Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session R06: Emergent Mechanics of Active, Robotic, and Living Materials IIFocus Live
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Sponsoring Units: DSOFT GSNP Chair: Corentin Coulais, Univ of Amsterdam; Jayson Paulose, University of Oregon Room: 06 |
Thursday, March 18, 2021 8:00AM - 8:12AM Live |
R06.00001: Locomotion without force: exploiting curved spaces Zeb Rocklin, James McInerney, Enes Aydin, Yasemin Ozkan-Aydin, Daniel I Goldman Conventional locomotion requires momentum exchange with the surrounding environment. As noted by Wisdom (2003), this relies on the commutativity of generators of translations and consequently locomotion can occur in curved spaces without the application of external forces, just as a falling cat can reorient itself. Here, we report the first experimental realization of this effect via a T-shaped robotic model with servos driving 34 gm masses via pinion-gears along three curved rack-type arms (lengths 59.5cm, 25cm, total mass 33 gm). The robot is confined to a spherical surface (radius 59.5 cm) via a carbon rod that rotates about a central pivot to allow low-friction swimming. The effect can be described via a gauge potential in which translations accumulate as a geometric (Berry) phase that depends on the path through shape space. This long-neglected effect is significant for locomotors that cover areas comparable to the inverse Gaussian curvature of the underlying surface--from macroscale robots to biological cells rearranging during embryogenesis. |
Thursday, March 18, 2021 8:12AM - 8:24AM Live |
R06.00002: Topological defects in non-reciprocal solids Colin Scheibner, Lara Braverman, Vincenzo Vitelli Topological defects in crystals are responsible for phenomena ranging from plastic deformation to defect-mediated melting. A crucial assumption in the elastic theory of topological defects is a symmetry between deformation and stress known as Maxwell-Betti (MB) reciprocity. However, in active media, MB reciprocity need not hold. In this talk, I will discuss the theory of topological defects in two-dimensional crystals that violate MB reciprocity. Intriguingly when MB reciprocity is broken, we find that the Peach-Koehler forces between two dislocations need not be equal and opposite even when the medium is freestanding, homogeneous, and isotropic. Beyond active solids, our theory lends insights into systems with microscopic transverse interactions such as vortex lattices and gyroelastic media. |
Thursday, March 18, 2021 8:24AM - 8:36AM Live |
R06.00003: Impact of active solids Martin Brandenbourger, Colin Scheibner, Vincenzo Vitelli, Corentin Coulais Active mechanical metamaterials have recently completely changed our ability to manipulate mechanical waves via, e.g., chiral edges modes, unidirectional transmission and self-amplification. However, little is known on how these properties could be translated to active metamaterials capable of large deformations. Here, we study 1D and 2D active metamaterials capable of large deformations by measuring their mechanical responses to dynamic impacts. We show that activity can be used to tune the bouncing angle and bouncing coefficient of active solids independently of their geometry and demonstrate that these specific properties can be rationalised by general odd elastic and non-hermitian skin waves models. This work opens avenues for novel active materials and devices with new mechanical properties based on out-of-equilibrium interactions such as antisymmetric shear coupling, self-oscillations, locomotion, etc. |
Thursday, March 18, 2021 8:36AM - 8:48AM Live |
R06.00004: Complete absorption of topologically protected waves Guido Baardink, Gino Cassella, Luke Neville, Anton Souslov, Paul Milewski Chiral edge states can transmit energy along imperfect interfaces in a topologically robust and unidirectional manner when protected by bulk-boundary correspondence. However, in continuum systems, the number of states at an interface can depend on boundary conditions. Here we design interfaces that host a net flux of the number of modes into a region, trapping incoming energy. As a realization, we present a model system of two topological fluids composed of counter-spinning particles, which are separated by a boundary that transitions from a fluid-fluid interface into a no-slip wall. In these fluids, chiral edge states disappear, which implies non-Hermiticity and leads to a novel interplay between topology and energy dissipation. Solving the fluid equations of motion, we find explicit expressions for the disappearing modes. We then conclude that energy dissipation is sped up by mode trapping. Instead of making efficient waveguides, our work shows how topology can be exploited for applications towards acoustic absorption, shielding, and soundproofing. |
