Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Y21: Elastic Instabilities and Pattern Formation |
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Sponsoring Units: DPOLY GSNP DFD Chair: Andrew Croll, North Dakota State University Room: 406 |
Friday, March 7, 2014 8:00AM - 8:12AM |
Y21.00001: Combined Bending, Stretching, and Wrinkling of Thin Sheets Katia Bertoldi, Michael Taylor, Benny Davidovitch Thin elastic sheets develop surface undulations, or wrinkles, in the presence of small compressive strain. In recent years, interest in thin sheets has greatly increased due to their relevance in a wide array of applications such as biological tissues, integrated circuits and solar sails. As a result, wrinkling has recently attracted considerable attention among engineers, physicists and biologists. Although the most basic buckling instability of uniaxially compressed plates was understood by Euler more than two centuries ago, recent experiments and simulations have shown significant deviations from predictions. Motivated by this puzzle we investigate wrinkling in a thin sheet under axisymmetric loading conditions and systematically compare numerical and analytical solutions. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y21.00002: The role of substrate pre-stretch on post-wrinkling bifurcations Anesia Auguste, Lihua Jin, Zhigang Suo, Ryan Hayward Wrinkles in compressed elastic bilayers, resulting from a balance between the bending energy of a stiff skin layer and the stretching energy of a softer substrate, have been applied in a variety of contexts including to change the wetting, optical, and adhesive properties of surfaces. Previous work has shown that at large compression, wrinkles transition into sub-harmonic modes and eventually form ridges or self-contacting folds due to the non-linearity of the substrate elasticity. However, our understanding of how pre-stretch of the substrate affects period doubling and other post-wrinkling bifurcations remains incomplete. We have performed a combined experimental and numerical study wherein the strain state in each layer can be independently varied. We find shifts in the critical strain for post-wrinkling bifurcation and, at high substrate pre-compression, the emergence of ``chaotic'' patterns with irregular spacings between the troughs that grow in amplitude. Our findings highlight the critical importance of substrate pre-stretch in determining the nature of post-wrinkling bifurcation modes. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y21.00003: Viscoelastic instability and detachment folds in soft elastomer interfaces Koushik Viswanathan, Anirban Mahato, Srinivasan Chandrasekar Physical contacts between a soft elastomer surface and a hard glassy polymer are largely governed by adhesion at the interface. Under application of sufficiently large tangential stress, relative motion occurs at the interface and compressive and tensile stresses develop at the leading and trailing edges of the contact respectively. This kinematic condition leads to a viscoelastic instability at the leading edge causing the elastomer surface to buckle and readhere --- a process governed both by the viscoelastic relaxation time of the elastomer and the sliding velocity $v$. Above a critical velocity $v_c$, detachment folds form ahead of the indenter and propagate through the contact region at velocities much greater than $v$. These are commonly referred to as Schallamach waves, after their discoverer, and are considered to be precursors to failure in soft materials. While their onset can be justified using linear elasticity, not much is known about their subsequent propagation. We present high-speed images of a glass-PDMS contact, and use front-tracking to estimate surface strains and the variation of wave velocity and generation frequency with $v$. A model for the dynamics of the wavefronts is also discussed, showing the onset and propagation of the instability. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y21.00004: Shape transitions in soft spheres regulated by elasticity Craig Fogle, Amy Rowat, Alex Levine, Joseph Rudnick Soft core shell structures abound in nature. Examples of these structures, comprised of a thin outer membrane bounding an elastic core, include raisins, gel-filled vesicles, and a variety of membrane-bound organelles in the cell. We study the elasticity-driven morphological transitions of spherical core shell structures when either their surface area is increased or their interior volume is decreased. We demonstrate a transition, which is related to the Euler buckling, from the spherical initial shape to a lower symmetry one. We discuss the dependence of the critical excess surface area (relative to that of a bounding sphere) for buckling, the internal stresses in the core, and the symmetry of the buckled state on the elastic parameters of the system. We compare these predictions to a variety of observed morphological transitions in hard and soft materials, and discuss extensions of this work to growing viscoelastic media. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y21.00005: Elastic instabilities in a model cerebral cortex David Mayett, Oksana Manyuhina, J.M. Schwarz Soft and biological systems exhibit elastic instabilities, such as buckling, folding and wrinkling, in the presence of external loads, growth, or both. The modeling of such systems calls for a continuum approach to account for the interplay between local elastic stresses and global growth profiles. It is this interplay that can lead to non-trivial geometries. We propose a model of the cerebral cortex, described as an anisotropic multi-layered material with two basic components (white matter and grey matter) undergoing differential growth. We explore the nature of buckling instabilities, assuming a compatibility between the growth and geometric deformation, by solving a nonlinear variational problem with a free interface. We expect that this simplified approach, based on a combination of geometry and elasticity, could give insight into the formation and splitting of folds observed during the development of the cerebral cortex. