Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session Z10: Invited Session: Elastic Instabilities and Pattern Formation in Structureless Solids |
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Sponsoring Units: DFD DPOLY GSNP Chair: Benny Davidovitch, University of Massachusetts Amherst Room: 309 |
Friday, March 22, 2013 11:15AM - 11:51AM |
Z10.00001: Coarsening of patterns from scale free instabilities in soft solids Invited Speaker: Evan Hohlfeld Soft materials such as rubbery solids have hidden, scale-free instabilities that are undetectable by linearized analysis, yet which have no energy barrier for onset. Examples include the nucleation of sharply creased surface folds resembling the sulci on the brain and the nucleation and growth of cavities. These instabilities can be understood as quasi-phase transitions: they have well defined binodal points, form via a nucleation and growth process, and have finite energies of transformation; however, there is no clear phase boundary dividing the ``nucleated phase'' from the surrounding elastomer. First anticipated by Weierstrass more than 100 years ago, our understanding of these instabilities---so called ``Weierstrass needles''---is now rapidly developing as an increasing number of physical examples are being identified. Recent experimental and theoretical work has continued to deepen the analogy between a Weierstrass needle and a more traditional phase transition. Along this line, I will present new results showing how the coarsening of a crease pattern can be understood as a form of Ostwald ripening. I will also discuss classes of systems which might support other examples of Weierstrass needles. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:27PM |
Z10.00002: Instabilities in axisymmetrically constrained sheets Invited Speaker: Jos\'e Bico We propose to describe three different situations where a circular sheet is submitted to axisymmetric loads resulting from capillary forces or constrained boundary conditions. In a first case, a thin annulus floating on water is radially compressed by a surface pressure induced by the addition of surfactant molecules outside the annulus. As a consequence the annulus is compressed in the orthoradial direction and wrinkles are observed beyond a critical load. In a second situation, a planar disk is deposited on an adhesive sphere. Can the sheet accommodate the change in gaussian curvature? Wrinkles actually appear at the edge of the disk if the diameter exceeds a critical value. A third experiment finally involves a planar disk squeezed in a spherical mold. While low confinement induces the formation of localized folds, these folds eventually evolve into a cascade of orthoradial wrinkles. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 1:03PM |
Z10.00003: The generation of stress-focusing features in confined elastic sheets Invited Speaker: Robert Schroll Crumpling is the canonical example of stress focusing in a confined elastic sheet. Subject to a large biaxial confinement, the sheet must bend in multiple directions, which induces Gaussian curvature and therefore strain. This strain is best accommodated by focusing the stress into small regions. In a crumpled sheet, multiple stress-focusing features appear apparently randomly. Here, I present two systems in which stress-focusing features are created in a controlled manner. In the first, a thin sheet is floated on a droplet of water. As the curvature of the droplet is increased, first wrinkles and then a focused features appear on the edge of the sheet. In the second, a focused feature appears at the transition between wrinkle patters of two different wavelengths. The degree of the focusing can be controlled by the confinement, the thickness, and the tension applied transverse to the confinement. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:39PM |
Z10.00004: Compression-triggered instabilities of multi-layer systems: From thin elastic membranes to lipid bilayers on flexible substrates Invited Speaker: Howard A. Stone Instabilities are triggered when elastic materials are subjected to compression. We explore new features of two distinct systems of this type. First, we describe a two-layer polymeric system under biaxial compressive stress, which exhibits a repetitive wrinkle-to-fold transition that subsequently generates a hierarchical network of folds during reorganization of the stress field. The folds delineate individual domains, and each domain subdivides into smaller ones over multiple generations. By modifying the boundary conditions and geometry, we demonstrate control over the final network morphology. Some analogies to the venation pattern of leaves are indicated. Second, motivated by the confined configurations common to cells, which are wrapped in lipid bilayer membranes, we study a lipid bilayer, coupled to an elastic sheet, and demonstrate that, upon straining, the confined lipid membrane is able to passively regulate its area. In particular, by stretching the elastic support, the bilayer laterally expands without rupture by fusing adhered lipid vesicles; upon compression, lipid tubes grow out of the membrane plane, thus reducing its area. These transformations are reversible, as we show using cycles of expansion and compression, and closely reproduce membrane processes found in cells during area regulation. The two distinct systems illustrate the influence of the substrate on finite amplitude shape changes, for which we describe the time-dependent shape evolution as the stress relaxes. This talk describes joint research with Manouk Abkarian, Marino Arroyo, Pilnam Kim, Mohammad Rahimi and Margarita Staykova. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 2:15PM |
Z10.00005: Electromechanical instability in soft materials: Theory, experiments and applications Invited Speaker: Zhigang Suo Subject to a voltage, a membrane of a dielectric elastomer reduces thickness and expands area, possibly straining over 100\%. The phenomenon is being developed as transducers for broad applications, including soft robots, adaptive optics, Braille displays, and electric generators. The behavior of dielectric elastomers is closely tied to electromechanical instability. This instability may limit the performance of devices, and may also be used to achieve giant actuation strains. This talk reviews the theory of dielectric elastomers, coupling large deformation and electric potential. The theory is developed within the framework of continuum mechanics and thermodynamics. The theory attempts to answer commonly asked questions. How do mechanics and electrostatics work together to generate large deformation? How efficiently can a material convert energy from one form to another? How do molecular processes affect macroscopic behavior? The theory is used to describe electromechanical instability, and is related to recent experiments. [Preview Abstract] |
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