Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session C57: Soft Interface Mechanics IIIFocus
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Sponsoring Units: GSOFT DBIO DPOLY Chair: Katharine Jensen, Williams Coll Room: LACC 518 |
Monday, March 5, 2018 2:30PM - 3:06PM |
C57.00001: Soft Adhesion & Friction: Compliance, Hysteresis, and Swelling Invited Speaker: Douglas Holmes Friction and adhesion are ubiquitous in daily life, playing important and essential roles in many natural processes and engineered technologies. Significant recent efforts have been devoted to understanding and mimicking adhesion mechanisms found in many organisms in nature, such as geckos and insects. In addition, soft and extensible materials have been used in state-of-the-art technologies, such as wearable sensors and stretchable electronics, in which the materials can deform under relatively small amounts of load, owing to a high effective compliance resulting from either low modulus or geometric design. Therefore it is necessary to understand the role of effective axial compliance on the adhesive and frictional properties of materials, from both experimental and theoretical points of view. In this talk, we will discuss the roles that compliance, hysteresis, and wet interfaces play in controlling soft adhesion and soft friction. In particular, we will examine the role of effective axial compliance on the friction of extensible strips and on the force capacity of adhesive bonds. Beginning with the classical fracture mechanics approach, we show how the characteristic shear lag length affects the frictional response of soft materials, and changes the failure mechanism of adhesive bonds from progressive to catastrophic. We will address the role of adhesion hysteresis on the kinetic coefficient of friction of soft elastomers. Finally, we will discuss how the adhesion of a soft interface is modified in the presence of fluid that swells the material - a process that changes both the bulk and interfacial properties of the material over time. |
Monday, March 5, 2018 3:06PM - 3:18PM |
C57.00002: Direct measurement of surface elasticity of strained soft solids Qin Xu, Robert Style, Katharine Jensen, Eric Dufresne Surface stress, also known as surface tension, is a fundamental material property of any interface. However, measurements of solid surface stress in traditional engineering materials, such as metals and oxides, have proven to be very challenging. Consequently, our understanding relies heavily on untested theories. Here, we take the advantage of high compliance and large deformability of a soft polymer gel to directly measure solid surface stress as a function of strain. Under both biaxial and uniaxial stretch, we find the surface stress depends on the strain via the surface elastic constants, which, remarkably, is even larger than the zero-strain surface tension [1, 2]. Further, we put forward a surface layer model to explain the origin of measured surface elasticity and strain-dependent surface stress for soft polymer gels. These results also suggest that solid surface stress, as a strain-dependent tensor, can play a dominant role in solid mechanics at much larger length scales than previously anticipated. |
Monday, March 5, 2018 3:18PM - 3:30PM |
C57.00003: Mechanically-excited surface waves on soft gels Joshua Bostwick, Xingchen Shao, John Saylor Mechanically-excited Faraday waves appear as surface patterns on soft agarose gels. We experimentally quantify the dispersion relationship of these waves for frequencies ranging from 40Hz to 200Hz and over a range of shear modulus where the surface energy (capillarity) is comparable to the elastic energy of the solid. Rayleigh waves and capillary-gravity waves are recovered as limiting cases. Non-dispersive Rayleigh waves are observed on stiffer gels above a critical shear modulus. For our softest gels, we find that surface tension affects the dispersion relationship. Gravitational forces appear through the self-weight of the gel and are also important. We show the experimental data fits well to a proposed dispersion relationship. Our results are highly relevant to cell printing and tissue engineering technologies which utilize bioinks driven to pinchoff. |
Monday, March 5, 2018 3:30PM - 3:42PM |
C57.00004: Crack formation during drying of a shrinkable porous medium Sujit Datta, H. Jeremy Cho, Nancy Lu The drying of shrinkable porous media, like fine-grained soils or packed hydrogels, arises in many environmental and agricultural settings. In many cases, these media abruptly crack while drying, which can be detrimental to applications requiring structural integrity. We perform drying experiments on packings of shrinkable particles to characterize the mechanics underlying crack formation. We find that the interplay between particle shrinkage, capillary cohesion, and adhesion with the substrate yields a rich array of drying behaviors and cracking patterns. Our results could ultimately yield guidelines for the control of crack formation during drying. |
Monday, March 5, 2018 3:42PM - 3:54PM |
