Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session E15: Extreme Mechanics |
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Sponsoring Units: GSNP Chair: Bryan Chen, University of Massachusetts Amherst Room: 274 |
Tuesday, March 14, 2017 8:00AM - 8:12AM |
E15.00001: Thermal stiffening of 2D elastic ribbons Duanduan Wan, Mark Bowick, David Nelson We use molecular dynamics to study the vibration of a thermally fluctuating 2D atomically-thin elastic membrane clamped at both ends. We identify the eigenmodes from peaks in the frequency domain of the time-dependent height and track the dependence of the eigenfrequency of a given mode on the bending rigidity of the membrane, taking care also of the subtle effects of thermal contraction in generating a tension. We find that the effective bending rigidity tends to a constant as the bare bending rigidity vanishes, supporting theoretical arguments that the macroscopic bending rigidity of the membrane as a whole arises from a strong renormalization of the microscopic bending rigidity. In the situation of one end clamped, we observe three phases: a crumpled phase, a phase of vibrating about the z$=$0 plane where the membrane initially lies, and a spontaneous symmetry breaking phase of vibrating about a tilt plane in the upper or lower half space. [Preview Abstract] |
Tuesday, March 14, 2017 8:12AM - 8:24AM |
E15.00002: Flutter-Limited Reconfiguration of a Flat Plate Bending in a Fluid Flow Frederick Gosselin, Fabien Sansas, Aviral Prakash, Awan Bhati, Eric Laurendeau Plants rely on their flexibility to change form and reduce their drag when subjected to fluid flow. Flexibility allows plants to reconfigure and reduce their drag, however it is well known that flexibility can also lead to a loss of stability and thus increased dynamical loads. Fluttering flags are a good example. In the present work, we consider the limitation to reconfiguration brought by a dynamic loss of stability in constant uniform flow. To understand the trade-off that flexibility brings to real plants in terms of drag reduction and loss of stability, we study an idealised two-dimensional system: a beam clamped at its centre and subjected to a normal flow. We combine wind tunnel experiments and numerical simulations to study how the beam bends in the flow statically when the flow velocity is increased until a critical value is reached and the beam starts fluttering. We observe the competition between reconfiguration and flutter in flat plates in a wind tunnel. We also adopt a computational approach coupling an ALE finite volume aerodynamics code to a finite difference solution of the large deformation beam equation. We find that for a lighter structure in a heavier fluid, the critical velocity is higher and more reconfiguration is possible without reaching an instability. [Preview Abstract] |
Tuesday, March 14, 2017 8:24AM - 8:36AM |
E15.00003: Over-damped elastic `snap-through' Michael Gomez, Derek E. Moulton, Dominic Vella Elastic `snap-through' occurs when a system is in an equilibrium state that either disappears or becomes unstable as a control parameter varies. The switch from one state to another is generally rapid and hence is used to generate fast motions in biology and engineering. While the conditions under which simple elastic objects undergo snap-through have been reasonably well studied, how fast snapping happens is much less well understood. Recently, it has been shown that snap-through can be subject to critical slowing down near the snapping transition, so that the dynamics may be slow even in the absence of viscous damping. Here, we study the interaction of snap-through with the flow of a viscous fluid. We begin by showing how snap-through may be used to create a channel whose hydraulic conductivity changes discontinuously in response to fluid flow. We then study the dynamics of snap-through for an elastic element embedded in a viscous fluid, which is typical of pull-in instabilities in micro-electromechanical systems (MEMS). [Preview Abstract] |
Tuesday, March 14, 2017 8:36AM - 8:48AM |
E15.00004: Crumpling in densely perforated membranes David Yllanes, Mark Bowick An outstanding problem in the statistical mechanics of two-dimensional membranes is a predicted crumpling transition when the bending stiffness is of the order of $kT$ (i.e., at a temperature much higher than the experimentally accessible regime). We propose a mechanism to tune this transition by modifying the bending stiffness of a graphene sheet through geometry. We have carried out extensive molecular dynamics simulations of perforated sheets with a dense array of holes and observed that the transition can be tuned by the fraction of removed area. The dependence of the transition point on the removed area is very strong but not sensitive to the particular arrangement of the holes. In addition, we have found that anisotropic arrays of holes produce two transition temperatures. The lower transition temperature corresponds to crumpling in only one dimension, along the easier axis, before the sheet crumples completely. The first anisotropic crumpling occurs at a significantly lower temperature and, therefore, adjusting the degree of anisotropy in the perforations may help bridge the gap to the experimentally accessible regime. [Preview Abstract] |
Tuesday, March 14, 2017 8:48AM - 9:00AM |
E15.00005: Wrinkling of floating monoatomic graphene sheets Herve Elettro, Francisco Melo Graphene is a carbon-based honeycomb structure only one atom thick that combines exceptional thermal, electrical, optical and mechanical properties. Whereas conventional bulk and thin film materials have been studied extensively, the key mechanical behavior of 2D materials (cracking, folding) are barely explored, mainly due to complexity of manipulation. Reaching quantitative understanding of these phenomena will prove valuable to the production of high-quality graphene at industrial scale, applicable in a wide range of technologies such as wearable bio-sensors and supercapacitors. In that state of mind, we investigate the complex behavior of graphene under compression and bending in a free-floating configuration. This adaptative support allows study of graphene intrinsic properties both at large and local scales. We have optimized preparation protocols for production of few defects mm scale floating samples. We use capillary confinement and micromechanical indentation to induce wrinkling, folding and tearing of monoatomic graphene sheets. Graphene samples are characterized by high-resolution optical microscopy combined with confocal Raman analysis to assess its physical quality and monoatomic thinness. Our results show exciting insights into the unique mechanics of 2D membranes. [Preview Abstract] |
