Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session H18: Function from Geometry: 3D Printing to Programmable Matter IFocus
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Sponsoring Units: GSOFT DPOLY GSNP FIAP Chair: Pierre-Thomas Brun, MIT Room: 277 |
Tuesday, March 14, 2017 2:30PM - 3:06PM |
H18.00001: Making Faces: Thin Nematic Elastomer Sheets in Theory and in Practice Invited Speaker: Hillel Aharoni Thin nematic elastomer sheets attain 3D configurations that depend on the nematic director field upon heating. Recent experiments from various groups demonstrate excellent control over the director fields embedded into such sheets, thus opening a door for achieving accurate and versatile designs of shape-shifting surfaces.\\ In this talk we describe the intrinsic geometry of such sheets at different temperatures, depending on their preprogrammed nematic director field. We focus on investigating the inverse problem -- constructing a director field that would induce a specified geometry. We provide analytical solutions for certain classes of desired geometries, and show how arbitrary geometries can be designed using approximate numerical methods. We show how further control over resultant shapes can be achieved by inscribing gradients in the director field across the sheet’s thickness, thus prescribing nontrivial local curvatures.\\ Finally, we combine these methods to create designs that are micropatterned onto a mold using photolithography, and then embedded via the mold into thin nematic elastomer sheets polymerized within it. Using this method we show success in experimentally producing flat sheets that, upon activation, take an arbitrary desired shape. [Preview Abstract] |
Tuesday, March 14, 2017 3:06PM - 3:18PM |
H18.00002: Molecular dynamics of reversible self-healing materials Ian Madden, Erik Luijten Hydrolyzable polymers have numerous industrial applications as degradable materials. Recent experimental work by Cheng and co-workers (\textit{Nat.\ Comm.}\ \textbf{5}, 3218 (2014)) has introduced the concept of hindered urea bond (HUB) chemistry to design self-healing systems. Important control parameters are the steric hindrance of the HUB structures, which is used to tune the hydrolytic degradation kinetics, and their density. We employ molecular dynamics simulations of polymeric interfaces to systematically explore the role of these properties in a coarse-grained model, and make direct comparison to experimental data. Our model provides direct insight into the self-healing process, permitting optimization of the control parameters. [Preview Abstract] |
Tuesday, March 14, 2017 3:18PM - 3:30PM |
H18.00003: The resolution limits of voxelated liquid crystal networks and elastomers Benjamin Kowalski, Timothy White Arbitrary director patterning within liquid crystal network films has assimilated functional materials responses in a monolith. Examples include complex 3D shape deformations and nonlinear mechanical responses. Fast, cheap, and rapidly reconfigurable patterning techniques are needed to fully realize the opportunity space. Here we demonstrate one-shot photopatterning at display resolution, using an off-the-shelf twisted-nematic spatial light modulator and simple projection optics. At high resolution, the inscribed director profile is dominated by elastic-mediated orientational relaxation, imposing a fundamental limit on achievable voxel size. A simple model for this effect is experimentally validated, and implications for device design are discussed. [Preview Abstract] |
Tuesday, March 14, 2017 3:30PM - 3:42PM |
H18.00004: Additive lattice kirigami Toen Castle, Daniel M. Sussman, Mike Tanis, Randall D. Kamien Kirigami uses bending, folding, cutting, and pasting to create complex three-dimensional (3D) structures from a flat sheet. In the case of lattice kirigami, this cutting and rejoining introduces defects into an underlying 2D lattice in the form of points of nonzero Gaussian curvature. I will present a general set of techniques which unify a wide variety of cut-and-paste actions under the rubric of lattice kirigami, including adding new material and rejoining material across arbitrary cuts in the sheet. Creating complex structures may require multiple overlapping kirigami cuts, where subsequent cuts are not performed on a locally flat lattice. Our additive kirigami method describes such cuts, providing a simple methodology and a set of techniques to build a huge variety of complex 3D shapes. [Preview Abstract] |
Tuesday, March 14, 2017 3:42PM - 3:54PM |
H18.00005: Surface Design Based on Discrete Conformal Transformations Carlos Duque, Christian Santangelo, Etienne Vouga Conformal transformations are angle-preserving maps from one domain to another. Although angles are preserved, the lengths between arbitrary points are not generally conserved. As a consequence there is always a given amount of distortion associated to any conformal map. Different uses of such transformations can be found in various fields, but have been used by us to program non-uniformly swellable gel sheets to buckle into prescribed three dimensional shapes. In this work we apply circle packings as a kind of discrete conformal map in order to find conformal maps from the sphere to the plane that can be used as nearly uniform swelling patterns to program non-Euclidean sheets to buckle into spheres. We explore the possibility of tuning the area distortion to fit the experimental range of minimum and maximum swelling by modifying the boundary of the planar domain through the introduction of different cutting schemes. [Preview Abstract] |
