Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session H47: Morphable StructuresFocus
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Sponsoring Units: GSNP Chair: Joel Marthelot, Massachusetts Institute of Technology-MIT Room: LACC 507 |
Tuesday, March 6, 2018 2:30PM - 3:06PM |
H47.00001: Kirigami-inspired Coiling of Plate-like Structures Invited Speaker: Sergio Pellegrino Coiling is a very efficient way of packaging thin and flat structures, but its applicability is subject to the fundamental restriction that the height of the coil cannot be smaller than the shortest length of the structure. We present a packaging scheme that allows a plate of any polygonal shape to be folded into a coil of arbitrarily small height. The key idea is to divide the plate into straight, concentric strips with a curved cross-section (e.g. tape springs), which are folded into a single nested coil. The strips have finite bending stiffness and are connected along the edges by slipping folds, to allow for arbitrary wall thickness of the strips. This packaging scheme is easily demonstrated on thin membranes, and its full potential is demonstrated through experiments on ultrathin carbon fiber structures. |
Tuesday, March 6, 2018 3:06PM - 3:18PM |
H47.00002: Shape Programming via Direct Laser Writing Anton Bauhofer, Sebastian Krödel, Jan Rys, Osama Bilal, Andrei Constantinescu, Chiara Daraio Photochemical shrinkage during laser-induced polymerization can cause residual stresses in materials that can be exploited for fabricating shape morphing structures. In this work, we show that direct laser writing, a method based on two-photon polymerization, can be used to create pre-programmed planar sheets that evolve into complex 3D geometries upon triggering. The method allows for direct production of structures in the cm-range with sub-micron resolution. |
Tuesday, March 6, 2018 3:18PM - 3:30PM |
H47.00003: Poking on Pasta Strainers: The Rigidity of Elastic Gridshells Changyeob Baek, Pedro Reis We study the linear mechanical response of a hemispherical elastic gridshell under point-load indentation. An elastic gridshell comprises an initially planar network of elastic rods that is actuated into a 3D shell-like structure by compressing its outer boundary. We have recently introduced a novel framework to design nearly hemispherical gridshells. Our designs harness the theory of Chebyshev nets, which describes the kinematics of a medium that is anisotropically inextensible. First, we lay a square grid of Nitinol rods over an etched acrylic mold, and, then, we pour an elastomeric polymer over the crossing points to produce joints that impose positional constraints. Compressing the boundary points of the originally flat grid yields a 3D shell-like geometry. We systematically vary the geometric parameters of the gridshells and measure their structural rigidity (force per unit indentation displacement). Combining experiments, simulations, and scaling analyses leads to a master curve that relates the structural rigidity of the gridshell to its geometric and material properties. Our results indicate that the mechanical response of the gridshell, and the underlying characteristic forces, are dictated by Euler's elastica instead of shell-related quantities. |
Tuesday, March 6, 2018 3:30PM - 3:42PM |
H47.00004: Shape-switching Undulating Sheets Anne Meeussen, Martin Van Hecke We present thin, undulating sheets that switch rapidly and reversibly between many distinct shapes. Local, hysteretic snap-through events underlie this shape-switching behaviour. Snapped-through 'defects' deform the sheet; their interactions, which we predict using a simple numerical model, govern the material's overall shape. We identify external forcing conditions that lead to the organized assembly of local defects, resulting in controllable shape-switching. |
Tuesday, March 6, 2018 3:42PM - 3:54PM |
H47.00005: Dragonfly wings-inspired deployable structures Joel Marthelot, Jonathan Schleifer, Pierre-Thomas Brun Maintaining the overall shape of a flat structure while increasing its surface area is a nontrivial challenge when designing soft structures. When the wing of an emerging dragonfly deploys in a couple of minutes, the expansion is guided by a network of veins, in which hemolymph is injected and subsequently solidifies to generate rigidity. During the deployment stage, the looping patterns of the edge of the wings are characteristic of differential growth. Inspired by this insect, we build a model experiment of the inflatable deployable structure composed of a tubular network of the veins. We first characterize the response of a unique looping tubular structure under pressure. We then characterize the in-plane expansion of the structure and study its correlation to the network geometry and the pressure applied in the system. A systematic variation of the geometric and elastic parameters allows us to search for an optimal design and operational conditions for maximal extension, while minimizing the input pressure. |