Tuesday, March 14, 2017 9:00AM - 9:12AM |
E15.00006: hyperbolic tearing path in brittle sheets Benoit Roman, Alejandro Ibarra, Francisco Melo Thin sheets are prone to bend out-of-plane when they are torn. Although non-linear plate elasticity is very diffcult to combine with fracture mechanics, experiments show that the fracture trajectory is very robust in brittle thin sheets, with oscillating, converging or spiral geometry. Here we show how simple arguments can be used to explain the fracture trajectory, considering anisotropic properties of the material. [Preview Abstract] |
Tuesday, March 14, 2017 9:12AM - 9:24AM |
E15.00007: Capillarity-induced phase separation in silicone elastomers and its consequence in droplet sliding dynamics Aurelie Hourlier-Fargette, Arnaud Antkowiak, Sebastien Neukirch Beyond the importance of understanding the motion of droplets on stiff surfaces, the recent development of soft materials has lead to a growing interest for capillarity problems where soft interfaces and supports come into play. Silicone elastomers are easy-to-make substrates, used in various research fields such as microfluidics, or elastocapillarity where experiments on slender bendable structures or thick soft substrates are performed. Here we focus on the dynamics of water-glycerol mixture droplets sliding down plates of such silicone elastomers, highlighting an unexpected behavior: we observe successively two sliding regimes with different constant speeds, and a sharp transition between them. We show that this behavior is due to the water droplet extracting uncrosslinked oligomers from the silicone elastomer through a capillarity-induced phase separation at the triple line. We further investigate the dynamics of this phase separation and its consequences on the wetting properties of the system. [Preview Abstract] |
Tuesday, March 14, 2017 9:24AM - 9:36AM |
E15.00008: Animating Soft Matter with the Elastic Leidenfrost Effect Scott Waitukaitis, Martin van Hecke, Anton Souslov, Corentin Coulais, Antal Zuiderduin Liquid droplets near hot surfaces don't boil, but instead float on a cushion of vapor created beneath them. This is the Leidenfrost Effect, and while it is well-studied for liquids and even hard solids such as dry ice, nothing is known about the behavior of soft solids under such conditions. I will show how this leads to a new phenomenon: the Elastic Leidenfrost Effect. By dropping hydrogel spheres onto a hot substrate, we observe not hovering, but instead sustained bouncing dynamics accompanied by violent screeching. With a variety of experimental techniques, I will show that the underlying physics of both the bouncing and the screeching relies on the coupling between vaporization and elastic deformation. Beyond the Leidenfrost Effect, this phenomenon unearths the broader concept of coupling activiation to deformation in soft materials and promises to impact fields ranging from granular physics and active matter to microfluidics and metamaterials. [Preview Abstract] |
Tuesday, March 14, 2017 9:36AM - 9:48AM |
E15.00009: Bifurcations of anisotropic rods and strips. Tian Yu, James Hanna Elastic strips with end constraints on position and orientation have a rich landscape of equilibria. This is a simple system in which complex results emerge from the competition between twist and writhe induced by global constraints. We compare experiments on strips of several widths with solutions of anisotropic Kirchhoff rod equations obtained by numerical continuation, and find that the latter naive model captures much of the bifurcation behavior of the real strips. [Preview Abstract] |
Tuesday, March 14, 2017 9:48AM - 10:00AM |
E15.00010: Ribbons of Infinite Length Gabriela Jaramillo, Shankar Venkataramani In recent years there has been a growing interest in the different ribbon configurations obtained after subjecting thin strips of acetate to tension and twist. In the work of Chopin J. et all, the case of a ribbon with clamped edges is studied and a phase diagram is obtained with a plethora of possible shapes ranging from helicoids to ribbon crystals. A natural question is then to ask if the boundary conditions promote some of the shapes seen in this diagram. To tackle this question we isolate the effects of the clamped edges by considering infinitely long ribbons. Our analysis suggests that in this case the preferred shape will be a spiral (or cylindrical) configuration. Similar results have been found in recent experiments involving ribbons subjected to similar loading conditions, but which are clamped only on a small region near the center line. [Preview Abstract] |
Tuesday, March 14, 2017 10:00AM - 10:12AM |
E15.00011: Elasticity and Fluctuations of Incompatible Nanoribbons Doron Grossman, Eran Sharon, Haim Diamant Geometrically incompatible ribbons are ubiquitous in nature, from the growing of biological tissues, to self assemblies of peptides and lipids. These exhibit unusual characteristics such shape bifurcations, and abnormal mechanical properties. When considering nano and micro ribbons, thermal fluctuations convert these properties into nontrivial statistics. We derive a reduced quasi-one-dimensional theory, which describes a wide range of incompatible elastic ribbons, and can be integrated into statistical mechanics formalism. Using it, we compute equilibrium configurations and statistical properties of two types of incompatible ribbons, with experimental significance: ribbons with positive spontaneous curvature, and ribbons with negative spontaneous curvature. The former, above a critical width, has a continuous family of degenerate configurations. In turn this causes the ribbons to behave as a random coils. The latter, however, exhibits a twisted-to-helical transition at a critical width, and behaves as an abnormal coil. It's persistence length is non-monotonic in the ribbon width and vanishes at a critical width, with principal modes of deformation different than compatible ribbons. Measurements of twisted ribbons made of chiral peptides, confirm some predictions of the model. [Preview Abstract] |
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