Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session T49: Focus Session: Reconfiguring and Actuating Soft Matter III: Shape |
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Sponsoring Units: GSOFT Chair: Karen Daniels, North Carolina State University Room: 217D |
Thursday, March 5, 2015 11:15AM - 11:27AM |
T49.00001: Differential Geometry Approach to Liquid-Crystal Elastomers and Glasses Jonathan Selinger, Thanh-Son Nguyen We present a theory for liquid-crystal elastomers and glasses using the language of differential geometry. The basic physics behind this approach is the Warner-Terentjev trace formula, supplemented by their semi-soft elastic term. By expressing the theory using differential geometry, we obtain several new insights. First, we can see that the theory of liquid-crystal elastomers is analogous to recent research on swellable gel sheets, which develop complex 3D shapes in response to programmable swelling patterns. Second, we can model the distinction between liquid-crystal elastomers and liquid-crystal glasses, depending on how strongly the director is locked into an orientation determined by the elastic distortion. Third, for soft elasticity, we can analyze the set of degenerate configurations in ideal elastomers, and see how the degeneracy is broken by the non-ideal semi-soft elasticity. Finally, for elastomers with blueprinted director configurations, we can assess which configurations remain stress-free as they distort under a temperature change. [Preview Abstract] |
Thursday, March 5, 2015 11:27AM - 11:39AM |
T49.00002: Wrinkling in thin nematic elastomers Madison Krieger, Marcelo Dias, Thomas Powers Coupling a nematic liquid crystal to a rubbery matrix can create a variety of surprising elastic effects. We derive a F\"{o}ppl-von K\'{a}rm\'{a}n type model for a very thin nematic elastomer, based on a phenomenological free energy. The model couples the order parameter to the elastic degrees of freedom of the plate. There are two categories of cross-linking which we explore: cross-linking in the isotropic phase, and cross-linking in the nematic phase. We consider the problem of wrinkling in scenarios of annular and rectangular sheets with tensile boundary conditions, where confinement causes a classical elastomer to buckle out-of-plane into a wrinkled configuration. [Preview Abstract] |
Thursday, March 5, 2015 11:39AM - 11:51AM |
T49.00003: Shape evolution in blueprinted liquid crystal polymer films: topological defects and artificial iris Andrew Konya, Robin Selinger Blueprinted nematic polymer networks are programmable solids with a non-uniform director field imposed at crosslinking. The pattern encodes a shape evolution trajectory triggered by heat or any stimulus that changes the magnitude of nematic order, inducing folding, curling, etc., a form of auto-origami [1]. We present 3D finite element simulation studies of blueprinted films with high order topological defects of positive or negative charge to study resulting actuation, and compare to experiments by White et al [2].~ Inspired by recent experiments by Zentel et al [3], we also model an artificial iris as a disk with a $+$1 defect and a circular hole. We consider a blueprinted director pattern that is purely radial, purely azimuthal, or at an intermediate angle, and model resulting actuation.~ Depending on the geometry, the iris may undergo expansion, contraction, and/or rotation. Interestingly, we find one geometry that actuates via pure rotation. We discuss potential applications in optics and robotics. Implementation of our 3-d finite element algorithm for NVIDIA CUDA-enabled GPU provides high speed performance. [1] de Haan et al DOI:10.1002/adfm.201302568 [2] ME McConney et al, DOI:10.1002/adma.201301891 [3] S Schuhladen et al, DOI:10.1002/adma.201402878 [Preview Abstract] |
Thursday, March 5, 2015 11:51AM - 12:03PM |
T49.00004: Thickness Effects on Piezoelectric Unimorphs under Various Boundary Conditions Taewoo Ha, Nanshu Lu Piezoelectric unimorphs are made of laminating a piezoelectric layer with an inert flexible substrate. We have developed a comprehensive understanding of the effects of unimorph thickness ratios on mechanical load induced voltages, charges, and energies, as well as voltage induced displacements of eight different boundary conditions, with both analytical and numerical means. By adopting an average $\sigma_{33}$ stress method, the effect of $d_{33}$ for both point and distributed loads is taken care of and our analytical equations are able to capture the FEM results which are obtained by COMSOL Piezoelectric Devices Module. Non-monotonic voltage and energy generation versus thickness ratio curves have been found for load-controlled energy generation scenarios. When the unimorph is actuated by voltage, non-uniform maximum deflection versus thickness ratio curves are also found. Our results reveal that the optimum thickness ratios for actuation are an order higher than the optimum thickness ratios for energy generation. In conclusion, closed-form analytical solutions are available for the thickness optimization of piezoelectric unimorphs depending on the application and boundary conditions. [Preview Abstract] |
Thursday, March 5, 2015 12:03PM - 12:15PM |
