermal fluctuations on the buckling instability of colloidal assemblies. Using laser tweezers to grab the ends of, and push on colloidal chains assembled with critical Casimir forces, we probe the buckling instability analogous to the Euler buckling of macroscopic beams, in the presence of thermal activation. We find that buckling fluctuations diverge upon approaching the buckling point, while their time scale diverges similar to critical slowing down. Molecular dynamic simulations allow quantitative extraction of the corresponding critical exponents, which we identify as the mean-field critical exponents. Using an analytically solvable minimal model, we elucidate the origin of these exponents. The surprisingly rich physics of this simple thermal mechanical system highlights the interplay of thermal fluctuations and elasticity in the buckling stability of micron-scale architectures such as biomaterials. |
Thursday, March 7, 2019 12:51PM - 1:03PM |
S58.00009: Memory effects in the attachment of thin films to liquid surfaces Deepak Kumar, Thomas Russell, Benjamin Davidovitch, Narayanan Menon We study the dynamics of depositing or peeling off a thin polymer film at an air-water surface. In our experiment, we control the velocity of the film withdrawal or attachment while measuring the force required to do so. The observed dynamics is hysteretic, wherein the force depends on the direction of motion even at the lowest attainable velocities, but the magnitude of the hysteresis loop appears to be independent of the velocity. When the motion of the film is halted, the relaxation to equilibrium configuration is extremely slow, and appears to be logarithmic in time. Quite remarkably, the relaxation dynamics retains the memory of velocity history. This behavior, involving hysteresis, slow relaxation and memory effects are hallmarks of glassy systems which have access to a large number of relaxation modes with a broad distribution of relaxation times. It is surprising that our simple system exhibits some of these features. |
Thursday, March 7, 2019 1:03PM - 1:15PM |
S58.00010: Motion of water droplets on oil infused surfaces Solomon Adera, Lilian Magermans, Mughees Khan, Joanna Aizenberg Controlling the motion of water droplets on solid surfaces has broad technological implications ranging from microfluidics to thermal management. Past approaches utilized topography and chemical composition gradients to manipulate the motion of droplets. Here we show that the motion of droplets on oil infused surfaces can be triggered via asymmetry of the wetting ridge. Our experimental results show that neighboring droplets exert force on each other through their wetting ridge. This interaction causes droplets to coalesce and self propel by releasing free surface energy. Importantly, our experiments show that the motion of water droplets on slippery oil infused surfaces is not random as has been hypothesized in past studies. Instead, the oleoplaning droplets select their path such that the force required to overcome viscous dissipation is minimum. Consequently, droplets move away from oil depleted regions where the lubrication oil thickness is small and the resistance to motion is large. This “sensing” ability of droplets to the thickness of lubrication oil enables them to maneuver by avoiding prior oil depleted paths. This study provides new mechanistic insight into the motion of water droplets on oil infused surfaces. |
Thursday, March 7, 2019 1:15PM - 1:27PM |
S58.00011: Two-wavelength Wrinkling Patterns in Chiral Liquid Crystal Surfaces Pardis Rofouie, Ziheng Wang, Alejandro Rey We present a model to investigate the formation of two-length scale surface patterns in biological and synthetic anisotropic soft matter through the high order interaction of anisotropic interfacial tension and capillarity at their free surfaces. Focusing on the chiral liquid crystal(CLC) material model, the generalized shape equation for anisotropic interfaces using the quartic anchoring energy is applied to understand the formation of two-length scale patterns, such as those found in floral petals. Analytical and numerical solutions are used to shed light on why and how simple anisotropic anchoring generates two-lengthscale wrinkles whose amplitudes are given in terms of anchoring coefficients. The proposed new nano-wrinkling mechanisms augment previous models dedicated to understand and mimic biological surface pattern formation. Symmetry relations and scaling laws are used to provide the explicit relations between the anchoring constants and surface profile of the two length scale wrinkles. These new findings establish a new paradigm for characterizing surface wrinkling in biological liquid crystals, and inspire the design of novel functional surface structures. |
Thursday, March 7, 2019 1:27PM - 1:39PM |
S58.00012: Sculpting high aspect ratio particles from oil-in-water emulsions Mathew Giso, Haoda Zhao, Patrick T Spicer, Tim Atherton Shape is a powerful route to control the transport and rheological properties of particulate media. Here, we present a process to sculpt elongated colloidal particles from an oil-in-water emulsion. Oil droplets are crystallized by reducing the temperature. The rate of cooling determines the rate of crystallization. With the addition of surfactants, it is possible to induce dewetting of the crystals by their own liquid phase. By tuning the relative rates of dewetting and crystallization, a rich variety of crystal shapes can be grown in an easily scalable process using controlled interfacial hydrodynamics. We explain these experimental findings using a non-equilibrium Monte Carlo model that captures both the crystallization and dewetting processes. Our results reproduce the wide range of shapes seen in experiment and provide insights to control their final morphology. Prospects for expanding the space of final shapes will also be discussed. |
Thursday, March 7, 2019 1:39PM - 1:51PM |
S58.00013: Defect screening in faceted emulsion droplets. Ireth García-Aguilar, Piermarco Fonda, Luca Giomi, Eli Sloutskin Oil emulsion droplets in water have been observed to spontaneously deform into polyhedral shapes at temperatures where the surfactant interface freezes while the bulk oil and water remain liquid. The interface monolayer crystalizes into a hexagonal lattice, which is topologically constrained to accomodate a certain number of defects, namely disclinations. Additional defects, called dislocations, are also expected to be found in crystals with large number of particles. Dislocations are not only thermally induced but, more importantly, they are known to screen the large stresses around disclinations by forming chain-like structures at their vicinity. We address the problem of faceting droplets by studying the interplay between the interface geometry and the arragement of dislocations. We have found that the coupling between the distribution of crystal defects, surface curvature and entropy is key to understand the temperature and size-dependent behaviour of the shape transformations. |
Thursday, March 7, 2019 1:51PM - 2:03PM |
S58.00014: Statistical Mechanics of Puckered Membranes Abigail Plummer, David R Nelson A triangular lattice of points connected by springs that resists bending and stretching provides a discrete model of a thin elastic plate. We consider such a surface with a superlattice of `impurities'-- sites that have longer springs connecting them to the rest of the lattice. In the continuum limit, this corresponds to a preferred metric with periodic dilations. These impurities tend to pucker either above or below the lattice. We regard this as an Ising-like degree of freedom, and characterize interactions between neighboring puckers. We find we can tune these puckered membranes from a `ferromagnetic' state to an `antiferromagnetic' state using elastic constants and superlattice structure, and investigate these states theoretically and numerically. |
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