Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session G29: Emergent Mechanics of Active, Robotic, and Living Materials IFocus

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Sponsoring Units: DSOFT GSNP Chair: Corentin Coulais, Univ of Amsterdam Room: 501 
Tuesday, March 3, 2020 11:15AM  11:51AM 
G29.00001: Transmisison hysteresis and edge modes in bounded spacetime composites Invited Speaker: Hussein Nassar The vibrational frequency response of bounded composites, e.g., metamaterials and sonic crystals, is often understood thanks to band diagrams established in the absence of boundaries. Introducing a pump wave that modulates in time the properties of the composite challenges the correspondence between the vibrations picture and the waves picture. The talk revisits this correspondence in the context of the nonreciprocal acoustics of spacetime composites. Specifically, we establish in the weak coupling regime how the hybridization of total bandgaps into pairs of oneway bandgaps triggers nonreciprocal hysteresis transmission loops in the spacefrequency domain and alters, qualitatively and quantitatively, the vibrational frequency response in the presence of reflecting boundaries. The theoretical analysis is assessed numerically and exploited to shed new light on previously obtained experimental data. Last, extrapolating our study to the strong coupling regime, transmission hysteresis is shown to explain the emergence of topological vibrational modes with oneway edgebulk and bulkedge transitions. 
Tuesday, March 3, 2020 11:51AM  12:03PM 
G29.00002: Motion via Bistability in Viscous Fluids Mohamed Zanaty, William Zunker, Jochen Mueller, Katia Bertoldi We study the behavior of bistable beams in viscous media and their application to aquatic locomotion. Specifically, we exploit asymmetric actuation of bistable beams, which leads to a nonreciprocal deformation path. As an example, we use double pinned axially compressed bistable beams. We actuate the beam asymmetrically by imposing an angular position on one of the beam extremities. 
Tuesday, March 3, 2020 12:03PM  12:15PM 
G29.00003: Nonreciprocal solitons in robotic materials Martin Brandenbourger, Hans Dekker, Corentin Coulais The recent development of robotic materials, which are assemblies of building blocks integrating sensors and actuators, enables the implementation of nonconservative interactions within mechanical systems. This opens the way to the emergence of unique large scale mechanical properties. 
Tuesday, March 3, 2020 12:15PM  12:27PM 
G29.00004: Topological locomotion Katia Bertoldi, Bolei Deng, Mohamed Zanaty We investigate how the propagation of the topological domain walls through a metamaterial based on the squares rotating mechanism can be harnessed to generate locomotion. First, we add internal elastic constraints to transform the system into a bistable system, with the two energy minima being the two symmetryrelated rotated phases. Such metamaterial supports topological solitons that take the deformed metamaterial from one static equilibrium position to another one. Remarkably, our results indicate that by combining the propagation of such finitewidth topological solitons with angledependent friction (that can be easily realized by putting the system on wheels) we can realize a crawler. 
Tuesday, March 3, 2020 12:27PM  12:39PM 
G29.00005: Topology by activity in nonHermitian mechanical metamaterials Colin Scheibner, William Thomas Mark Irvine, Vincenzo Vitelli A simple view of an active solid is a network of masses interacting via nonconservative bonds. We examine 2D topological lattices equipped with nonreciprocal active bonds that inherit two key properties from their passive counterparts: their interactions conserve linear momentum and depend only on the relative positions of the masses. We characterize the flow of Berry curvature and exceptional points in the resulting nonHermitian band structure as activity is tuned from the passive limit to the brink of instability. Along this flow, we find a discrete onset of topologically charged bands whose activity threshold can be controlled via zeromode deformations of the underlying lattice. Simulations and analytical calculations reveal the emergence of persistent edge modes influenced by the interplay of a topologically protected penetration depth and an effective quality factor derived from nonHermitian gain and loss. Our work sheds light on nonHermitian band theory and the design of active metamaterials that conserve linear momentum. 
Tuesday, March 3, 2020 12:39PM  12:51PM 
G29.00006: The topology of nonlinear mechanical systems PoWei Lo, Michael J Lawler, Christian Santangelo, Bryan G Chen, Krishanu Roychowdhury, ChaoMing Jian Following the pioneering work of Kane and Lubensky (and others), commendable advancements have been made in the field of topological mechanics. The majority of the work, however, concerns the topology of linear zero modes primarily engaging the framework of linear response theory (often drawing parallels to electronic responses in topological insulators). We, in the present work, attempt an extension to accommodate nonlinear effects that are more natural to occur in realistic mechanical systems and feature topologically protected zero modes. Invoking the tools of differential geometry, we present an exact theory to demonstrate this topology. Our theory (inspired by topological quantum field thoery), remarkably, predicts the existence of a Ztype topological invariant which arises only from the nonlinearities and does not demand any symmetry imposition (unlike the linear zero modes). We further include example systems to illustrate the physics. 