T49.00005: A Three-Dimensional Phase Diagram of Growth-Induced Surface Instabilities Qiming Wang, Xuanhe Zhao A variety of fascinating morphological patterns arise on surfaces of growing, developing or aging tissues, organs and microorganism colonies. These patterns can be classified into creases, wrinkles, folds, period-doubles, ridges and delaminated-buckles according to their distinctive topographical characteristics. One universal mechanism for the pattern formation has been long believed to be the mismatch strains between biological layers with different expanding or shrinking rates, which induce mechanical instabilities. However, a general model that accounts for the formation and evolution of these various surface-instability patterns still does not exist. Here, we take biological structures at their current states as thermodynamic systems, treat each instability-pattern as a thermodynamic phase, and construct a unified phase diagram that can quantitatively predict various types of growth-induced surface instabilities. We further validate the phase diagram with our experiments on surface instabilities induced by mismatch strains as well as the reported data on growth-induced instabilities in various biological systems. It is expected that the unified phase diagram will not only advance the understanding of biological morphogenesis, but also significantly facilitate the design of new materials and structures by rationally harnessing surface instabilities. [Preview Abstract] |
Thursday, March 5, 2015 12:15PM - 12:27PM |
T49.00006: Mechanical Responses of a Polymer Graphene-sheet Nano-sandwich Xiguang Li, Juliusz Warzywoda, Gregory McKenna The interfacial mechanics and reinforcement by graphene sheets in polymer matrix nanocomposites are important to their understanding. However, the methods available for their investigation remain a challenge. Here we report on a novel study in which the mechanical responses of a nano-sandwich model structure made of a single graphene sheet sandwiched between ultrathin polymer layers are determined using a nano-bubble inflation method. The stress-strain behavior of the graphene nano-sandwich shows that significant reinforcement is obtained at small strains and that the method also provides a measurement of the interfacial shear strength. In addition, the study provides data related to internal stresses that develop between the graphene layer and the polymer sandwich faces. [Preview Abstract] |
Thursday, March 5, 2015 12:27PM - 1:03PM |
T49.00007: Reconfiguring and Actuating Liquid Metals, Gels, and Polymers Invited Speaker: Michael Dickey This talk will describe efforts in our research group to control the shape and function of soft materials (liquid metals, polymers and hydrogels) for applications that include reconfigurable electronics, soft robots, and self-folding origami. The research harnesses interfacial phenomena, micro fabrication, patterning, and thin films. The talk with discuss the underlying fundamental science that enables the following: \begin{enumerate} \item Shape reconfigurable liquid metal alloys based on gallium. The metal is a liquid at room-temperature with low-viscosity (water-like) and can be micromolded due to a thin, oxide skin that forms rapidly on its surface. The metal can be patterned in a number of ways including injection into microchannels or by direct-write 3D printing. Recently, we discovered that the oxide may be the best surfactant ever reported and can be removed or deposited using electrochemistry in electrolyte as a new method to control and reconfigure the shape of the metal. \item Self-folding polymers sheets that change shape in response to light. These sheets are a form of shape memory polymers that are compatible with 2D patterning techniques including lithography, inkjet printing, and roll to roll processing. The appeal of this work is converting 2D patterns into 3D shapes in a hands free manner. \item New methods for actuating hydrogels by patterning ions in these gels. This reversible process can imprint topography in the hydrogel using modest voltages, tune its local mechanical properties to create physically-reinforcing exoskeletons, and generate stresses sufficient to actuate or fold hydrogels over large distances within seconds. \end{enumerate} [Preview Abstract] |
Thursday, March 5, 2015 1:03PM - 1:15PM |
T49.00008: Osmotic robotics: Reversible shape change driven by local osmolarity gradients Tao Zhang, Duanduan Wan, J. M. Schwarz, Mark J. Bowick Nature provides us with many examples of biomaterials that can change their shape in response to external stimuli and/or in response to varying internal stresses embedded within the structure of the biomaterial. Inspired by such biomaterials as well as recent experiments, we consider a three-dimensional network of aqueous droplets joined by single lipid bilayers to form a cohesive, tissue-like material. The droplets in these droplet networks can be programmed with different osmolarities. These osmolarity gradients generate internal stresses via local flows and the network then folds into designed structures. Using molecular dynamics simulations, we study the formation of shapes ranging from rings to spirals to tetrahedra and determine the optimal range of parameters for such structures. We also add an osmotic interaction with a dynamic environment, i.e. external stimuli, to realize a reversible folding-unfolding process. This finding contributes towards the development of osmotic robotics in bio-inspired materials. [Preview Abstract] |