Tuesday, March 14, 2017 3:54PM - 4:06PM |
H18.00006: Localizing linear and nonlinear elastic responses in liquid crystal elastomers Anesia D. Auguste, Benjamin A. Kowalski, Timothy J. White Liquid crystal elastomers (LCE) contain rod-like rigid units (mesogens) which exhibit and maintain orientational or positional order. Under a stimulus, LCEs exhibit dramatic shape and/or optical changes that can be utilized in aerospace applications, optics, or medicine. Here, we prepared patterned elastomers with distinct local elastic properties by controlling the alignment of the mesogens through ink-jet printing. The mechanical response of LCEs is sensitive to the direction of the applied force with respect to director which is the preferred orientation direction of the mesogens. The material exhibits `soft' elasticity when the mesogens realign to the stretching direction leading to deformation occuring at nearly constant stress. The orientation of the mesogens in homeotropically aligned LCEs, where the mesogens are aligned normal to the substrate, enables and allows for `omnidirectional' soft elasticity in which the LCE exhibits nonlinear elasticity in any and all deformation directions compare to the anisotropic behavior of a planar aligned LCE. The ability to control the local elastic response allows us to create designer functional monoliths with various Poisson's ratios which may find use in flexible hybrid devices. [Preview Abstract] |
Tuesday, March 14, 2017 4:06PM - 4:18PM |
H18.00007: Large deflections of a hydrogel rod caused by internal phase separation Michael S. Dimitriyev, Paul M. Goldbart, Ya-Wen Chang, Anton Souslov, Alberto Fernandez-Nieves Hydrogels are soft materials that consist of a cross-linked polymer matrix capable of undergoing large volume changes via absorption of a solvent. As with binary mixtures, hydrogels can undergo a macroscopic phase separation transition to create a more swollen region and a less swollen one. We address this transition in the case of an initially swollen hydrogel, in a slender-rod geometry, possibly curved, which is heated to a temperature at which one would expect deswelling of the entire sample. However, the rapidity of the rise in temperature inhibits the system from expelling solvent through the rod's surface, so that re-equilibration takes place at fixed solvent volume. Owing to this constraint and the system's elasticity, the solvent-poor region fails to fully deswell, and the hydrogel partitions into an incompletely deswollen region and an excessively swollen one, determined by stress balance and a lever rule. Because the polymer network remains contiguous the rod undergoes a macroscopic shape change. When the partitioning is constant along the rod, the interface-orientation is a Goldstone mode that couples to the rod’s bending and twisting degrees of freedom and as a result, a large deflection of the rod occurs. [Preview Abstract] |
Tuesday, March 14, 2017 4:18PM - 4:30PM |
H18.00008: Shape Changing Thin Films Powered by DNA Strand Exchange Tae Soup Shim, Zaki Estephan, Zhaoxia Qian, David Chenoweth, Daeyeon Lee, So-Jung Park, John Crocker Active materials that respond to physical and chemical stimuli can be used to build dynamic thin-film micromachines that lie at the interface between biological systems and engineered devices. In principle, the specific hybridization of DNA can be used to form a library of independent, chemically driven actuators for use in such microrobotic applications and could lead to device capabilities that are not possible with polymer- or metal-layer-based approaches. Here, we report shape changing films that are powered by DNA strand exchange reactions with two different domains that can respond to distinct chemical signals. The films are formed from DNA-grafted gold nanoparticles using a layer-by-layer deposition process. Films consisting of an active and a passive layer show rapid, reversible curling in response to stimulus DNA strands added to solution. Films consisting of two independently addressable active layers display a complex suite of repeatable transformations, involving eight mechanochemical states and incorporating self-righting behavior. [Preview Abstract] |
Tuesday, March 14, 2017 4:30PM - 4:42PM |
