Robust boundary flow in chiral active fluid

Spontaneous separation of attractive chiral mixtures

The separation of chiral matter has garnered significant attention due to its wide-ranging applications in biological and chemical processes. In prior researches, particle interactions were predominantly repulsive, but the indiscriminate attraction among particles under attractive interactions makes the separation of mixtures more difficult. The question of whether chiral mixed particles, characterized by attractive effects, can undergo spontaneous separation, remains unresolved. We study a binary mixture of chiral (counterclockwise or clockwise) active particles with attractive interactions. It is demonstrated that attractive chiral particles can undergo spontaneous separation without the aid of any specific strategies. The key factor driving the separation is the attractive interactions, enabling the formation of stable clusters of particles with same chirality. There exist optimal parameters (self-propelled velocity, angular velocity, and packing fraction) at which the separation is optimal. Our results may contribute to a deeper understanding of the mechanisms behind chiral matter separation and potentially catalyze further experimental investigations in this field.

Phase separation, edge currents, and Hall effect for active matter with Magnus dynamics

We examine run-and-tumble disks in two-dimensional systems where the particles also have a Magnus component to their dynamics. For increased activity, we find that the system forms a motility-induced phase-separated (MIPS) state with chiral edge flow around the clusters, where the direction of the current is correlated with the sign of the Magnus term. The stability of the MIPS state is non-monotonic as a function of increasing Magnus term amplitude, with the MIPS region first extending down to lower activities followed by a break up of MIPS at large Magnus amplitudes into a gel-like state. We examine the dynamics in the presence of quenched disorder and a uniform drive and find that the bulk flow exhibits a drive-dependent Hall angle. This is a result of the side jump effect produced by scattering from the pinning sites and is similar to the behavior found for skyrmions in chiral magnets with quenched disorder.

Programming tunable active dynamics in a self-propelled robot

We present a scheme for producing tunable active dynamics in a self-propelled robotic device. The robot moves using the differential drive mechanism where two wheels can vary their instantaneous velocities independently. These velocities are calculated by equating robot's equations of motion in two dimensions with well-established active particle models and encoded into the robot's microcontroller. We demonstrate that the robot can depict active Brownian, run and tumble, and Brownian dynamics with a wide range of parameters. The resulting motion analyzed using particle tracking shows excellent agreement with the theoretically predicted trajectories. Later, we show that its motion can be switched between different dynamics using light intensity as an external parameter. Intriguingly, we demonstrate that the robot can efficiently navigate through many obstacles by performing stochastic reorientations driven by the gradient in light intensity towards a desired location, namely the target. This work opens an avenue for designing tunable active systems with the potential of revealing the physics of active matter and its application for bio- and nature-inspired robotics.

Experimental and numerical study of a second-order transition in the behavior of confined self-propelled particles

In this paper, we conduct experimental investigations on the behavior of confined self-propelled particles within a circular arena, employing small commercial robots capable of locomotion, communication, and information processing. These robots execute circular trajectories, which can be clockwise or counterclockwise, based on two internal states. Using a majority-based stochastic decision algorithm, each robot can reverse its direction based on the states of two neighboring robots. By manipulating a control parameter governing the interaction, the system exhibits a transition from a state where all robots rotate randomly to one where they rotate uniformly in the same direction. Moreover, this transition significantly impacts the trajectories of the robots. To extend our findings to larger systems, we introduce a mathematical model enabling characterization of the order transition type and the resulting trajectories. Our results reveal a second-order transition from active Brownian to chiral motion.

Phase Coexistence and Edge Currents in the Chiral Lennard-Jones Fluid

We study a model chiral fluid in two dimensions composed of Brownian disks interacting via a Lennard-Jones potential and a nonconservative transverse force, mimicking colloids spinning at a given rate. The system exhibits a phase separation between a chiral liquid and a dilute gas phase that can be characterized using a thermodynamic framework. We compute the equations of state and show that the surface tension controls interface corrections to the coexisting pressure predicted from the equal-area construction. Transverse forces increase surface tension and generate edge currents at the liquid-gas interface. The analysis of these currents shows that the rotational viscosity introduced in chiral hydrodynamics is consistent with microscopic bulk mechanical measurements. Chirality can also break the solid phase, giving rise to a dense fluid made of rotating hexatic patches. Our Letter paves the way for the development of the statistical mechanics of chiral particles assemblies.

Density-mediated spin correlations drive edge-to-bulk flow transition in active chiral matter

We demonstrate that edge currents develop in active chiral matter due to boundary shielding over a wide range of densities corresponding to a gas, fluid, and crystal. The system is composed of spinning disk-shaped grains with chirally arranged tilted legs confined in a circular vibrating chamber. The edge currents are shown to increasingly drive circulating bulk flows with area fraction as percolating clusters develop due to increasing spin-coupling between neighbors mediated by frictional contacts. Edge currents are observed even in the dilute limit. While, at low area fraction, the average flux vanishes except within a distance that is of the order of a particle diameter of the boundary, the penetration depth grows with increasing area fraction until a solid-body rotation is achieved corresponding to the highest packing, where the particles are fully caged with hexagonal order and spin in phase with the entire packing. A coarse-grained model, based on the increased collisional interlocking of the particles with area fraction and the emergence of order, captures the observed flow fields.