Thursday, March 5, 2015 1:15PM - 1:27PM |
T49.00009: Free-Standing Temperature-Sensitive Hydrogel-Particle Membranes from Evaporating Drops Tim Still, Peter Yunker, Kevin Aptowicz, Kasey Hanson, Zoey Davidson, Matthew Lohr, A.G. Yodh We demonstrate a simple method using evaporating colloidal drops to prepare temperature-sensitive membranes composed of micron-sized colloidal hydrogel particles that are up to a few particle diameters thick. Sessile droplets of hydrogel particle suspension were evaporated on silicon wafers. The radially outward flows that drive the common coffee-ring effect push hydrogel particles towards the drop edge wherein the particles attach to the air-water interface. Most of these microgel particles move radially inward along the interface and coat the drop surface. The particles are then cross-linked, forming a membrane. The resultant thin films exhibit a temperature-responsiveness characteristic of the individual particles, permitting modulation of membrane size, shape, and optical transmission. We understand the optical properties using a Mie scattering model and an assumed membrane structure. [Preview Abstract] |
Thursday, March 5, 2015 1:27PM - 1:39PM |
T49.00010: Growth morphologies in active elastic bilayers David Mayett, Shiladitya Banerjee, J. M. Schwarz, M. Cristina Marchetti Many biological systems exhibit elastic instabilities ranging from buckling to folding to wrinkling. Such instabilities are typically driven by growth of the system. We explore the deformation properties of a layer of growing elastic material resting on a passive elastic substrate of finite thickness. We first show that there exists a mapping between the well-known Rodriguez formulation of growth and an active model where growth is incorporated via a component of the stress tensor describing the proliferation of active units in the elastic medium. Motivated by such systems as the epithelial cells making up the lining of the small intestines and sitting on top of the elastic stroma and the cerebral cortex of the brain that rests on the underlying white matter, we then use analytical and numerical approaches to show how the morphologies observed in different systems can be accounted for by different functional forms of the activity. Our active model of growth in elastic bilayer systems provides a simple, unified framework to classify the zoo of morphologies observed across seemingly different biological systems. [Preview Abstract] |
Thursday, March 5, 2015 1:39PM - 1:51PM |
T49.00011: Evolution to increase the matrix composition of clinical biofilm infections makes them stiffer, consistent with a mechanical fitness benefit Megan Davis-Fields, Kristin Kovach, Vernita Gordon \textit{Pseudomonas aeruginosa }is an opportunistic, bacterial pathogen that forms biofilms in long-term infections. Biofilms are aggregates of bacteria in a matrix composed of extracellular polymeric substance (EPS). Biofilm \textit{P. aeruginosa} infections in the lungs of cystic fibrosis patients can persist for decades, ample time for the infecting microbes to evolve. Evolutionary pressures include clearance by antibiotics and the immune system; being within a biofilm makes the bacteria more resistant to both of these. To date, most research has focused on chemical benefits conferred on the biofilm by the EPS matrix. Other researchers have recently found that long-term lung infections of \textit{P. aeruginosa }increase production of Psl, one type of EPS polysaccharide. Increasing Psl must therefore confer some benefit to \textit{P. aeruginosa }in the lung. We do bulk rheological measurements of biofilms grown from chronological clinical isolates from cystic fibrosis patients and find that strains that have evolved higher production of Psl have increased storage modulus -- $i.e.$, they are stiffer. From others' estimates of the stresses that phagocytotic cells can apply, we estimate that the stiffening we measure could provide a mechanical benefit to biofilms, helping them avoid immunological clearance. [Preview Abstract] |
Thursday, March 5, 2015 1:51PM - 2:03PM |
T49.00012: Switchable and Tunable Aerodynamic Drag on Cylinders Mark Guttag, Pedro Reis We report results on the performance of Smart Morphable Surfaces (Smporhs) that can be coated onto cylindrical structures to actively reduce their aerodynamic drag. Our system comprises of an elastomeric substrate that contains a series of optimally designed undersurface cavities that, once depressurized, lead to a dramatic deformation of the surface topography, on demand. Our design is inspired by the morphology of the giant cactus (\textit{Carnegiea gigantea}) which possesses an array of axial grooves, which are thought to help reduce aerodynamic drag, thereby enhancing the structural robustness of the plant under wind loading. We perform systematic wind tunnel tests on cylinders covered with our Smorphs and characterize their aerodynamic performance. The switchable and tunable nature of our system offers substantial advantages for aerodynamic performance when compared to static topographies, due to their operation over a wider range of flow conditions. [Preview Abstract] |
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