Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session V59: Actuation in Soft Matter III |
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Sponsoring Units: GSOFT Chair: Willem Van Rees, Massachusetts Institute of Technology Room: BCEC 257B |
Thursday, March 7, 2019 2:30PM - 2:42PM |
V59.00001: Mechanics and inverse-design of thin shape-shifting structures Willem Van Rees, Lakshminarayanan Mahadevan Recent progress in additive manufacturing and materials engineering has led to a surge of interest in shape-changing plate and shell-like structures. Such structures are typically printed in a planar configuration and, when exposed to an ambient stimulus such as heat or humidity, swell into a desired three-dimensional geometry. Viewed through the lens of differential geometry and elasticity, the application of the physical stimulus can be understood as a local change in the metric of a two dimensional surface embedded in three dimensions. In this talk we present our numerical approach for simulating the elastic response to such a metric change for thin structures. We also show our theoretical contributions on the inverse-design of shape-shifting bilayers, and discuss how these developments have led to the design and experimental realization of a 4D printed lattice that can undergo complex shape changes. |
Thursday, March 7, 2019 2:42PM - 2:54PM |
V59.00002: The shape of a rose petal Lauren Niu, Lakshminarayanan Mahadevan The rose petal has a distinctive set of sharp bends along the edges of its outer petals. To understand the origin of this growth pattern, we dissect rose petals to show that the their Gaussian curvature changes from positive to negative toward the outer edges. Using scaling and stability analysis, we show that a curved “sonic" line controls the shape and location of edge buckling, which is itself driven by in-plane growth. We verify our results by numerically solving the nonlinear elastic equations associated with the growing surface. Our analysis suggests a method of designing buckled edge structures with controlled in-plane growth. |
Thursday, March 7, 2019 2:54PM - 3:06PM |
V59.00003: Topography Driven Surface Renewal Luka Pocivavsek, Joseph Pugar, Sachin Velankar, Enrique Cerda Natural surfaces excel in self-renewal and preventing bio-fouling, while synthetic materials placed in contact with complex fluids quickly foul. We present a novel biophysics inspired mechanism for surface renewal using actuating surface topography, generated by wrinkling. We calculate a critical surface curvature, given by an intrinsic characteristic length scale of the fouling layer that accounts for its effective flexural or bending stiffness and adhesion energy, beyond which surface renewal occurs. The effective bending stiffness includes the elasticity and thickness of the fouling patch, but also the boundary layer depth of the imposed wrinkled topography. The analytical scaling laws are validated using finite element simulations and physical experiments. Our data span over five orders of magnitude in critical curvatures and are well normalized by the analytically calculated scaling. Moreover, our numerics suggests an energy release mechanism whereby stored elastic energy in the fouling layer drives surface renewal. The strategy is broadly applicable to any surface with tunable topography and fouling layers with elastic response. |
Thursday, March 7, 2019 3:06PM - 3:18PM |
V59.00004: Differential swelling of boundary-prescribed patterns Carlos Duque, Christian Santangelo Non-Euclidean plates, thin elastic sheets that grow or shrink inhomogeneously, can be thought as geometrically frustrated surfaces that are unable to completely eliminate in-plane stress and are forced to adopt interesting 3D rest configurations as means of reducing their elastic energy cost. We use conformal flattening methods as a natural framework to prescribe isotropic, nonuniform growth patterns on elastic sheets as a way of making them buckle into a given target shape. We tune the ratio of maximal to minimal area distortion required by modifying the planar domain shape and produce 3D patterns that range from ellipsoids and Gaussian bumps to undulating spheres and corroborate the results using finite thickness simulations of growing sheets. Though surfaces with both positive and negative Gaussian curvature behave differently from those with only one sign of Gaussian curvature, we discuss what seems to be a more general composition property of optimal swelling patterns. |
Thursday, March 7, 2019 3:18PM - 3:30PM |
V59.00005: Twist-induced snapping in a bent elastic ribbon Tomohiko Sano, Hirofumi Wada Snapping a slender structure is utilized in a wide range of natural and man-made systems, to achieve rapid movement without relying on muscle-like elements. Although several mechanisms for elastic energy storage and rapid release have been studied in detail, a general understanding to design such a kinetic system is a key challenge in mechanics. Here we study a twist-driven buckling and flip dynamics of a geometrically constrained ribbon by combining experiments, simulations, and analytical theory. We identify two distinct types of shape transitions; a narrow ribbon snaps, whereas a wide ribbon forms a pair of localized helices. We construct a phase diagram and explain the origin of the boundary determined by geometry. We quantify effects of gravity and timescale dictating the flipping. Our study reveals the unique role of twist-bend coupling on the fast dynamics of a thin constrained structure, which has implications for a wide range of biophysical and applied physical problems. |
Thursday, March 7, 2019 3:30PM - 3:42PM |