Thursday, March 18, 2021 8:48AM - 9:00AM Live |
R06.00005: Active Elastic Materials : from emerging collective motion to autonomous actuation Paul Baconnier, Claudio Hernández, Gustavo During, Corentin Coulais, Vincent Démery, Olivier Dauchot A population of motile units does not only display collective motion at all scale; it can also generate active forces when embedded in an elastic matrix. A typical biological instance of such processes is that of migrating cells that transmit forces to the extracellular matrix via actin filament contractions. Mimicking such processes is a natural path towards the design of autonomous functional materials. With the help of centimetric model of self-propelled particles, we construct the first experimental model system of an active elastic material, and study its emerging behaviors in various mechanical conditions. We find that self-propelled particles acting at the nodes of an elastic structure spontaneously organize and actuate a variety of mechanical functions, including rigid body motion, mechanisms actuation and selected excitation of vibrational mode. Such active solids present very peculiar dynamical behaviors in strong violation of the equilibrium principles, and open the way toward the autonomous actuation of more advanced synthetic networks, including shape-morphing, topological and nonlinear meta materials, with predesigned functionality. |
Thursday, March 18, 2021 9:00AM - 9:12AM Live |
R06.00006: Signal manipulation via dynamic dispersion tuning through a flat band in a phononic metamaterial Pragalv Karki, Jayson Paulose The ability to trap and manipulate a signal is crucial for technologies ranging from sensors to computing devices. In this talk, we will demonstrate how to stop and reverse a sound pulse in a new class of programmable metamaterials. We show that this functionality is enabled by tuning the dispersion relation of a periodic metamaterial in real-time, a mechanism we term dynamic dispersion tuning. The necessary change in dispersion is achieved by the change in sign of an effective coupling between fundamental modes, which generically leads to a nearly dispersionless, or flat, band at the transition point. We show how adiabatic tuning of the band dispersion can immobilize and reverse the propagation of a sound pulse in simulations of a one-dimensional resonator chain. |
Thursday, March 18, 2021 9:12AM - 9:24AM Live |
R06.00007: Marine Sponge Tissue Displays Both Dynamic and Diverse Mechanics Emile Kraus, Lauren Mellenthin, Sara Siwiecki, Dawei Song, Paul Janmey, Alison Sweeney Sponges are animals that utilize active and passive filtration to consume bacteria from the surrounding water and eject wastes and recycled carbon. They have evolved to inhabit nearly every type of flow regime, and are an ancient phylum, apparently predating the evolution of nerves and muscles. Sponge connective tissue is composed of a diverse set of collagens with an embedded scaffolding of siliceous spicules. It has long been suspected that there is a feedback between the mechanics of this tissue in environmental flows and the sponge’s body form. However, the physical behavior of sponge tissue has not previously been explored in any detail. Here we perform rheological experiments on a diverse set of sponge species and fit the data using hyperelastic constitutive models. Our results show that living sponge tissue displays anisotropic elasticity. The degree and direction of the anisotropy are both correlated with a given species’ overall growth form. The ability of this soft material system to grow tall in strong currents can be explained by the presence of silica-spicule-reinforced collagen fibers that allow dynamic strengthening along an axis aligned with the net force on the animal due to drag and lift in current. |
Thursday, March 18, 2021 9:24AM - 9:36AM Live |
R06.00008: 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 bio-inspired 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 pseudo-spin state and as well chiral mechanical response. This constitutes a mechanical analogue of the quantum spin Hall effect. |
Thursday, March 18, 2021 9:36AM - 10:12AM Live |
