Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session T28: Focus Session: Shells, Plates, and Thin Films |
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Sponsoring Units: GSNP Chair: Katia Bertoldi, Harvard University Room: 336 |
Thursday, March 21, 2013 8:00AM - 8:12AM |
T28.00001: Buckling in 2D periodic, soft and porous structures: effect of pore shape and lattice pattern Sicong Shan, Katia Bertoldi, Jongmin Shim, Johannes T.B. Overvelde, Sung Hoon Kang Adaptive structures allowing dramatic shape changes offer unique opportunities for the design of responsive and reconfigurable devices. Traditional morphing and foldable structures with stiff structural members and mechanical joints remains a challenge in manufacturing at small length scales. Soft structures where the folding mechanisms are induced by a mechanical instability represent a new class of novel adaptive materials which can be easily manufactured over a wide range of length scales. More specifically, soft porous structures with deliberately designed patterns can significantly change their architecture in response to diverse stimuli, opening avenues for reconfigurable devices that change their shapes to respond to their environment. While so far only two-dimensional periodic porous structures with circular holes arranged on a square or triangular lattice have been investigated, here we investigate both numerically and experimentally the effects of pore shape and lattice pattern on the macroscopic properties of the structures. Our results show that both the pore shape and lattice pattern can be used to effectively design desired materials and pave the way for the development of a new class of soft, active and reconfigurable devices over a wide range of length scales. [Preview Abstract] |
Thursday, March 21, 2013 8:12AM - 8:24AM |
T28.00002: Localization in thin shells under indentation Alice Nasto, Amin Adjari, Arnaud Lazarus, Ashkan Vaziri, Pedro Reis We perform a hybrid experimental and numerical investigation of deformation in thin spherical elastic shells under indentation. Past the initial linear response, an inverted cap develops as a Pegorelov circular ridge. For further indentation, this ridge loses axis-symmetry and sharp points of localized curvature form, which we refer to as 's-cones' (for shell-cones), in contrast with their developable cousins in plates, 'd-cones'. We quantify how the formation and evolution of s-cones is affected by systematically varying the indenter's curvature. In our precision model experiments, rapid prototyping is used to fabricate elastomeric shells and rigid indenters of various shapes. The mechanical response is quantified through load-displacement comparison tests and the deformation process is further characterized through digital imaging. In parallel, the experimental results are contrasted against nonlinear Finite Element simulations, which enable us to explore the role of friction at the shell-indenter contacts and characterize the relative strain energy focusing properties at different loci of localization. Our combined experimental and computational approach allows us to gain invaluable physical insight towards rationalizing this geometrically nonlinear process. [Preview Abstract] |
Thursday, March 21, 2013 8:24AM - 9:00AM |
T28.00003: Eggstreme Mechanics of Thin Shells Invited Speaker: Pedro Reis I will present a series of experimental explorations on the rich mechanical behavior of thin elastic shells, subject to different forms of loading. First, I will discuss the geometry-induced rigidity of non-spherical pressurized shells under indentation, that can be used for non-destructive testing. I will proceed by characterizing the emergence and evolution of point and linear-like loci of localization on thin shells indented well into the nonlinear regime. I will then present a new mechanism that utilizes the compression of a thin-shell/soft-core system for switchable and tunable wrinkling on curved surfaces, that can be exploited for active aerodynamic drag control. Finally, I shall introduce the framework for buckling-induced folding (or ``Buckligami'') that involves functional structural transformations of patterned shells that can be excited to achieve encapsulation, flexure and twist. The main common feature underlying these series of examples is the prominence of geometry in dictating the complex mechanical behavior of slender soft structures, thereby making our results relevant and applicable over a wide range of length scales. Moreover, our findings suggest that we rethink our relationship with mechanical instabilities which, rather than modes of failure, can be embraced as opportunities for functionality that are scalable, reversible, and robust. [Preview Abstract] |
Thursday, March 21, 2013 9:00AM - 9:12AM |
