Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session R58: Soft Interface Mechanics IFocus
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Sponsoring Units: GSOFT DPOLY GSNP DBIO Chair: Katharine Jensen, Williams College Room: BCEC 257A |
Thursday, March 7, 2019 8:00AM - 8:12AM |
R58.00001: Influence of Elastic Modulus of a Soft Elastomer on Adhesion to Rough Surfaces Siddhesh Dalvi, Abhijeet Gujrati, Ali N Dhinojwala, Tevis Jacobs, Lars Pastewka In the field of adhesion science, it is of practical importance to understand how roughness affects adhesion. This is of direct relevance in designing adhesives for applications in the areas of microelectronics, biomedical devices, and composites. The main idea is to have the adhesives conform to the roughness and approach close enough for van der Waals (or other electrostatic) interactions to be effective. The first complete mathematical formulation for roughness was put forward by Greenwood and Williamson using a single-length-scale model describing the surfaces as hemispherical asperities with a Gaussian distribution of heights. Bo Persson has proposed a multi-scale model to calculate the adhesion energy as a function of modulus and roughness, as described using the power spectral density of topography. Here we have performed contact and fracture mechanics experiments for smooth cross-linked polydimethylsiloxane with varying elastic modulus on polycrystalline diamond surfaces with varying roughness, which have been characterized to obtain accurate multi-scale power spectral densities. We will present the comparison between the predicted adhesion energies calculated using Persson’s theory and the experimental adhesion energy measured during approach and separation. |
Thursday, March 7, 2019 8:12AM - 8:24AM |
R58.00002: Gluing polymer interfaces with nanoparticles: insights from molecular dynamics Nicola Molinari, Boris Kozinsky, Stefano Angioletti-Uberti In recent years, nanoparticles have been shown to have the potential to answer the centuries-old question of how to mechanically strengthen an interface between soft materials. A deeper understanding of the mechanism and parameters resulting in the mechanical reinforcement, or lack thereof, would help rationalize experimental findings and, ultimately, guide the design of adhesive solutions. Building on our previous work on polymer in bad solvent conditions [DOI: 10.1021/acs.nanolett.8b00586], we investigate the trends, and the fundamental drivers of, the mechanical properties of polymer interfaces glued with spherical nanoparticles. Using a coarse-grained model, we simulate polymer network structures with different degrees of swelling. The interface between two networks is filled with nanoparticles, and the overall system is then uni-axially strained to construct the stress-strain curve for a given set of parameters. The results from this investigation help disentangling the contributions to the strengthening of the interface and can be used as a guide to experimental design of nanoparticles-based adhesives. |
Thursday, March 7, 2019 8:24AM - 8:36AM |
R58.00003: Ultra-soft textured substrates Martin Coux, John Kolinski Droplets deposited on soft elastomers or gels with elastic moduli ~ 1 – 10 kPa induce a micron-scale elastic deformation at the contact line. This significantly modifies the wetting properties of the solid - for example, a droplet deposited on an inclined soft surface will slide downward driven by its weight at velocities orders of magnitude less than a droplet on a similarly inclined rigid solid. Here, we confine the contact between water and a soft silicone gel to micrometric patches by texturing the solid, inducing superhydrophobicity for materials with elastic moduli as low as 3 kPa. The coupling between surface texture and surface compliance strongly influences the behavior of the droplet at both macro- and micro- scales, leading to a giant speed up during descent, and potentially affecting the stability of the Cassie state. |
Thursday, March 7, 2019 8:36AM - 8:48AM |
R58.00004: Hydrogel menisci: Shape and interaction Anupam Pandey, Charlotte Nawijn, Jacco Snoeijer The interface of a soft hydrogel is easily deformed when it is in contact with particles, |
Thursday, March 7, 2019 8:48AM - 9:00AM |
R58.00005: Equilibration time, swelling, and the tangential contact-line force on a partially immersed gel thread Shih-Yuan Chen, Aaron R Bardall, Michael Shearer, Karen Daniels When a liquid contacts a sufficiently soft material, the surface tension of the liquid deforms the substrate at the contact line, forming a ridge. The height of this ridge is on the order of γ/E, known as the elastocapillary length. The precise stress boundary condition at the contact line has a significant effect on this mechanism, but is as yet unresolved. A complicating factor is that liquids also interact with gels by swelling and/or contamination from the uncrosslinked oil in the gel. Hence, it is crucial to distinguish these different mechanisms when conducting an elastocapillary experiment. In our talk, we present experiments with a poly(vinyl siloxane) gel thread partially immersed in liquids of different wettability and swellability. We show that when making stress-strain measurements via fluorescent bead microscopy, the positions of the beads only equilibrate after 12 to 24 hrs. This timescale is incompatible with the use of ethanol as the immersion fluid, since it swells the PVS thread; we find that using FC-40 allows the thread to reach equilibrium without swelling. Once these two effects are resolved, the evidence for a tangential force at the contact line is weak. |
Thursday, March 7, 2019 9:00AM - 9:12AM |
R58.00006: ABSTRACT WITHDRAWN
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Thursday, March 7, 2019 9:12AM - 9:24AM |