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y21.00006: Interaction of two cracks in plastic sheets Stephane Santucci, Marie-Julie Dalbe, Juha Koivisto, Loic Vanel, Osvanny Ramos, Mikko Alava We study experimentally the interaction of two cracks in a plastic sheet submitted to uniaxial stress at a constant imposed velocity. We can observe a repulsive regime, where the cracks deviate with an angle, which depends on the geometrical parameters as well as on the material studied. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y21.00007: Hierarchically UVO patterned elastomeric and thermoplastic structures Ying Chen, Manish Kulkarni, Allan Marshall, Alamgir Karim We demonstrate a simple yet versatile method to fabricate tunable hierarchical micro-nanostructures on flexible Poly(dimethylsiloxane) (PDMS) elastomer and thermoplastic polymer surface by a two-step process. Nanoscale patterned PDMS was obtained by imprinting compact disc (CD)/digital video disc (DVD) patterns. The second micro pattern was superposed by selective densification of PDMS by exposing to ultraviolet-ozone radiation (UVO) through micro-patterned TEM grid as a mask. The nanoscale patterns are preserved through UVO exposure step leading to formation of deep hierarchical patterns, so that for a 19 um square mesh, the micro pattern has a depth of 600nm with 6h PDMS UVO exposure time. This simple method can be promoted to fabricate hierarchical structures of thermoplastic materials (such as polystyrene), from which the mechanism of capillary imprinting and thermal stability of hierarchical patterns are investigated. This study is potentially important to various applications ranging from biomimetic scaffolds to solar cell. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y21.00008: Axi-symmetric patterns of active polar filaments on spherical and composite surfaces Pragya Srivastava, Madan Rao Experiments performed on Fission Yeast cells of cylindrical and spherical shapes, rod-shaped bacteria and reconstituted cylindrical liposomes suggest the influence of cell geometry on patterning of cortical actin. A theoretical model based on active hydrodynamic description of cortical actin that includes curvature-orientation coupling predicts spontaneous formation of acto-myosin rings, cables and nodes on cylindrical and spherical geometries [P. Srivastava et al, PRL {\bf 110}, 168104(2013)]. Stability and dynamics of these patterns is also affected by the cellular shape and has been observed in experiments performed on Fission Yeast cells of spherical shape. Motivated by this, we study the stability and dynamics of axi-symmetric patterns of active polar filaments on the surfaces of spherical, saddle shaped and conical geometry and classify the stable steady state patterns on these surfaces. Based on the analysis of the fluorescence images of Myosin-II during ring slippage we propose a simple mechanical model for ring-sliding based on force balance and make quantitative comparison with the experiments performed on Fission Yeast cells. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y21.00009: Tuning Surface Wettability Using Single Layered and Hierarchically Ordered Arrays of Spherical Colloidal Particles Ali Dhinojwala, Ila Badge, Sarang Bhawalkar, Li Jia A control over wetting properties of a surface can be achieved by tuning surface roughness and surface chemistry. In this study, we formed single level and dual hierarchical roughness with hexagonal non-contiguously close packed (HNCP) patterns of spherical particles using colloidal lithography. Surface chemistry was controlled using plasma-enhanced chemical vapour deposition (PECVD). A hexagonal unit cell model, which is representative of the HNCP pattern, was used to predict the contact angles. The predictions of this model were in good agreement with experimentally measured contact angles. The systematic thermodynamic analysis of wetting properties is important when using structured surfaces at different hydrostatic pressures, relative humidity, temperature fluctuations or prolonged exposure to water. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y21.00010: ABSTRACT WITHDRAWN |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y21.00011: Shape Programming through Hierarchic Crystallization of Semicrystalline Elastomers Qiaoxi Li, Jing Zhou, Sara Turner, Valerie Ashby, Jan-Michael Carrillo, Andrey Dobrynin, Sergei Sheiko Hierarchic organization of semi-crystalline morphology has proved to be key to encoding different shapes at different stages of the crystallization process. We have studied shape transformations as a new tool to gain insights of a crystallization process and then translated the hierarchic crystallization into programmable shape transformations. Reversible transitions between multiple shapes has been achieved through partial melting of a crystalline scaffold, leaving a latent template, which inverts shape recovery by steering crystallization along kinetically preferred pathways replicating the scaffold. A composite model has been applied to interpret the relationship between shape, elastic modulus and crystallinity of semi-crystalline elastomers, assuming morphological transition between isolated crystallites, clusters, and percolated scaffold. [Preview Abstract] |
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