Rotation and separation of chiral active particles in a ring-shaped channel

Transport of chiral active particles is numerically investigated in a two-dimensional ring-shaped channel. The ring-shaped channel is transversal asymmetric and can induce the directed transport (rotation) of chiral active particles. For the particles with small chirality, they slide along the outer boundary of the channel. For the particles with large chirality, the particles move along some small local circular orbits and can also exhibit directed rotation. Moreover, the rotation effect can be strongly enhanced by modifying the inner boundary geometry. Based on the study of particle rotation, we further study the separation of active particles with different chiralities. It is found that the particles with different chiralities may be distributed in different regions of the ring-shaped channel. Interestingly, these particles can be completely separated by shifting the channel's inner boundary or adding a blocking plate in the channel. Our results may be useful for understanding relevant experimental phenomena and provide a scheme for the separation of binary mixtures.

Odd viscosity-induced Hall-like transport of an active chiral fluid

Broken time-reversal and parity symmetries in active spinner fluids imply a nondissipative "odd viscosity," engendering phenomena unattainable in traditional passive or active fluids. Here we show that the odd viscosity itself can lead to a Hall-like transport when the active chiral fluid flows through a quenched matrix of obstacles, reminiscent of the anomalous Hall effect. The Hall-like velocity depends significantly on the spinner activity and longitudinal flow due to the interplay between odd viscosity and spinner-obstacle collisions. Our findings underscore the importance of odd viscosity in active chiral matter and elucidate its essential role in the anomalous Hall-like effect.

Active Ornstein-Uhlenbeck model for self-propelled particles with inertia

Self-propelled particles, which convert energy into mechanical motion, exhibit inertia if they have a macroscopic size or move inside a gaseous medium, in contrast to micron-sized overdamped particles immersed in a viscous fluid. Here we study an extension of the active Ornstein-Uhlenbeck model, in which self-propulsion is described by colored noise, to access these inertial effects. We summarize and discuss analytical solutions of the particle's mean-squared displacement and velocity autocorrelation function for several settings ranging from a free particle to various external influences, like a linear or harmonic potential and coupling to another particle via a harmonic spring. Taking into account the particular role of the initial particle velocity in a nonstationary setup, we observe all dynamical exponents between zero and four. After the typical inertial time, determined by the particle's mass, the results inherently revert to the behavior of an overdamped particle with the exception of the harmonically confined systems, in which the overall displacement is enhanced by inertia. We further consider an underdamped model for an active particle with a time-dependent mass, which critically affects the displacement in the intermediate time-regime. Most strikingly, for a sufficiently large rate of mass accumulation, the particle's motion is completely governed by inertial effects as it remains superdiffusive for all times.

Emergent spiral vortex of confined biased active particles

Confinement is known to have profound effects on the collective dynamics of many active systems. Here, we investigate a modeled active system in circular confinement consisting of biased active particles, where the direction of active force deviates a biased angle from the principle orientation of the anisotropic interaction. We find that such particles can spontaneously form a spiral vortex with two concentric and counter-rotating regions near the boundary. The emerged vortex can be measured by the vortex order parameter which shows nonmonotonic dependencies on both the biased angle and the strength of the anisotropic interaction. Our work can provide an understanding of such dynamic behaviors and enable different strategies for designing ordered collective behaviors.

Behavior of chiral active nematics confined to nanoscopic circular region

We performed molecular dynamic simulations of a model active nematic confined to a two-dimensional nanoscopic circular region under both tangential and radial anchoring boundary conditions. This active material is assumed to be composed of elongated chiral particles which interact with each other by means of isotropic Lennard-Jones and anisotropic Maier-Saupe-like potentials. These particles have the lateral appendage emitting a jet of some substance generated by a certain internal chemical reaction. As a result, such elongated particles are exposed to both the reactive self-propelled force and the torque that provide an additional translational movement of particles and a self-rotation with respect to their geometric centers. It has been found that the chiral active nematics under consideration form time-dependent vortex-like structures with two +1/2 topological defects which are similar to experimentally observed structures in active materials.

Topologically Protected Transport of Cargo in a Chiral Active Fluid Aided by Odd-Viscosity-Enhanced Depletion Interactions

The discovery of topological edge states that unidirectionally propagate along the boundary of system without backscattering has enabled the development of new design principles for material or information transport. Here, we show that the topological edge flow supported by the chiral active fluid composed of spinners can even robustly transport an immersed intruder with the aid of the spinner-mediated depletion interaction between the intruder and boundary. Importantly, the effective interaction significantly depends on the dissipationless odd viscosity of the chiral active fluid, which originates from the spinning-induced breaking of time-reversal and parity symmetries, rendering the transport controllable. Our findings propose a novel avenue for robust cargo transport and could open a range of new possibilities throughout biological and microfluidic systems.