H18.00009: Baromorphs Emmanuel Siefert, José Bico, Etienne Reyssat, Benoit Roman A pneumatic network of millimetric channels is embedded into elastomer plates. Upon inflation or suction, we observe that initially planar sheets destabilize into 3D shapes with non-zero Gaussian curvature. The difference in air pressure between the inside and outside of the channels induces anisotropic strains which leads to variations in the metric of the plate and triggers buckling. We use the coupling of pressure driven pneumatic networks with mechanical instabilities of plates to design structures with programmed 3D shapes. The actuation of these sheets is reversible and shape changes occur in typically one second. We present the results of combined experimental and minimal models on these pressure-actuated structures/objects/devices. [Preview Abstract] |
Tuesday, March 14, 2017 4:42PM - 4:54PM |
H18.00010: Growth and Pattern Formation in Thin Elastic Shells Salem Al Mosleh, Christian Santangelo Heterogeneous growth plays an important role in shape and pattern formation in thin elastic shells, for example blooming lilies, rippling leaves, swelling polymer films and rod-like E. coli. In many of these examples the local growth could be coupled to the local geometry which poses interesting questions regarding stability and regulation. We model the growth process as a quasi-static time evolution of the metric, with fast elastic relaxation of the shape. A growth law is given by a coupling between the shape of the shell and the time derivative of the metric. We characterize the possible growth laws consistent with shell symmetries and study the stable shapes emerging from various growth laws. Finally we discuss possible applications to biological and experimental systems. [Preview Abstract] |
Tuesday, March 14, 2017 4:54PM - 5:06PM |
H18.00011: Rational design of reconfigurable prismatic architected materials. Katia Bertoldi, Johannes Overvelde, Chuck Hoberman, James Weaver Advances in fabrication technologies are enabling the production of architected materials with unprecedented properties. While most of these materials are characterized by a fixed geometry,an intriguing avenue is to incorporate internal mechanisms capable of recon\textunderscore guring their spatial architecture, therefore enabling tunable functionality. Inspired by the structural diversity and foldability of the prismatic geometries that can be constructed using the snapology origami-technique, here we introduce a robust design strategy based on space-filling polyhedra to create 3D reconfigurable materials comprising a periodic assembly of rigid plates and elastic hinges. Guided by numerical analysis and physical prototypes, we systematically explore the mobility of the designed structures and identify a wide range of qualitatively di\textunderscore erent deformations and internal rearrangements. Given that the underlying principles are scale-independent, our strategy can be applied to design the next generation of reconfigurable structures and materials, ranging from transformable meter-scale architectures to nanoscale tunable photonic systems.. [Preview Abstract] |
Tuesday, March 14, 2017 5:06PM - 5:18PM |
H18.00012: Exploring the limits of multifunctionality in adaptable networks: comparing flow networks to mechanical metamaterials Jason W. Rocks, Henrik Ronellenfitsch, Eleni Katifori, Andrea J. Liu, Sidney R. Nagel Previous work shows that spring networks are both adaptable and robust - via selective bond pruning, specific functions can be programmed precisely, efficiently and robustly [Rocks et. al., 2016, arXiv:1607.08562]. These functions include localized but long-range-correlated deformations reminiscent of allostery in proteins. Analogous functionality can be introduced into flow networks by controlling the current through a bond in response to a current applied elsewhere in the network. Here we explore the limits of multifunctionality. How many separate independent functions can be simultaneously tuned successfully into a network and how many different targets can be controlled by a single source? These questions can be classified as constraint-satisfaction problems that we study in both mechanical and flow networks. [Preview Abstract] |
Tuesday, March 14, 2017 5:18PM - 5:30PM |
H18.00013: Experiments on Hyperuniformity induced by Random Organization Sam Wilken, Rodrigo Guerra, David Pine, Paul Chaikin Periodically sheared dilute, non-Brownian suspensions explore new configurations through collisions in an otherwise reversible flow. Below a critical strain, the particles remain active until they find a configuration with no collisions and reach an absorbing state. Recent simulations by Hexner and Levine have shown that the configuration of particles in the critically absorbing state is hyperuniform. The structure factor of a hyperuniform system goes to zero as approaches zero ($S(q \rightarrow 0) \rightarrow 0$), as opposed to a constant positive value for the same suspension away from the critical state. Simulations predict a power law behavior of S(q) for length scales larger than the particle separation. We built a compact, uni-axial shear cell in order to shear dilute suspensions while using small-angle light scattering to measure S(q) from angles of 0.1$^{\circ}$ to 1.5$^{\circ}$. We are able to identify hyperuniform structures via light scattering for colloidal suspensions of up to 40\% volume fraction at the critically absorbing state. [Preview Abstract] |
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