Session J29: Emergent Mechanics of Active, Robotic, and Living Materials II

Rectification of energy and motion in non-equilibrium parity violating metamaterials

Uncovering new mechanisms for rectification of stochastic fluctuations has been a longstanding problem in non-equilibrium statistical mechanics. Here, using a model parity violating metamaterial that is allowed to interact with a bath of active energy consuming particles, we uncover new mechanisms for rectification of energy and motion. Our model active metamaterial can generate energy flows through an object in the absence of any temperature gradient. The nonreciprocal microscopic fluctuations responsible for generating the energy flows can further be used to power locomotion in, or exert forces on, a viscous fluid. Taken together, our analytical and numerical results elucidate how the geometry and inter-particle interactions of the parity violating material can couple with the non-equilibrium fluctuations of an active bath and enable rectification of energy and motion.   

Robotic swarms as adaptive active matter

The field of active matter has been closely tied to swarming behaviors from its origins. However, a key feature of natural swarms, feedback regulation of activity in space and time, is usually absent in active matter models. Here, we study a model of active matter with adaptive activity inspired by recent experiments on mechanically coupled robotic swarms. We allow for self-propulsion to be modulated over space and time based on local stresses and strains and study how such self-regulated activity modifies transport and the jamming phase diagram.   

Rheology of Active Polymer-like T. Tubifex Worms

Of all complex fluids, it is probably the rheology of polymers we understand best. In-depth insight into the entanglement and reptation of individual polymers allows us to predict for instance the shear-thinning rheology and the behaviour in virtually any flow situation of practical importance. The situation is markedly different when we move from passive to active polymers where the coupling of filament activity, hydrodynamic interactions, and conformations open the way to a plethora of novel structural and dynamical features. Here we experimentally study the rheology of long, slender and entangled living worms (Tubifex tubifex) and propose this system as a new type of active polymer. Its level of activity can be controlled by changing the temperature or by adding small amounts of alcohol to make the worms temporarily inactive. This allow us to unlock existing experimental limitations and to unravel the recent fundamental questions on the mechanical and flow properties of such assembly.   

Collective Behavior of Worm Blobs

We study the aggregation of blackworms( Lumbriculus variegatus ) into large ensembles of entangled, living “blobs” composed of thousands of slender bodies knotted together. To understand the mechanism and advantages of aggregation in these worm blobs, we systematically expose them to various environmental stresses including light and temperature. The diameter of the worm blob can be controlled by both light stimulus history and light intensity. At low light intensity, the blob dilates; conversely, increasing the light intensity contracts the blob and leads to a more entangled and tightly packed state. This behavior also affects the collective movement under thermal stress. Under high light intensity (>1500 Lux) we find that a 5 g (~600 hundred) worm blob placed under a linear temperature gradient between 15 to 50°C stays as a blob and moves collectively to the cold side with a speed of 0.35± 0.01 cm/min. In contrast, if the light intensity is reduced to 400 lux, the worm blob dissipates and individual worms crawl to the cold side with a speed of 0.21± 0.03 cm/min. We find that the number of surviving worms increases when they move as a blob.   

Predicting Crowd Dynamics Using Local Structure

Unstable and active disordered materials exhibit interesting collective properties and nontrivial dynamics. While the behavior of amorphous solids under shear is relatively well-understood, the instabilities in active systems remain difficult to characterize and predict. In the context of dense crowd dynamics, existing work has analyzed position fluctuations in a self-propelled particle (SPP) model to identify Goldstone modes and soft spots in models for human crowds. This analysis requires time-resolved trajectory information in order to form predictions for collective behavior, which can be cumbersome. To address this issue, we have developed a novel method to generate static packings in an artificial potential that reproduce the packing structures in a class of point-of-interest active SPP crowd simulations. These static packings then allow us to precisely identify local structural defects that govern dynamical group behavior, so that we can predict the locations of material-like failures in dense, active SPP models. Unlike previous methods, these predictions can be derived from a single snapshot and could be relevant to preventing dangerous emergent phenomena in real crowd systems.   

