Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session X14: Focus Session: Extreme Mechanics: Elasticity and Deformation IV |
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Sponsoring Units: GSNP Chair: Narayanan Menon, University of Massachusetts, Amherst Room: D227 |
Thursday, March 24, 2011 2:30PM - 3:06PM |
X14.00001: Harnessing Instabilities in Polymers under Electric Fields Invited Speaker: Subject to a voltage, a layer of a polymer reduces thickness and expands area, so the same voltage will induce an even higher electric field. The positive feedback may cause the polymer to thin down drastically, resulting in an electrical breakdown. This electromechanical instability has been long recognized in the electrical power industry as a major failure mode for polymer insulators. In this talk, we will present recent new observations and understandings of the electromechanical instability. For example, what will happen if the polymer is bonded on a rigid substrate to prevent the area expansion? We show that a new mode of instability will set in. Once the electric field reaches a critical value, the initially flat surface suddenly folds upon itself, deforming into a pattern of creases. As the electric field further rises, the creases increase in size and decrease in density, and strikingly evolve into holes in the polymer. The critical electric field for the creasing instability scales with square root of the polymer's modulus. We show that linear stability analysis overestimates the critical electric field for the instability. A theoretical model has been developed to predict the critical field by comparing the potential energies in the creased and flat states. The theoretical prediction matches consistently with experimental results. We further show that the instability can be harnessed with promising applications in many areas including high-breakdown-field organic capacitors, electrostatic lithography, dynamic pattern formations, fabrication of semi-permeable membranes, and energy harvesting. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X14.00002: Ultrasoft Electronics for Hyperelastic Strain, Pressure, and Direct Curvature Sensing Carmel Majidi, Rebecca Kramer, Robert Wood Progress in soft robotics, wearable computing, and programmable matter demands a new class of ultrasoft electronics for tactile control, contact detection, and deformation mapping. This next generation of sensors will remain electrically functional under extreme deformation without influencing the natural mechanics of the host system. Ultrasoft strain and pressure sensing has previously been demonstrated with elastomer sheets (eg. PDMS, silicone rubber) embedded with microchannels of conductive liquid (mercury, eGaIn). Building on these efforts, we introduce a novel method for direct curvature sensing that registers the location and intensity of surface curvature. An elastomer sheet is embedded with micropatterned cavities and microchannels of conductive liquid. Bending the elastomer or placing it on a curved surface leads to a change in channel cross-section and a corresponding change in its electrical resistance. In contrast to conventional methods of curvature sensing, this approach does not depend on semi-rigid components or differential strain measurement. Direct curvature sensing completes the portfolio of sensing elements required to completely map hyperelastic deformation for future soft robotics and computing. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X14.00003: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X14.00004: Tensile shock waves in rubber Krishnaswamy Ravi-Chandar, Johnathan Niemczura We examine the propagation of waves of finite deformation in rubbers through experiments and analysis; in particular attention is focused on the propagation of one-dimensional tensile shock waves in strips of latex and nitrile rubber. Tensile wave propagation experiments were conducted at high strain-rates by holding one end fixed and displacing the other end at a constant velocity. A high-speed video camera was used to monitor the motion and to determine the evolution of strain and particle velocity in rubber strips. Shock waves have been generated under tensile impact in pre-stretched rubber strips; analysis of the response yields the tensile shock adiabat for rubbers. The propagation of shocks is analyzed by developing an analogy with the theory of detonation; it is shown that the condition for shock propagation can be determined using the Chapman-Jouguet shock condition. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X14.00005: Kinetic features of pattern transformation and recovery in periodic hydrogel membranes Xuelian Zhu, Rong Dong, Ji Feng, Chi-Mon Chen, Shu Yang Pattern transformation triggered by mechanical instabilities is an attractive bottom-up method to create complex structures over a wide range of length scales. However, how to dynamically control the transformation and its recovery is yet to be studied. Here, we present a systematic study of the kinetic pattern transformation and its recovery using a model system from poly(2-hydroxyethyl methacrylate) hydrogel membrane with a square lattice of micron-sized cylindrical holes. The hydrogel membrane undergoes (1) a breathing mode (i.e. the hole reduces size but retains the shape) when exposed to DI-water; (2) a phase transition to a diamond plate pattern driven by capillarity during drying process; and (3) a recovery upon re-exposure to water. During drying, many antiphase boundaries (APBs) appear in the diamond plate pattern, which then act as embryos that determine the kinetic path for recovery. The boundary morphology (either random or aligned) can be manipulated by the moving speed of the waterfront. To reveal the underlying mechanism of pattern transformation and APB arrangement, as well as the role of APB in recovery, we utilized the dynamic Monte Carlo method to simulate the kinetic process of pattern transformation and recovery, which qualitatively matched well with the experiments. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X14.00006: Cavitation in elastomeric solids: A defect-growth theory Oscar Lopez-Pamies, Martin Idiart, Toshio Nakamura A new theory is introduced to study the phenomenon of cavitation in soft solids that, contrary to existing approaches, simultaneously: (i) applies to large (including compressible and anisotropic) classes of nonlinear elastic solids, (ii) allows to consider general 3D loading conditions with arbitrary triaxiality, and (iii) incorporates direct information on the initial shape, spatial distribution, and mechanical properties of the underlying defects at which cavitation can initiate. The basic idea is to cast cavitation in elastomeric solids as the homogenization problem of nonlinear elastic materials containing random distributions of zero-volume cavities, or defects. In spite of the generality of the proposed approach, the relevant calculations amount to solving tractable Hamilton-Jacobi equations, in which the initial size of the cavities plays the role of ``time'' and the applied load plays the role of ``space.'' [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X14.00007: The aerodynamics of jumping rope Jeffrey Aristoff, Howard Stone We present the results of a combined theoretical and experimental investigation of the motion of a rotating string that is held at both ends (i.e. a jump rope). In particular, we determine how the surrounding fluid affects the shape of the string at high Reynolds numbers: the string bends toward the axis of rotation, thereby reducing its total drag. We derive a pair of coupled non-linear differential equations that describe the shape, the numerical solution of which compares well with asymptotic approximations and experiments. Implications for successful skipping will be discussed. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X14.00008: Helical Root Buckling: A Transient Mechanism for Stiff Interface Penetration Jesse Silverberg, Roslyn Noar, Michael Packer, Maria Harrison, Itai Cohen, Chris Henley, Sharon Gerbode Tilling in agriculture is commonly used to loosen the topmost layer of soil and promote healthy plant growth. As roots navigate this mechanically heterogeneous environment, they encounter interfaces between the compliant soil and the underlying compacted soil. Inspired by this problem, we used 3D time-lapse imaging of Medicago Truncatula plants to study root growth in two-layered transparent hydrogels. The layers are mechanically distinct; the top layer is more compliant than the bottom. We observe that the roots form a transient helical structure as they attempt to penetrate the bi-layer interface. Interpreting this phenotype as a form of buckling due to root elongation, we measured the helix size as a function of the surrounding gel modulus. Our measurements show that by twisting the root tip during growth, the helical structure recruits the surrounding medium for an enhanced penetration force allowing the plants access to the lower layer of gel. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X14.00009: Increasing Digging Efficiency Using Two Biologically-Inspired Techniques Dawn Wendell, Peko Hosoi The mechanics of digging through granular materials often neglect the inhomogeneities present in granular packings. This work reports on two biologically-inspired mechanisms that aim to increase the efficiency of digging through granular materials by taking advantage of the variety of forces found in granular packings. First, flexible diggers demonstrate that a slight increase in flexibility can lead to more efficient digging using a completely passive mechanism. Secondly, a digger with an actuated tip is investigated to find optimum parameters for energy efficient digging with actuated mechanisms. [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X14.00010: Pattern switches in granular crystals Katia Bertoldi, JongMin Shim, Fatih Goncu, Stephen Willshaw, Tom Mullin, Stefan Luding We report an experimental and numerical study of a pattern transformation in a regular array of macroscopic cylindrical particles with contrasting dimensions and stiffnesses. The initial structure is a square lattice with a pair of large (soft) and small (hard) particles at each lattice site. The application of a uniaxial compression produces a new periodic structure and the transformation principally depends on the size ratio of the particles. At small ratios it is homogeneous and approximately reversible i.e. the initial geometry is almost recovered after unloading. In contrast, when the size ratio is increased the final pattern is reached after a sudden rearrangement of the particles which involves the formation of a shear band. The structural reorganization of the granular crystal will have a significant effect on wave propagation properties and we suggest that this could have interesting applications in phononic and photonic crystals. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X14.00011: Evidence for a mechanical instability, via folding, of the vein network in leaves Pilnam Kim, Manouk Abkarian, Howard A. Stone The venation pattern of leaves is the archetype of a self-organized transport network whose efficiency and robustness stems from the connectivity of its hierarchical branching structure, but whose underlying principles of formation are not understood. Here we propose that the folding instability of the inner tissues of the leaf provides such a hierarchical venation pattern. Using a multi-layered polymeric system under an equibiaxial compressive stress, which mimics both growth and the layered structure of a leaf tissue, we show that a repetitive wrinkling-to-folding transition can achieve a hierarchical network of folds by continual, local reorganization of the stress field. We find that the resulting network topology, including closed loops, is the result of a spontaneous evolution of both terminal and segmental branching of the fold network and shares basic topological properties with venation patterns. This folding transition gives new insights into the role of mechanical stress as a possible feedback mechanism for cell differentiation in early veins. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X14.00012: Minimal resonances in annular non-Euclidean strips Bryan Chen, Christian Santangelo Differential growth processes play a prominent role in shaping leaves and biological tissues. Using both analytical and numerical calculations, we consider the shapes of closed, elastic strips which have been subjected to an inhomogeneous pattern of swelling. The stretching and bending energies of a closed strip are frustrated by compatibility constraints between the curvatures and metric of the strip. To analyze this frustration, we study the class of ``conical'' closed strips with a prescribed metric tensor on their center line. The resulting strip shapes can be classified according to their number of wrinkles and the prescribed pattern of swelling. We use this class of strips as a variational ansatz to obtain the minimal energy shapes of closed strips and find excellent agreement with the results of a numerical bead-spring model. We derive and test a surprising resonance condition for strips with minimal bending energy along the strip center line to exist. [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X14.00013: Deflation of elastic surfaces Catherine Quilliet The deflation of elastic spherical surfaces has been numerically investigated, and show very different types of deformations according the range of elastic parameters, some of them being quantitatively understood through simple theoretical considerations. In particular, the role of the Poisson ratio is closely investigated. This work allowed to retrieve various shapes observed on hollow deformable shells (from colloidal to centimeter scale), on lipid vesicles, or on some simple biological objects. Conversely, it shows how high deformations can tell observers about mechanical properties of a body. Such investigations have been extended to other geometries, in order to provide clues to understand deformations of vegetal or animal tissues. [Preview Abstract] |
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