C57.00005: Interfacial and bulk effects in packed microgel yielding Tapomoy Bhattacharjee, Thomas Angelini Microgels made from crosslinked polyelectrolytes are used as a rheological modifier of numerous different products. Packed microgels are also used to study the origins of jamming and glassy behavior in soft-sphere model systems. The rheological behavior of microgels has been studied thoroughly at the macroscopic scale; at the particle-scale, jamming, yielding, and glassy-dynamics have been carefully investigated through experiment, theory, and simulation. Yielding of granular materials in general is controlled by interfacial friction between neighboring particles and by particle elasticity. However, in packed microgels, the relative contribution of friction and elastic deformation is not known. Here we show that the macroscopic yielding behaviors in packed microgels is not controlled by gel-gel friction, and instead can be predicted from the classic polyelectrolyte physics scaling laws that control single-microgel elasticity and relaxations. Specifically, we find that the yield stress is the elastic stress associated with single particle deformations during rearrangements and the shear-rate at which microgels transition from a solid to a fluidized state is controlled by the diffusive relaxation of microgel deformation during re-arrangements. |
Monday, March 5, 2018 3:54PM - 4:06PM |
C57.00006: Impact Propagation and Absorption Mechanisms in Two Dimensional Colloidal Monolayers Jinwoong Cha, Ivo Buttinoni, Wei-Hsun Lin, Stéphane Job, Chiara Daraio, Lucio Isa We report experimental and numerical studies on the propagation and absorption of localized mechanical pulses in two dimensional colloidal monolayers. The colloidal monolayers consist of hexagonal-packed and disordered arrays of SiO2 microspheres (radius R = 3.69 um) immersed in solvent with different viscosities. We triggered the localized mechanical pulses by creating an isotropic pressure wave from pulsed laser ablation (532nm, 4ns) of a single gold-coated SiO2 Janus microsphere and monitored the impact propagation at single particle level using a microscope system equipped with a high speed camera (300kHz frame rate). We modeled the dynamics of the systems considering hydrodynamic interactions, particle elasticity, and intermolecular forces and showed a good agreement with experimental observations. We will discuss the dependence of laser power, solvent viscosities, and spatial arrangement on the stress propagation. |
Monday, March 5, 2018 4:06PM - 4:18PM |
C57.00007: Viscoelastic Imaging of Living Cells Using a Noncontact Long-Needle Atomic Force Microscope Dongshi Guan, Elisabeth Charlaix, Robert Z. Qi, Penger Tong Imaging of surface topography and elasticity of living cells can provide new insights into the roles played by the cells’ volumetric and mechanical properties and their response to external forces in regulating the essential cellular events and functions. Atomic force microscopy (AFM) would be a natural means to this end, but is designed for operation in air and does not function well in a liquid biological environment. Here, we report a unique technique of noncontact viscoelastic imaging of live cells using AFM with a long-needle glass probe. Because only the probe tip is placed in a liquid medium near the cell surface, the AFM cantilever in air functions well under dual-frequency modulation, retaining its high-quality resonant modes. The probe tip interacts with the cell surface through a minute hydrodynamic flow in the nanometer-thin gap region between them without physical contact. Quantitative measurements of the cell height, volume, and Young’s modulus are conducted simultaneously. The experiment demonstrates that the long-needle AFM has a wide range of applications in the study of soft interface mechanics and mechanobiology. |
Monday, March 5, 2018 4:18PM - 4:30PM |
C57.00008: Slip of Hexadecane on Organic Friction Modifier Monolayers James Ewen, Sridhar Kannam, Daniele Dini, Billy Todd Organic friction modifiers (OFMs) are amphiphilic surfactants added to lubricants in order to reduce friction and wear. They achieve this by preventing the contact of surface asperities and perhaps also by promoting boundary slip. The structure, flow, and friction behaviour of nanometer-thick OFM films are challenging to probe directly through experiment. In this study, large-scale nonequilibrium molecular dynamics (NEMD) simulations have been used to provide unique insights into the physical origins of friction reduction by OFMs under a wide range of conditions. The OFM films are shown to be resilient to high pressures and sliding velocities, as well as nanoscale surface roughness. Both slip and friction are shown to be sensitive to the amount of interdigitation between OFM and hexadecane layers. The formation of films with a high surface coverage is shown to be critical to form smooth sliding interfaces, promote slip and ultimately reduce friction. Slip and friction trends from the NEMD simulations are consistent with experimental results. |
Monday, March 5, 2018 4:30PM - 4:42PM |
