Session G20: Emergent Mechanics of Active, Robotic, and Living Materials II

Non-dispersive, non-reciprocal amplification of wavepackets in sonic metamaterials

Active metamaterials harbor sound processing capabilities that are impossible in passive structures. However, the desirable response is often restricted to narrow frequency ranges, limiting its utility for signals composed of a mix of frequencies. For instance, parametric amplification--injecting energy into sound waves via periodic modulation of elastic stiffnesses--is typically restricted to specific multiples of the modulation frequency. Inspired by a recent proposal in optics, we describe a mechanism for one-way amplification of sound waves across an entire frequency band using a traveling-wave modulation of local stiffnesses. When the speed of the modulation wave approaches that of the speed of sound in the metamaterial--a regime called the sonic limit--nearly all modes in the forward-propagating acoustic band are amplified, whereas no amplification occurs in the reverse-propagating band. We find wide ranges of parameters for which the strength of the effect is nearly uniform across the band, enabling amplification of wavepackets while preserving their speed, shape, and spectral content. Our mechanism provides a route to designing acoustic metamaterials which can propagate wave pulses without losses or distortion across a wide range of frequencies.   

Light-driven Synchronization of Active Particles in Marangoni Optical Traps

We demonstrate a versatile platform of photoresponsive active particles that exhibit synchronized oscillation and rotation. The active particles, hydrogel nanocomposite disks, are patterned with gold nanoparticles (AuNPs) that enable a photothermal response. When confined to an air-water interface, spatially inhomogeneous illumination combined with the AuNP-patterning allows for the modulation of the Marangoni force on the particles. We observe distinct modes of light-driven active motion, including oscillation, spinning, and orbiting of individual particles around a trap. When multiple oscillators and rotors are arranged in proximity, we observe strong synchronization arising from thermal interactions between the particles. We study how the nature of the synchronization can be reconfigured using the geometry of the particles and the illumination patterns. Our experiments offer promise for understanding the collective behavior of active matter that arises from different modes of broken symmetry and motions.   

Microrobot Controlled Electrodeposition Metamaterial

We show progress towards developing a metamaterial made from a swarm of microscale robots.  The robots, in solution, electrodeposit nickel on their bodies with local application of energy.  The material can actively adapt it's structural properties through growth and removal of nickel in the space between robots.  We demonstrate that electrodeposition of nickel on microscale electrodes is achievable, that adjacent growth fronts can be merged to form bonds between robots, and that electrodeposited nickel can be entirely removed from the electrodes.  These results pave the way for a new breed of adaptable metallic metamaterial.   

Crowd-surfing particles sorted by magnetic artificial cilia

One of the major challenges in modern robotics is controlling micromanipulation by active and adaptive materials. In the respiratory system, such actuation enables pathogen clearance by means of motile cilia. While various types of artificial cilia have been engineered recently, they often involve complex manufacturing protocols and focus on transporting liquids only. Here, soft magnetic carpets are created via an easy self-assembly route based on the Rosensweig instability. These carpets can transport not only liquids but also solid objects that are larger and heavier than the artificial cilia, using a crowd-surfing effect. This amphibious transportation is locally and reconfigurably tunable by simple micromagnets or advanced programmable magnetic fields with a high degree of spatial resolution. Two surprising cargo reversal effects are identified and modeled due to collective ciliary motion and nontrivial elastohydrodynamics. While the active carpets are generally applicable to integrated control systems for transport, mixing, and sorting, these effects can also be exploited for microfluidic viscosimetry and elastometry.   