Tuesday, March 3, 2020 12:51PM  1:03PM 
G29.00007: Mechanical metamaterial inspired by biological tissues Xinzhi Li, Dapeng Bi We introduce an amorphous mechanical metamaterial inspired by how cells pack in biological tissues. The spatial heterogeneity in the local stiffness of these materials has been recently shown to impact the mechanics of confluent biological tissues and cancer tumors (Li et al Phys. Rev. Lett. 123, 058101 (2019)). Here we use this bioinspired model as a design template and show that this heterogeneity can give rise to amorphous cellular solids with large, tunable phononic bandgaps. Unlike in phononic crystals, the band gaps here are directionally isotropic due to their complete lack of positional order. The size of the bandgap can be tuned by a combination of local stiffness heterogeneity and the local elasticity modulus. Finally, we also investigated the possibility of introducing a topological nature in the mechanical response using a hexagonal lattice version of the same model. By tuning the local elasticity moduli, we demonstrate the emergence of a pseudospin state and as well chiral mechanical response. This constitutes a mechanical analogue of the quantum spin Hall effect. 
Tuesday, March 3, 2020 1:03PM  1:15PM 
G29.00008: Modeling tunable acoustic transport in driven graphene nanoresonator arrays Pragalv Karki, David Miller, Andrew D Blaikie, Brittany Carter, Benjamin J Aleman, Jayson Paulose Arrays of graphene nanoelectromechanical resonators show promise as a platform for precise, programmable manipulation of acoustic waves at the nanoscale. Besides harboring technologically relevant resonant frequencies and high quality factor, graphene resonators display a remarkable tunability of the local membrane tension through electrostatic backgating or laser heating, which can be used to modulate vibrational properties. Motivated by experiments, we present a theoretical and computational study of the acoustic properties of coupled graphene resonators with dynamical spatiotemporal modulation of the tension. Starting from the theory of thin elastic plates, we develop a reduced description of the collective modes arising from the coupling of the fundamental modes on individual resonators and evaluate the effects of externally driven changes in tension on wave propagation. We will also describe early efforts at calibrating and testing the model against experimental measurements of coupled resonator acoustic modes. Our results pave the way towards designing driven graphene nanoelectromechanical resonator arrays with a manifestly outofequilibrium acoustic response. 
Tuesday, March 3, 2020 1:15PM  1:27PM 
G29.00009: Transitions from conservative to nonconservative regimes in optical binding of colloidal matter. Dominique Davenport, Dustin Kleckner We study light generated interparticle interactions known as optical binding. These forces are mediated by the particle light scattering making it uniquely responsive to simple changes in the parameter space. For instance, we find that many colloidal particles placed in a weakly focused laser beam can selfassemble into higherorder and multileveled structures which can be tuned with simple alterations. We share experimental results which conclude a dramatic change in assembly patterns by changing particle index of refraction; namely, a switch between a conservative pairwise regime to highly nonconservative manybody regime. 
Tuesday, March 3, 2020 1:27PM  1:39PM 
G29.00010: What kinds of forces does optical binding produce? Dustin Kleckner, Dominique Davenport Optical binding is an attractive tool for controlling colloidal forces because it offers a highly tunable method of producing interparticle forces without chemical modifications. Previous studies of optically bound particles have tended to focus on small numbers of particles in relatively static configurations. By contrast, our experimental studies of many wavelength sized particles exhibit novel dynamics which are indicative of driven, nonconservative forces. I will discuss how these forces arise, and how they might be controlled to study colloidal dynamics in novel regimes. 
Tuesday, March 3, 2020 1:39PM  1:51PM 
G29.00011: The odd flows of a colloidal chiral fluid Ephraim Bililign, Vishal H Soni, Sofia Magkiriadou, Stefano Sacanna, Denis Bartolo, Michael John Shelley, William Thomas Mark Irvine We report the assembly of a chiral fluid composed of millions of spinning colloidal magnets. By activating the fluid at the single unit level, we observe macroscopic flows with no counterpart in conventional fluids. Odd viscous stresses drive the propagation of unidirectional freesurface waves damped by odd (or Hall) viscosity. Further, the competition between odd stress and cohesive forces results in intermittent bulk flows, blurring the distinction between solid and liquid. 
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