C57.00009: Precursors to Molecular Slip on Smooth Hydrophobic Surfaces Justin Pye, Clay Wood, Justin Burton Experiments and simulations suggest that simple liquids can experience slip while flowing near a smooth, solid surface. Hydrophobic surfaces are expected to enhance slip at high shear rates due to a depleted density in the liquid near the solid. Here we show how precursors to molecular slip can be observed in the complex response of a liquid to oscillatory shear. We measure both the change in frequency and bandwidth of a quartz crystal microbalance (QCM) during the growth of a single drop of water immersed in an ambient liquid. By varying the hydrophobicity of the surface using self-assembled monolayers, our results show little or no slip for water on all surfaces, yet excess transverse motion near hydrophobic surfaces due to elastic deformation of liquid molecules in local potential wells. We also show that this dynamic effect can be easily missed in simulations with finite-ranged interaction potentials. |
Monday, March 5, 2018 4:42PM - 4:54PM |
C57.00010: Melting and Structural Transitions of Interacting Particle Arrays on Soft Shells. Amit Rajnarayan Singh, Luigi Perotti, Robijn Bruinsma, Joseph Rudnick, Jeff Eldredge We report on a numerical study of the finite-temperature phase diagram of discrete particles with orientation-dependent interactions whose groundstate is a closed shell. According to continuum theory, a closed shell can undergo a buckling transition between a spherical and an icosahedral state as a function of the ratio of the stretching and bending moduli (the Föppl-von-Kármán (FvK) number). This buckling transition is reproduced by the discrete particle model. However, we find that at low temperatures the icosahedral structure is stable only at large FvK numbers. For low FvK numbers, it narrowly competes with a variety of different ordered structures, in particular when the pair-potential has a narrow width. When the temperature is increased, the various low FvK structures melt easily into a fluid state. If the temperature is raised further then the liquid state shell collapses into a 3D structure. On the other hand, the more stable high FvK icosahedral states never melt before they collapse. Our simulations emphasize the extraordinary stability of an icosahedral shell at large FvK numbers. |
Monday, March 5, 2018 4:54PM - 5:06PM |
C57.00011: Experimental study of shear-bands of drying latex Zhiyu Jiang, Hao Huang, H Daniel Ou-Yang Shear-bands are commonly seen during a latex drying even before close packing, suggesting the rate of water evaporation exceeds the rate of the local structural relaxation. The structural relaxation could be modified by the size of particles and the friction of the contact points. To verify the suggestion, we used particles with different sizes and different materials. We modified the interfacial friction by surfactant. In addition, we control the ambient vapor pressure to vary the water evaporation rate. We used optical coherence tomography (OCT) to image the detail structures of shear-bands. The results of this study help us understand how to control the dynamics of the structural relaxation the rate of water evaporation. |
Monday, March 5, 2018 5:06PM - 5:18PM |
C57.00012: Rifts in Rafts Kha-I To, Sidney Nagel Two-dimensional particle rafts are single-layers of aggregated sub-millimeter polydisperse particles floating at an air-fluid interface. The material failure of such rafts under an applied extensional load has a morphology that appears to be distinct from other known fracture modes. At low strain rates, the system fails like a ductile solid and it breaks at the point where a neck pinches off. However, at higher shear rates, numerous small-scale cracks are distributed diffusively throughout the entire system. We observe a dependence of the characteristics of this distributed failure on the surface tension and viscosity of the underlying fluid. These results indicate that the fluid plays an important role in controlling the velocity-dependent fracture. |
Monday, March 5, 2018 5:18PM - 5:30PM |
C57.00013: Nanoparticles Organization Controls Their Potency as Universal Glues for Polymer Interfaces. Nicola Molinari, Stefano Angioletti-Uberti In recent years, nanoparticles (NP) have been shown to have the potential to answer the centuries-old question of how to mechanically strengthen an interface between soft materials. The wide range of tuneable parameters and properties make glues composed of nano-sized particle appealing to a variety of fields and applications. The design of new adhesive, however, would benefit from a deeper understanding of the parameters that concur to determine the final adhesion strength. We used a coarse-grained model of polymer melts and nanoparticles, and molecular dynamics simulations to shine light on the interplay of three key properties, namely NP size, NP-polymer interaction strength, and NP density. Our main result shows a non-monotonous strengthening of the mechanical response depending on the nanoparticle concentration at the interface. Furthermore, our findings are in good qualitative agreement with a simple analytical model of the melt adhesive energy that we developed. 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. |
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