T28.00004: Buckling Instability of Dielectric Elastomeric Plates for Soft, Bio-Compatible Microfluidic Pumps Behrouz Tavakol, Michael Bozlar, Guillaume Froehlicher, Christian Punckt, Howard A. Stone, Ilhan Aksay, Douglas Holmes Dielectric elastomers are well-known for their superior stretchability and permittivity. A fully-clamped thin elastomer will buckle when it is compressed by applying sufficient electric potentials to its sides. When embedded within soft, silicone rubbers, these advanced materials can provide a means for a bio-compatible pumping mechanism that can be used to inject bio-fluids with desired flow rates into microfluidic devices, tissues, and organs of interest. We have incorporated a dielectric film that is sandwiched between two thin, flexible, solid electrodes into a microfluidic device and utilized a voltage-induced out-of-plane buckling instability for pumping of fluids. We experimentally quantify the voltage-induced plate buckling and measure the fluid flow rate when the structure is embedded in a microchannel. Additionally, we offer an analytical prediction that uses plate buckling theory to estimate the flow rate as a function of applied voltage. [Preview Abstract] |
Thursday, March 21, 2013 9:12AM - 9:24AM |
T28.00005: Ordering in a crumpled elastic sheet Anne Dominique Cambou, Narayanan Menon We experimentally study the conformations of polydimethylsiloxane (PDMS) sheets crumpled in a cylinder at volume fractions ranging from 3{\%} to 40{\%}. The PDMS sheets show no plasticity, and slide with low friction as they are immersed in an index-matching fluid to allow imaging in the interior. We crumple the sheet either axially with a piston, or radially by shrinking the radius of the cylinder. We focus on the development of local nematic order created by facets stacking together. Either the flat piston or the curved cylindrical wall promotes global alignment of these stacks. We compare our results to previous experiments on aluminum foil confined in a sphere to understand the role of plasticity and friction on the ordering in crumpled sheets. [Preview Abstract] |
Thursday, March 21, 2013 9:24AM - 9:36AM |
T28.00006: Whirling Skirts James Hanna, Jemal Guven, Martin Michael M{\"{u}}ller Steady wave patterns may be observed on a rotating skirt. These patterns display a well-defined dihedral symmetry and are marked by strikingly sharp features. We capture these with a minimal model of traveling waves on an inextensible, flexible, rotating generalized-conical sheet. Conservation laws associated with the dynamics are used to reduce the Euler-Lagrange equations to a quadrature describing a particle in a potential. Analytical solutions are obtained; these are quantized by the extrinsic closure of the skirt. Coriolis forces play an essential role in establishing these configurations. [Preview Abstract] |
Thursday, March 21, 2013 9:36AM - 9:48AM |
T28.00007: Mechanical response of creases network in thin sheets Benjamin Thiria, Fr\'ed\'eric Lechenault, Mokthar Adda-Bedia In a recent study [Thiria \& Adda-Bedia, PRL, 2011], it has been shown that the local plastic zone (crease) created during thin-film folding exhibits a logarithmic mechanical response typical to aging. It was found that the related relaxation processes could be described by an Arrhenius law with a typical time scale intrinsic to the material. Here we present an extension to this study by adding collective behaviors and topology (or geometry) to the system . The systems considered consist in in-line series of folds and origami-like 2D patterns. We present the global behavior and mechanical properties (aging, rigidity) of multi-folded thin sheets as a function of the experimental parameters (material, thickness, fold preparation and geometrical characteristic). [Preview Abstract] |
Thursday, March 21, 2013 9:48AM - 10:00AM |
T28.00008: Folding of thin film on a highly pre-strained elastomer Atsushi Takei, Hiroyuki Fujita Multi-layered systems composed of a rigid thin film and an elastomeric base are ubiquitous in Nature and technology. When the rigid thin film is deposited on the stretched elastomeric base, periodical patterns appear on its surface in releasing the pre-strain. If the pre-strain is small ($\sim$ 10\%), sinusoidal wavy patterns appear entirely on the surface as known in literature. On the other hand, with the large pre-strain ($\sim$ 50\%), the deformation is localized, and foldings are engendered. We studied this phenomenon experimentally using a balloon structure composed of a PDMS chamber and a thin organic membrane Parylene. Firstly the chamber is filled with oil and inflated like a balloon. Then, keeping its pressure, the organic membrane is deposited on the surface. By changing the pressure inside the chamber during the deposition, the pre-strain can be ranged over 50\%. In this study, we demonstrate the pre-strain dependency on the morphology in one dimensional and two dimensional models. We also present that with the balloon structure the surface roughness can be tuned by changing the pressure and that it can be applied to a tunable hydrophobic surface. [Preview Abstract] |
Thursday, March 21, 2013 10:00AM - 10:12AM |