R06.00009: Coarse-grained kinematics of origami tessellations Invited Speaker: Hussein Nassar Morphing shells hold a special position at the interface between structures and mechanisms: while they exhibit high rigidity under certain types of loadings, other loadings trigger global and highly nonlinear transformations of shape altering elongations and curvatures. Origami tessellations are one example of such shells where the interplay between soft creases and stiff panels produces diverse, however restricted, curved geometries out of an initially flat sheet and are therefore of interest in the design of robotic and actuated materials and structures. Here, we develop tools suitable for the analysis of the coarse grained kinematics of origami tessellations in the limit of fine crease patterns. In particular, we draw connections between the underlying geometry of the pattern and emergent mechanical properties such as the in-plane and out-of-plane Poisson's coefficients and comment on issues pertaining to stability, boundary control and the spatial distribution of soft modes. |
Thursday, March 18, 2021 10:12AM - 10:24AM Live |
R06.00010: Active elastocapillarity Jack Binysh, Anton Souslov Active solids use elastic coupling between energy-consuming elements to achieve functionality inaccessible at equilibrium. For two-dimensional active surfaces, powerful experimental techniques allow for exquisite control over spatial patterning and fuel delivery. However, achieving such control in three-dimensional objects presents a challenge. Here, we develop a continuum theory that describes an active surface wrapped around a passive soft solid. The competition between active surface stresses and bulk elasticity leads to a broad range of previously unexplored phenomena, which we dub active elastocapillarity. In passive materials, positive surface tension rounds out corners and drives every shape towards a sphere. By contrast, activity can send the surface tension negative, which allows us to tune the target shape using elasticity. We discover that in these reconfigurable objects, material nonlinearity controls reversible switching and snap-through transitions between anisotropic shapes. Even for stable surfaces, a signature of activity arises in the negative group velocity of surface Rayleigh waves. These phenomena offer insights into living cellular membranes and underpin novel design principles across scales from robotic metamaterials down to shape-shifting nanoparticles. |
Thursday, March 18, 2021 10:24AM - 10:36AM Live |
R06.00011: Topology protects robust global cycles in stochastic systems Evelyn Tang, Jaime Agudo-Canalejo, Ramin Golestanian Living systems can exhibit time-scales much longer than those of the underlying components, as well as emergent dynamical and collective behavior. How such robust global behavior is subserved by stochastic constituents remains unclear. Here, we present biologically plausible motifs from which two-dimensional stochastic networks can be constructed. The motifs represent out-of-equilibrium cycles on the microscopic scale, which support macroscopic edge currents in configuration space, a consequence of the topological Zak phase. Uniquely non-Hermitian properties of the system are seen in the emergence of exceptional points or the non-zero vorticity and doubled periodicity of edge states. Our framework enables a wealth of dynamical phenomena such as a global clock, dynamical growth and de-growth, as well as synchronization, similar to observations that are quite prevalent in biology. Our models suggest new insights into the theoretical framework of non-Hermitian physics, and pave the way for the prediction of new states in both classical and quantum systems. |
Thursday, March 18, 2021 10:36AM - 10:48AM Live |
R06.00012: Bioinspired Soft Composites with Deformation-induced Coloration Ahmad Rafsanjani, Erik Poloni, Vadim Place, David Ferretti, Andre Studart Fast dynamic control of skin coloration in the animal kingdom inspires soft robotic materials that can vibrantly adapt to their environment. Nature’s soft experts such as cephalopods and some fish species rely on iridescent cells called iridophores for camouflage and communication. The nanostructure of iridophores formed by alternating stacks of thin crystalline platelets and cytoplasm sheets disrupts the incident light and selectively reflects specific colors. Here, we draw a high-level inspiration from active iridophores and introduce an architected soft composite material that can reversibly change its color under pure deformation. The soft composite consists of an elastomer matrix filled with reflective ceramic platelets that are magnetized with iron oxide nanoparticles and aligned in a low magnetic field. We exploit shear instabilities to reversibly tilt reflective platelets upon stretching and dramatically change the appearance of the material even with a low concentration of platelets. Deformation-induced coloration in soft materials enables visual signaling and stress mapping at the material level that is suitable for sensing and feedback control of soft robots. |
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