The dynamics of in-silico active filamentous elastic swimmers explored using Brownian dynamics simulations

Slender elastic filaments when continuously deformed by active or actuating forces fields can move persistently. In filament-motor assays for instance, animating forces act directionally along the filament; forces thus follow the ensuing filament motion thereby driving and sustaining the deformation. Here, we study the spatiotemporal dynamics of a computationally minimal swimmer - an active elastic filament attached to a viscous cargo that can move in a plane. Locomotion is achieved via competition between activity, elasticity, dissipation and boundary constraints. Examination of the emergent phase space allows us to identify three distinct stable locomoting forms attained by the filament-cargo complex - straight, rotation, and oscillatory flutter. We show that transitions between these states may be trigerred by ramping or dampening noise, by tuning global elasticity and by adjusting the type or softness of the connection between the cargo (head) and the filament (tail). Furthermore, these in-silico swimmers move as soft blobs with effective spatial extent primarily determined by a combination of activity and elasticity. Our results allow for a nuanced understanding of the patterns seen in assays and also offer rules that may guide the design of synthetic soft microswimmers.   

Controlled reversal of vortex chirality in populations of colloidal rollers

Chiral active liquids composed of spinning individual units represent a new class of active materials where both energy and angular momentum is injected at the microscopic level. Spontaneous emergence of particle flocks and global polar states are prime examples of remarkable collective dynamics and self-organization recently observed in active chiral liquids. Formation of the globally correlated polar states in such systems proceeds through an emergence of a macroscopic steadily rotating vortex that spontaneously selects a clockwise or counterclockwise global chiral state. We demonstrate in experiments and simulations that active chiral liquids in a collective vortex state exhibit memory and the subsequent formation of the polar states is not random. Our results provide new fundamental insights into mechanisms of formation of collective polar states in active systems.   

Mechanics of ultrasonically levitated active granular membranes

We explore granular rafts in an acoustic trap consisting of hundreds of macroscopic particles. These close-packed rafts are self-assembled by a sonic depletion force mediated by scattering, which establishes short-range attractions between the constituent particles [1,2]. We show that droplets of this granular fluid display emergent surface tension and elasticity. These droplets interact with the acoustic field, inducing forces and torques that drive coalescence, deformations, and break-up. We use a persistent torque in the acoustic field to extract the droplet surface tension. At the same time, active fluctuations in the acoustic field act as an effective temperature, driving the droplet to explore its configurational space. Microstructural measurements, and the fluctuation spectra of the droplet perimeter, reveal the far-from-equilibrium dynamics of this granular active membrane.

References:
[1] MX Lim, A Souslov, V Vitelli, and HM Jaeger: “Cluster formation by acoustic forces and active fluctuations in levitated granular matter”. Nature Physics 15,460 (2019)
[2] MX Lim, KA Murphy, and HM Jaeger, “Edges control clustering in levitated granular matter”. Granular Matter 21, 77 (2019)   

Impact of wall constraint on the dynamics of self-propelled particles

The presence of wall constraint strongly affects the motions of dispersed particles in a fluid. A striking example can be seen in the dynamics of self-propelled particles near fluid/solid boundaries where the single (collective) motion of such particle(s) depend sensitively on the detailed flow profile around them. In the present study, we investigate the dynamics of two popular types of self-propelled particles, i.e., spherical micro-swimmers (squirmers) and rolling spheres (Quincke rollers) on a flat plate and by means of direct numerical simulation of fluid/particle composite systems with fully resolving the hydrodynamics.   

Construction of non-equilibrium structures by insect aggregations: the case of fire ants

Active matter networks are ubiquitous in nature and synthetic materials, ranging from molecular-scale polymers to macroscale swarms of social insects. These materials are characterized by their transient, reversible crosslinks and non-equilibrium state. Due to these reversible bonds and non-equilibrium states, active matter networks can exhibit directed viscous flow and morphogenesis. One such set of active matter networks is the aggregations formed by the bodies of red imported fire ant (Solenopsis invicta). These ants aggregate into floating rafts when placed in water, and will form steady-state, convective towers if given vertical rods around which to nucleate. In this presentation, we explore the mechanical rules individual fire ants follow on the surfaces of these towers by measuring their statistical velocity distribution, parking rates, and unparking rates, through imaging analysis. We then present and numerically employ a theoretical framework, that bridges these physical rules to the emergent behavior, to replicate the global morphologies observed. Thus, verifying that the postulated physical rules explain the global morphological response of this active matter system and generally informing our understanding of what drives these collective mechanical phenomena.