R58.00007: Adhesion-Based Measurements of Strain-Dependent Surface Stress in Soft Solids Jeremy Thaller, Minwoo Kang, Katharine Jensen Surface stress, or surface tension, is a fundamental material property of interfaces. However, our understanding of solid surface tension and how it changes with deformation remains limited because it is difficult to measure in traditional stiff materials. Soft materials, including gels, provide a unique opportunity to measure these fundamental properties because they can be stretched elastically to very large strains. In this work, we use an adhesion-based approach to measure the solid surface stress of compliant materials as a function of applied strain. This approach is applicable to a wide range of soft materials. |
Thursday, March 7, 2019 9:24AM - 9:36AM |
R58.00008: Interfacially Driven Plastic Deformation of Soft Solids Christopher O'Bryan, Thomas Angelini Capillary forces acting on the surface of a solid result in elastic deformation on the scale of the elastocapillary length. While these elastic deformations may be negligible in stiff materials, they can dominate the behavior of soft solids, including rubbers, hydrogels, and biological tissues. In the extreme case, capillary forces may exceed the material’s yield stress, resulting in plastic deformation near the contact line. Although this plastocapillary effect has been theorized, experimental exploration into this phenomenon have remained limited. Understanding the role of interfacial forces on the deformation of soft surfaces is necessary as new methods for manufacturing soft materials with low moduli and yield stresses are developed. Here, we investigate interfacial instabilities of 3D printed microbeams, using jammed granular microgels swollen in aqueous and organic solvents as both sacrificial support materials and as printed inks. By leveraging the highly tunable material properties of these microgel systems and the structural control offered by 3D printing, we systematically explore a range of yield stresses and beam widths to understand how plastocapillary effects can drive mechanical instabilities and material failure in soft solids. |
Thursday, March 7, 2019 9:36AM - 9:48AM |
R58.00009: Poking a peach: Is testing for ripeness confounded by the skin? Dominic Vella, Finn Box, Mokhtar Adda-Bedia To test whether soft fruit is ripe, one often pokes with a finger. However, the flesh of the fruit itself is significantly softer than when the skin remains intact. In this talk we will address the question of when the indentation stiffness of a soft material coated by a thin, but significantly stiffer, material is affected by the presence of the coating. We demonstrate a substrate-dominated regime and a mixed regime (in which both coating and substrate are important) and show that the size of the indenter is a key parameter separating the two. |
Thursday, March 7, 2019 9:48AM - 10:24AM |
R58.00010: Direct observations of dewetting relaxation at the interface of soft gels Invited Speaker: Qin Xu Wetting of liquid droplets on soft gels (E~3KPa) deforms gel interface significantly: a wetting ridge grows under the balance between liquid surface tension and solid surface stress near contact line. Using interference microscopy and fast camera imaging, we directly observe the recovery of gel surface from this wetting-induced deformation after removing liquid droplet. We show experimentally that surface relaxation of soft gels cannot be simply modelled by their viscoelastic rheology. Instead, the relaxation behaviour is a consequence of interplay between different restoring stresses near the interface, including solid capillarity and elasticity, and resisting stresses from the bulk, such as osmotic pressure across the network and local viscous stresses. By measuring scaling of wetting shapes under relaxation, we can determine which stress dominates the dissipation. Our results further extend the current understanding of elastocapillarity with the effect of bulk porosity, and bring new insight into the study of wetting dynamics on soft materials. |
Thursday, March 7, 2019 10:24AM - 10:36AM |
R58.00011: Surface waves and wakes on soft solids Robert Haussman, Aditi Chakrabarti, L Mahadevan Understanding the response of soft matter surfaces to dynamical disturbances is critical for a variety of applications, such as ultrasound and needleless injections. Motivated by this, here we explore the dynamical response of a very large, very soft solid due to impulsive and moving pressure disturbances on its surface. In particular, we describe the conditions under which Kelvin wake-like patterns should appear and derive perturbative corrections due to soft elasticity about the well-known inviscid limit, and describe the results of experiments to probe this limit. |
Thursday, March 7, 2019 10:36AM - 10:48AM |
R58.00012: Measuring the dispersion relation of capillary waves using differential dynamic microscopy Jing Wang, Ryan J. McGorty With a recently-developed method for measuring capillary wave dynamics at the interface of fluids we examine non-equilibrium interfacial fluctuations. We use a colloidal-polymer mixture that separates into colloid-rich and colloid-poor fluid phases with an ultra-low surface tension and capillary velocities on the order of a micron per second. We use differential dynamic microscopy (DDM) to measure the two-dimensional dispersion relation of capillary waves at spatial frequencies spanning over an order of magnitude. Using temperature-responsive colloidal particles (pNIPAM) to tune the phase diagram we investigate the interfacial fluctuations between non-equilibrium phases. |
Thursday, March 7, 2019 10:48AM - 11:00AM |
R58.00013: How suction cups fail Chon U Chan, Michelle Walsh, L Mahadevan Suction cups occur in nature and technology, and are widely used in a range of situations across many length scales. They often cannot afford to fail owing to catastrophic consequences associated with examples such as lifting large glass sheets, and in manipulating delicate tissues and organisms. But how do they fail? We report an experimental study that captures the failure dynamics of soft suction cups that involves an interplay of elastic deformation, viscous lubrication flows and interfacial instabilities over time scales that range from milliseconds to hours. Our study allows us to propose a design to prolong the holding time till failure. |
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