Emergent Beating in Colloidal Matter: Stabilization via Symmetry Breaking

Emergent order is routinely observed in far-from-equilibrium collectives as varied as swarms of robots and active colloidal suspensions. Here, we study the self-organized behaviors of a colloidal system of active Janus particles (~250um) suspended at the air-liquid interface of a hydrogen peroxide droplet. The particles consist of an upward facing SU8 side and a downward facing Pt patch that grows oxygen bubbles by catalyzing the decomposition of the peroxide bath. An individual particle's bubble growth is self-limited by the resulting decrease in exposed surface area of the Pt patch, leading to stable bubble growth that does not tend to burst spontaneously. Once other particles are introduced, however, their respective bubbles can merge into larger shared bubbles capable of bursting. We show that a pair of particles exhibit robustly periodic beating despite the well-known chaotic dynamics of bubble collapse. As additional particles are introduced, the system gradually transitions from periodic beating into uncorrelated random bursts in time. However, we find that breaking permutation symmetry by increasing the Pt patch size of a single particle in the collective allows the system to access configurations where many-body order is possible, and recover the robust periodic beating behavior originally observed solely in the two-particle setting. Using techniques from dynamical systems, stochastic processes, and nonequilibrium thermodynamics, we characterize the different behavioral regimes of the system, the nature of transitions between regimes, and offer hypotheses for the role of symmetry-breaking in the emergent periodicity of the system.   

Robotic swimming in curved space via geometric phase

In flat space, locomotion requires momentum exchange with the surrounding environment. However, in curved space, the non-commutativity of translations can permit locomotion without momentum exchange, just as rotational non-commutivity allows falling cats and lizards to change their orientations. Here we illustrate this principle experimentally via a robot that locomotes via shape changes without any additional forces while confined to a spherical surface with a radius of 0.3 m. Permitting the robot to rotate about the vertical axis, we minimize external forces, particularly friction and gravity. We then observe an initial angular displacement per cycle matching the geometric (Berry) phase induced by the robot's shape changes for various patterns and magnitudes of such changes. A gait with each stroke displacing a robot's internal components 30 degrees advances the robot 0.6 degrees per cycle. In contrast with the ideal, force-free case, frictional dissipation and weak gravitational forces eventually arrest the robot, while also imbuing it with momentum in the opposite direction.  Our work demonstrates how the interaction between environmental curvature, active driving and geometric phases yields rich, exotic phenomena.   

Re-programmable non-reciprocal metamaterials

Asymmetric structures of nonlinear elements have been previously shown to break reciprocity in passive metamaterials by utilizing bistability to permit the propagation of transition waves in one direction. The configuration of these metamaterials typically limits the propagation of the wave to a single direction, speed, and occurrence. Here we present a simple design for a repulsive lattice of magnetic dipoles which allow for all of these parameters to be tuned, enabled, or disabled through the use of a simple mechanical switch. We present a theoretical map of geometries that allow for reciprocal and non-reciprocal wave propagation. We verify this using numerical simulations and demonstrate that the constant velocity of the transition waves are dependant on the material geometry regardless of the magnitude of impulse. In experiments we validate our numerical findings in regards to the constant velocity of the wave, reversibility of the switch mechanism, and the ability to direct the wave along a curved path. This result suggests the potential use of our material for tuning wave speed.   

Motile dislocations melt an odd crystal

2d crystals melt as a result of the proliferation and annihilation of dislocations ruled by The competition between thermal fluctuations and potential forces. We show that introducing transverse interactions dramatically alters  dislocations dynamics and phase behavior. An isolated dislocation moves ballistically when transverse forces are present. 

Crystal stability is then chiefly determined by the competition between dislocation motility and elastic interactions. These dynamics produce a new crystalline whorl state that we consistently observe in minimal simulations and a colloidal spinner experiment.   

Simulations of Elastic Active Matter

Active matter encompass a wide range of non-equilibrium systems in which the constituents can convert other forms of energy into mechanical work. Active matter can give rise to intriguing phenomena that do not arise in equilibrium systems. One example is collective dynamics, which have been extensively examined in terms of swarming, flocking and spontaneous flows in active fluids. In this work, we rely on particle-based simulations to investigate the elastic properties of active matter. We find two activity-driven elastic modes in a two-dimensional system.  Our calculations are complemented by theoretical analyses and exhibit excellent agreement with bacteria-based experiments. Our results uncover the physics of the collective dynamics in active solids, deepening our understanding of active matter.   