V59.00006: Self-folded pleated structures in twisted elastic sheets Arshad Kudrolli, Andreea Panaitescu, Julien Chopin We examine with experiments the post-wrinkling growth of curvature condensation and self-folded structures in thin sheets which are twisted about their axis while held under tension. Using x-ray imaging, we show that the initial sinusoidal wrinkles are found to increasing focus curvature resulting in spontaneous development of pleated structures. Unlike purely tensional wrinkles which decrease in amplitude after an application of a critical strain, the wrinkles are observed to persist and approach a finite value given by the mode number of the primary instability. The number of pleats can be controlled by varying the applied tension and the aspect ratio of the sheet with greater width to length ratios giving rise to more wrinkles and pleats. |
Thursday, March 7, 2019 3:42PM - 3:54PM |
V59.00007: Instability of an active-elastic chain Tingyuan Zheng, Martin Brandenbourger, Corentin Coulais While active materials have drawn an explosion of interest recently, the researches are almost exclusively focused on active fluids. |
Thursday, March 7, 2019 3:54PM - 4:06PM |
V59.00008: Dynamics of optical and anisotropic mechanical responses in hydrogels C. Nadir Kaplan, Peter A. Korevaar, Alison Grinthal, Joanna Aizenberg Materials that perform complex chemical sensing are ubiquitous in living systems, and would transform developments in biomedicine, environmental monitoring, and many other areas. We introduce a continuum mechanical framework that predicts the minimal set of components needed to integrate dynamical signal processing capabilities into simple hydrogels without structural modifications. For a common polyacrylic acid hydrogel, with copper cations and acid as representative chemical stimuli, the theory explains the experimentally observed (i) two-dimensional traveling waves of copper induced blue color selectively indicating a slowly progressing acid stimulus, (ii) anisotropic mechanical responses depending on the direction of acid progression, associated with tilting of an array of embedded passive microplates that report local hydrogel deformations. These results suggest simple hydrogels have a much larger sensing space than is currently made use of. |
Thursday, March 7, 2019 4:06PM - 4:18PM |
V59.00009: Active structuring of a particle film on a droplet's surface and examples of its applications Khobaib Khobaib, Alexander Mikkelsen, Zbigniew Rozynek Particle films have a broad range of applications, for instance in smart materials as constituent of strain sensors, actuators, or smart windows. Such films can be assembled and actively controlled by acoustic, magnetic, or electric fields. In this work, we used electric fields to actively compress and stretch particle film monolayers on a droplet interface. Particle manipulation was achieved through the synergetic action of electric field-induced droplet deformation and electrohydrodynamic flows. We studied in detail how droplet deformation and particle structuring are influenced by parameters such as electric field strength, ionic conductivity, and viscosity. We found that the time for particles to compress at the droplet interface was strongly influenced by the electric field, and that it scaled as E-4. Moreover, hysteresis effects were observed both in the magnitude of droplet deformation and in particle expansion and contraction. Looking towards practical applications, we also demonstrated how particles at droplet interfaces manipulated by electric fields could be used as an active optical lens, a tool for detecting of material encapsulated in particle-covered droplets, or for fabrication of porous materials used in biological applications. |
Thursday, March 7, 2019 4:18PM - 4:30PM |
V59.00010: Development of hydrogel-based cell stretching devices as in vitro model of atherosclerotic vessel walls Qi Lu, Weiguo Huang, Aritra Nath Kundu, Maria Fernanda Gencoglu, Shelly Peyton, Ryan Hayward Atherosclerosis, the hardening of arteries, is the leading cause of heart attacks and strokes. The disease is worsened by smooth muscle cell (SMC) dedifferentiation, but the cause is difficult to study in vitro because it involves coupling between multiple physical and chemical factors. We sought to improve SMC culture models by creating a device with independent control over substrate stiffness, mechanical stretch and cell attachment. This device was fabricated by embedding micro-heaters under temperature-responsive hydrogel with patterned creases on the surface, and one SMC was seeded between two neighboring creases by photo-lithographically patterned attachment of peptides in desired location. The actuation is achieved by driving the heaters at 1 Hz, which mimics the resting pulse rate, causing the cell to be cyclically stretched and released by the repeatable deepening and relaxation of the creases. The cell is stretched by a strain of 5-10%, which is comparable to the stretch ratio that it experiences in physiological environment. This device can be systematically engineered, avoids macroscopic deformation and can potentially be scaled to high-throughput arrays. |
Thursday, March 7, 2019 4:30PM - 4:42PM |
V59.00011: Actuated Fibre Networks to Study Physical Principles of Multicelluler Organization Fazil Uslu, Christopher D. Davidson, Nikolaos Bouklas, Brendon Baker, Mahmut Selman Sakar Cells continuously sense and respond to passive mechanical and topographical properties of the surrounding matrix as well as active forces transmitted through the same matrix. Cell-generated forces in deformable fibrous matrices enable long-range intercellular communication and drive collective cell behavior. Our aim is to quantitatively describe how matrix architecture and mechanics influence transmission of forces, and elucidate principles of physical organization instantiated by remodeling of a fibrillar substrate. We developed a microrobotic manipulation platform along with an experimentally validated finite element modeling framework to systematically manipulate and monitor stress on engineered fibre networks with tunable properties. Our approach allows application of spatiotemporally defined deformations using magnetically controlled microactuators and mapping of stress using simulations of materials, for which we performed measurements using confocal imaging, atomic force microscopy and MEMS force sensors. Preliminary results show that cells do respond to the signals generated by the synthetic actuators and a two-way communication can be established through the fibres using real-time feedback provided by time-lapse microscopy. |