Tuesday, March 6, 2018 3:54PM - 4:06PM |
H47.00006: Structural response of thin folded membranes Buwaneth Dharmadasa, Chinthaka Mallikarachchi, Francisco Lopez Jimenez
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Tuesday, March 6, 2018 4:06PM - 4:18PM |
H47.00007: Reprogramming the Elastic Properties of Mechanical Metamaterials by Amplifying Imperfections. Giorgio Oliveri, Johannes Overvelde Researchers have started to explore the use of compliance in the design of soft robotic devices that have the potential to be more robust, adaptable and safer for human interaction than traditional rigid systems. However, the field of soft robotics is still in its development phase and advances in control and tunability of soft actuators’ response are needed. A promising direction to embed and control multiple functionalities in a single actuator is to use mechanical metamaterials. In these designed materials, elastic instabilities are often harnessed to enable switching between two modes of deformation. By embedding these metamaterials in soft actuators, they could benefit in a similar way from elastic instabilities associated with different functionalities. We aim to develop an actuated pneumatic mechanical metamaterial that can switch effectively between multiple deformation modes. We will enhance the sensitivity of the unstable response to imperfections, and harness small reversible shape changes that can potentially lead to different post-buckling deformations. This research will open up exciting opportunities for the design of soft robots with different and improved functionalities, bringing these soft systems closer to real world applications. |
Tuesday, March 6, 2018 4:18PM - 4:30PM |
H47.00008: Buckling instability for directional control in biomimetic soft robots Weicheng Huang, Mohammad Khalid Jawed We report a data-driven method to control the locomotion of a bacteria-inspired soft robot by using buckling instability. The soft robot is composed of a spherical head and a helical elastic rod rotating in low Reynolds fluid. We use an experimentally validated fluid-structure interaction simulator that combines Discrete Elastic Rods algorithm with Resistive Force Theory. The robot follows a straight path below a threshold rotational speed. Buckling ensues in the rod at this threshold and the robot takes a nonlinear trajectory in its post-buckling phase. Even though the simulator can predict the trajectory, solving the inverse problem of following a given path simply by controlling the angular velocity poses a challenge for traditional analytical tools. This led us to adopt data-driven techniques from the machine learning community and exploit the robustness and speed of the simulator to use it as a data generator. We demonstrate that the soft robot can follow a prescribed path only by tuning its angular velocity. Our results may shed light on the original microscopic system of bacterial locomotion that inspired our robot design. |
Tuesday, March 6, 2018 4:30PM - 4:42PM |
H47.00009: Baromorphs - Dynamically controlled bio-inspired shape-morphing Emmanuel Siefert, Jose Bico, Etienne Reyssat, Benoit Roman We introduce a new pneumatic system based on meso-structured elastomer plates that undergo fast, reversible and controllable shape-morphing. These baromorph plates include a network of airways whose geometry is controlled during the casting of a single material. Upon inflation or suction, initially planar sheets destabilize into 3D shapes with non-zero Gaussian curvature. Air pressure in the channels induces anisotropic strains which leads to variations in the metric of the plate and triggers buckling. The coupling of pressure driven pneumatic networks with mechanical instabilities of plates is used to design structures with programmed 3D shapes. Each object can be easily controlled dynamically. Combined experimental measurements and theoretical minimal model allow us to rationalize, predict and program the pressure dependent shapes obtained. |
Tuesday, March 6, 2018 4:42PM - 4:54PM |