Odd elastic response of colloidal spinner crystals

The elastic response of two-dimensional solids assembled from passive units that interact isotropically is completely determined by their bulk and shear moduli. Driving these units out of equilibrium enables the addition of isotropic transverse interactions. This generically expands the allowed mechanics to include intrinsic chiral stresses and moduli unconstrained by energy conservation. To each of these additions are associated new phenomena, from the self-propulsion of dislocations driven by odd stress to the anomalous coupling of crystalline deformations enabled by odd elasticity. We ground our discussion using coarse-grained microscopic models, molecular dynamics simulations, and colloidal experiments.   

Collision Operators for Mesoscale Simulations of Flocks

Flocking is the quintessential example of emergent collective behaviour in active systems, seen across scales from bacterial dynamics to drone swarms. The agent-based Vicsek model, and its hydrodynamic limit, continue revealing exciting physics, including new non-equilibrium phases. However, understanding and simulating flocks at the mesoscale, i.e., between the microscopic and hydrodynamic limits, remains a significant challenge. We utilize the Multi-Particle Collision Dynamics framework and develop a set of collision operators to simulate flocking at the mesoscale. Comparing the scalar order parameters, density fluctuations, and autocorrelation functions to those obtained from agent-based models, we show that our proposed operators reproduce the dynamics and out-of-equilibrium phases of the Vicsek model and its variants. We expect our algorithm to reveal exciting dynamics at the mesoscale between micro and macroscopic limits of flocking, offer a pathway to coarse-graining other active systems, and allow extensions to more complex flocking behaviours.   

Active and robotic materials: from self-amplified waves to self-sustained deformations

Active materials denote materials in which local active forces generate work. They correspond to many biological systems in which local chemical reactions trigger cyclic or feedback-based self-deformations. The recent development of robotics now enables the integration of such paradigm in artificial materials, which offers new ways for materials to interact with their environment. So far, in many of these systems, it remains difficult to predict which mechanical properties emerge from the presence of local active forces. Here, we use structured model experiments combining passive elasticity and local active forces to develop models predicting the mechanical properties of such active materials and demonstrate how the interplay between dissipation, restoring forces and active forces controls their dynamics. For small amplitudes of active forces, these active materials are characterized by new elastic moduli and wave phenomena, such as spatially asymmetric standing waves at all frequencies. For larger amplitudes of the active forces, waves are self-amplified, which destabilizes the material. Yet, for the right balance of each force in presence, active materials can stabilize in a state of self-deformation emerging from nonlinear self-sustained work cycles. We harness these inherently nonlinear cycles to power and control processes such as impact control and locomotion. Beyond distributed robotics, our work suggests how these nonlinear work cycles and associated functionalities can emerge as solutions of the non-conservative dynamics of active and biological materials.   

Polar Fluctuations Lead to Extensile Nematic Behaviour in Confluent Tissues

Collective active nematic behaviour has been found to mediate a growing number of important biological processes, such as cell extrusion in epithelial monolayers or the formation of layers in Myxococcus xanthus colonies. Intriguingly, isolated epithelial cells display polar motility and generate contractile nematic stresses when elongated, but exhibit extensile nematic behaviour when part of a tissue. How these cells can exhibit active nematic behaviour at the tissue level is poorly understood and deciphering the mechanisms behind this behaviour is necessary for elucidating fundamental biological processes. Here, we resolve this cellular to tissue level disconnect with a linearized hydrodynamic theory that applies universally in the small fluctuation regime to tissues in both fluid and solid states. We show that polar fluctuations generically generate extensile stresses in confluent tissues, and so can drive extensile collective behaviour in cells that are contractile in isolation. We then validate our theory by demonstrating the appearance of extensile nematic defects in both fluid and solid cell-resolution models with polar active forces. Our results also demonstrate that materials with no inherently nematic active forces can exhibit active nematic collective behaviour, and that the active nematic signatures observed in epithelial tissues can naturally emerge downstream of fundamentally polar processes.