Thursday, March 7, 2019 4:42PM - 4:54PM |
V59.00012: Fracturing of marginally stable structures: fiber networks and topological metamaterials Leyou Zhang, D. Zeb Rocklin, Leonard M Sander, Xiaoming Mao We present simulation results on fracture mechanics of two lattice models on the verge of mechanical instability. For both models, we show that the fracture and failure mechanism is distinctive from traditional brittle solids: stress does not concentrate on crack tips. In the first model which is a randomly diluted triangular lattice under isostaticity, we observed that nonlinear alignments of fiber chains lead to a steady state in which new load-bearing fiber chains emerge to replace those lost to fracture. We show that the stress concentration is dissipated and eventually prevented when the rigidity of the model decreases to zero. In the second model which is a kagome lattice with topologically protected states of self stress on devised domain walls, we show that stress concentrates on domain walls instead of cracks, leading to delayed catastrophic failure. Suitable boundary conditions for experiments will also be discussed. |
(Author Not Attending)
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V59.00013: C. ELEGANS NEMATODES DEFORM LIKE ELASTIC RODS. Octavio Albarran, Peter Weist, Eugenia Butkevich, Renata Garces, Christoph Schmidt To perform undulatory locomotion, C. elegans nematodes generate forces with their body-wall muscles acting on the surrounding environment and against their own body bending resistance. The knowledge of the passive elastic body response is crucial to understand the dynamic functions of the worm. It has been hypothesized that the worm kinematics can be understood in terms of linear viscoleastic beam theory. However, to date, mechanical studies in the large deformation regime, typical of native undulatory locomotion, are lacking. We here present a micro-needle-based experiment imposing large strains. Living worms were kept straight by clamping their extremities onto agar plates. We laterally displaced the centers of worms with a glass cantilever of known spring constant and directly probed the linear beam model. To separate passive responses from muscle activity, we varied the contraction-relaxation state of the muscles using pharmacological interference. We provide a synthesis of the typical range of magnitudes of the worm’s material parameters for different muscles states. |
Thursday, March 7, 2019 5:06PM - 5:18PM |
V59.00014: Frictional Pinning of Amorphous Contacts with 2D and 3D Elastic Substrates Joseph Monti, Mark Owen Robbins Disorder at the interface of an amorphous slider on a crystalline substrate leads to vanishing friction for large rigid contacts. The inclusion of elasticity introduces an energetic contribution that competes with the random interfacial potential and allows frictional pinning over a domain with characteristic radius ac. The interfacial force scales linearly with ac, while the elastic restoring force from deformation on the scale of ac depends upon the dimension of the substrate. For a 2D substrate, the restoring force is independent of ac, while it grows linearly with ac in 3D. Comparison of the interfacial and elastic contributions shows that the pinning force is proportional to ac in 2D. The 3D case is marginal because the forces scale with the same power of ac. Extensive simulations are used to test the scaling of friction with disorder strength and system size. In 3D, we find an exponential decrease of the pinning force and increase in ac with increasing substrate stiffness. In 2D, the expected power law scaling is observed. Substrates of finite thickness show a crossover in scaling with contact size that may be important in understanding solid lubricants with a plate structure. |
Thursday, March 7, 2019 5:18PM - 5:30PM |
V59.00015: Global and Local Mechanical Properties of Responsive Microgels Adsorbed to Solid-Liquid Interfaces Marie Friederike Schulte, Andrea Scotti, Monia Brugnoni, Steffen Bochenek, Ahmed Mourran, Walter Richtering Microgels are highly interfacial active, although not being amphiphilic. They readily adsorb to liquid/liquid, liquid/air or solid interfaces and deform [1]. We studied microgels with unique internal structures, (i) one with a rigid silica core and a PNIPAM shell, (ii) hollow microgels obtained by dissolving the silica core, and (iii) ultra-low cross-linked microgels. These responsive microgels were investigated at the solid-liquid interface by scanning force microscopy. An important parameter is the size of the probe compared to the dimensions of the microgel [2]. Colloidal probe experiments lead to a compression of the whole microgel and provide information on their global mechanical properties. Changing the probe dimensions, we will demonstrate that indentation experiments using sharp tips lead to a local penetration of the porous swollen microgel network. Therefore, force-distance curve measurements enable to probe the segment density distribution orthogonal to the substrate of adsorbed single microgels. |
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