H47.00010: Robotic Networks of Soft Linear Actuators Nathan Usevitch, Zachary Hammond, Mac Schwager, Allison Okamura, Elliot Hawkes We consider the design of high elongation pneumatic linear actuators and control methods for robots made up of many of these linear actuators connected at universal joints. The result is a robot with natural compliance that is capable of dramatically changing its size and shape. The actuators are pneumatically powered and composed primarily of flexible but inextensible materials. To control robots formed by connecting many such actuators at universal joints, we derive the differential kinematics that relate changes in the actuator lengths to changes in the positions of the actuator endpoints, and show that controllability of the robot is equivalent to the infinitesimal rigidity of the underlying graph. Control methods are developed for two applications: locomotion and shape morphing. The control algorithm in both cases greedily minimizes an objective function while ensuring physical feasibility. For locomotion, the objective function is the motion of the robot’s center of mass along a prescribed trajectory. For shape morphing, the objective function is the distance between the robot's surface and a target shape represented by a point cloud. We present simulation results for both capabilities. |
Tuesday, March 6, 2018 4:54PM - 5:06PM |
H47.00011: Programming 3D shapes from 2D hydrogel sheets with patterned metric and curvature Ying Zhou, Christian Santangelo, Ryan Hayward Hydrogels are capable of undergoing large strains via swelling/deswelling in response to a variety of stimuli, making them ideal candidates for shape morphing systems. A number of recent studies have shown how patterning variations in swelling within the plane of thin sheets can be used to define complex 3D shapes with prescribed Gaussian curvatures. However, few methods exist to controllably break degeneracy between nearly isometric states in these materials. Here, we rely on photocrosslinking of pendent benzophenone groups to pattern the crosslinking density and swelling degree of a poly(diethylacrylamide) (PDEAm) by UV irradiation. Using a digital micromirror array device (DMD), we pattern differential swelling in PDEAm thin films by maskless photolithography, successfully programming the shape morphing from 2D flat films to non-Euclidean surfaces in 3D. Furthermore, we introduce gradients in crosslinking through the film thickness by introducing a UV absorber, thus defining a preferential buckling direction during shape morphing of PDEAm films. With double-sided lithographic exposure, we control both in-plane and out-of-plane differential swelling in the films and demonstrate the patterning of buckling directions of each repeating unit in a corrugated surface. |
Tuesday, March 6, 2018 5:06PM - 5:18PM |
H47.00012: Mechanical instabilities in the origami hypar Evgueni Filipov, Maria Redoutey This talk will highlight the mechanical characteristics of the origami hypar which is a bistable thin sheet structure with symmetric stable states. We use analytical and experimental studies to obtain the force-displacement characteristic of the structure, and to explore both the global and local behaviors. For the hypar to reconfigure between the stable states the thin sheet which is initially patterned with acute and reflex folds needs to fully flatten. However, depending on the geometry of the structure, local buckling can also emerge and can thereby allow portions of the origami to remain folded throughout the reconfiguration. A parametric study on different materials, sheet thickness, and fold line spacing is used to further evaluate and quantify the mechanical characteristics of the origami hypar. |
Tuesday, March 6, 2018 5:18PM - 5:30PM |
H47.00013: A Versatile and Robust Soft Rolling Robot Driven by Shape Memory Alloy Xiaonan Huang, Mohammad Khalid Jawed, Amarbold Batzorig, Carmel Majidi We present a soft rolling robot that is capable of fast locomotion on different terrains at over 1 body length per second. The palm-sized robot can endure falls from elevated heights and retain functionality after 40% compression of its body size. This highly deformable structure is composed of seven thermal actuators arranged in a star-shaped configuration. Tribological interactions with the ground are controlled by placing silicone rubber “shoes” at the tip of each vertex. Actuators are fabricated out of shape memory alloy wires that are sandwiched between two layers of silicone-coated thermally conductive rubber. One of the layers is pre-stretched resulting in a curved shape of the actuator. As the actuator is activated, it straightens and this deformation shifts the center mass leading to a rolling motion. Additionally, we developed a simulation tool based on the Discrete Elastic Rods modeling environment to inform the design and predict the locomotion of the robot. We vary the geometric and material parameters of the robot in the simulation and quantify its locomotion, with the aim of developing a more general-purpose, computationally-driven tool for designing soft robots. |
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