T28.00009: Experiments with a particle film: Evidence for force chain buckling Andrew B. Croll, Bekele J. Gurmessa, David Carey, Antoinette Tordesillas Granular materials are a unique state of matter that, when loaded, focus stress on a small subset of their total volume. Accurate modeling of the regions of high stress, the force chains, is critical to understanding the overall material behaviour. Progress in modeling the transition from static to fluid has recently been made by considering the onset of the transition as originating with the buckling and failure of a force chain. There is currently little direct experimental evidence for such behaviour. Here we use a simplified model system in which a set of solid particles, packed into a monolayer, is adhered to a soft substrate and compressed. We observe buckling and the emergence of a single dominant lengthscale, much in analogy to the well known ``wrinkling'' instability of a continuum plate. However, several tests show the behaviour observed in our system to be uniquely granular in nature. Finally, we show how many features of our experiment are in agreement with recent predictions of the force chain buckling model. [Preview Abstract] |
Thursday, March 21, 2013 10:12AM - 10:24AM |
T28.00010: Mechanics without Muscles: Fast Motion of the Venus flytrap and Bio-inspired Robotics Zi Chen, Qiaohang Guo, Huang Zheng, Wei Li, Yiting Ding, Guiping Su, Junjie Lin, Yuxin Liu, Wenzhe Chen, Larry Taber The rapid motion of plants has intrigued scientists for centuries. Plants have neither nerves nor muscles, yet the Venus flytrap can move in a fraction of a second to capture insects. Darwin did a first systematic study on the trap closure mechanism, and called this plant ``one of the most wonderful in the world''. Several physical mechanisms have since been proposed, such as the rapid loss of turgor pressure, an irreversible acid-induced wall loosening mechanism, and tsnap-through instability, but no unanimous agreement is reached. We propose a coupled mechanical bistable mechanism that explains the rapid closure of the Venus flytrap, consistent with experimental observations. Such bistable behaviors are theoretically modeled and validated with experiments. Biomimetic flytrap robots are also fabricated according to the learnt principles. It is thus promising to design smart bio-mimetic materials and devices with snapping mechanisms as sensors, actuators, artificial muscles and biomedical devices. [Preview Abstract] |
Thursday, March 21, 2013 10:24AM - 10:36AM |
T28.00011: Auto-origami with liquid crystal elastomers: a simulation study Andrew Konya, Robin Selinger Liquid crystal elastomers (LCE) undergo shape transformations induced by stimuli such as heating/cooling or illumination. When a non-uniform director field is imposed on a sample during crosslinking, it encodes a complex actuation trajectory which may include a combination of bends, twists, and folds along with changes in Gaussian curvature. Taking a materials-by-design approach, we perform finite element simulations to explore director geometries which produce such auto-origami behavior. By cataloging and assembling a variety of basic motifs including those identified by Modes and Warner [1], we design director geometries that yield a variety of target structures. Assembling a sample with domains of two LCE materials with different isotropic-nematic transition temperatures provides a means for sequencing steps in the resulting actuation choreography on heating/cooling. [1] CD Modes and M Warner, Phys. Rev. E84, 021711 (2011) [Preview Abstract] |
Thursday, March 21, 2013 10:36AM - 10:48AM |
T28.00012: Mechanics of morphogenesis during cell sheet movements Glenn Edwards, Heng Lu, Adam Sokolow, Dan Kiehart We have been investigating the mechanics of dorsal closure, a stage of \textit{Drosophila} embryogenesis. Over 2-3 hours a ``hole'' in the dorsal surface changes its 2-D geometry from an ellipse to an eye shape, which eventually closes edge to edge. This hole initially is filled with a monolayer of amnioserosa cells, a transient tissue under tension. Beyond the dorsal hole are two flanks of epithelial tissue, also under tension, which are zipped together at each ``corner of the eye.'' The net result of dorsal closure is to form a continuous epithelium on the outer surface of the embryo. High-resolution, \textit{in vivo} images of amnioserosa cells will be presented. Experimental time series of apical shape changes have been assessed with the methods of signal analysis to quantify a band of reversible oscillations and a set of ingression processes. A generalized-force model was formulated to account for changes in cross-sectional areas. High-resolution, 3-D images of dorsal closure also will be presented. The amnioserosa was observed to bulge outwards, where the asymmetric dome was analyzed with Laplace's formula to quantify the turgor pressure. The 3-D zipping process includes substantial remodeling of tissue interfaces and significant intracellular remodeling. [Preview Abstract] |
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