Time-dependent inertia of self-propelled particles: The Langevin rocket

Many self-propelled objects are large enough to exhibit inertial effects but still suffer from environmental fluctuations. The corresponding basic equations of motion are governed by active Langevin dynamics, which involve inertia, friction, and stochastic noise for both the translational and orientational degrees of freedom coupled via the self-propulsion along the particle orientation. In this paper, we generalize the active Langevin model to time-dependent parameters and explicitly discuss the effect of time-dependent inertia for achiral and chiral particles. Realizations of this situation are manifold, ranging from minirockets (which are self-propelled by burning their own mass), to dust particles in plasma (which lose mass by evaporating material), to walkers with expiring activity. Here we present analytical solutions for several dynamical correlation functions, such as mean-square displacement and orientational and velocity autocorrelation functions. If the parameters exhibit a slow power law in time, we obtain anomalous superdiffusion with a nontrivial dynamical exponent. Finally, we constitute the "Langevin rocket" model by including orientational fluctuations in the traditional Tsiolkovsky rocket equation. We calculate the mean reach of the Langevin rocket and discuss different mass ejection strategies to maximize it. Our results can be tested in experiments on macroscopic robotic or living particles or in self-propelled mesoscopic objects moving in media of low viscosity, such as complex plasma.

Sorting and Extraction of Self-Propelled Chiral Particles by Polarized Wall Currents

The dynamics of self-propelled particles with curved trajectories is investigated. Two modes are observed, a bulk mode with a quasicircular motion and a surface mode with the particles following the walls. The surface mode is the only mode of ballistic transport and the particle current is polar and depends on the particles' chirality. We show that a robust sorting and extraction occurs when the particles explore a domain with two exit gates collecting selectively the particles circling left and right. With a counterslope, the extraction rate is found to increase while the sorting error is reduced.

Dynamical Crystallites of Active Chiral Particles

One of the intrinsic characteristics of far-from-equilibrium systems is the nonrelaxational nature of the system dynamics, which leads to novel properties that cannot be understood and described by conventional pathways based on thermodynamic potentials. Of particular interest are the formation and evolution of ordered patterns composed of active particles that exhibit collective behavior. Here we examine such a type of nonpotential active system, focusing on effects of coupling and competition between chiral particle self-propulsion and self-spinning. It leads to the transition between three bulk dynamical regimes dominated by collective translative motion, spinning-induced structural arrest, and dynamical frustration. In addition, a persistently dynamical state of self-rotating crystallites is identified as a result of a localized-delocalized transition induced by the crystal-melt interface. The mechanism for the breaking of localized bulk states can also be utilized to achieve self-shearing or self-flow of active crystalline layers.

Exceptional non-Hermitian topological edge mode and its application to active matter

Topological materials exhibit edge-localized scattering-free modes protected by their nontrivial bulk topology through the bulk-edge correspondence in Hermitian systems. While topological phenomena have recently been much investigated in non-Hermitian systems with dissipations and injections, the fundamental principle of their edge modes has not fully been established. Here, we reveal that, in non-Hermitian systems, robust gapless edge modes can ubiquitously appear owing to a mechanism that is distinct from bulk topology, thus indicating the breakdown of the bulk-edge correspondence. The robustness of these edge modes originates from yet another topological structure accompanying the branchpoint singularity around an exceptional point, at which eigenvectors coalesce and the Hamiltonian becomes nondiagonalizable. Their characteristic complex eigenenergy spectra are applicable to realize lasing wave packets that propagate along the edge of the sample. We numerically confirm the emergence and the robustness of the proposed edge modes in the prototypical lattice models. Furthermore, we show that these edge modes appear in a model of chiral active matter based on the hydrodynamic description, demonstrating that active matter can exhibit an inherently non-Hermitian topological feature. The proposed general mechanism would serve as an alternative designing principle to realize scattering-free edge current in non-Hermitian devices, going beyond the existing frameworks of non-Hermitian topological phases.

Oscillating collective motion of active rotors in confinement

Due to its inherent out-of-equilibrium nature, active matter in confinement may exhibit collective behavior absent in unconfined systems. Extensive studies have indicated that hydrodynamic or steric interactions between active particles and boundary play an important role in the emergence of collective behavior. However, besides introducing external couplings at the single-particle level, the confinement also induces an inhomogeneous density distribution due to particle-position correlations, whose effect on collective behavior remains unclear. Here, we investigate this effect in a minimal chiral active matter composed of self-spinning rotors through simulation, experiment, and theory. We find that the density inhomogeneity leads to a position-dependent frictional stress that results from interrotor friction and couples the spin to the translation of the particles, which can then drive a striking spatially oscillating collective motion of the chiral active matter along the confinement boundary. Moreover, depending on the oscillation properties, the collective behavior has three different modes as the packing fraction varies. The structural origins of the transitions between the different modes are well identified by the percolation of solid-like regions or the occurrence of defect-induced particle rearrangement. Our results thus show that the confinement-induced inhomogeneity, dynamic structure, and compressibility have significant influences on collective behavior of active matter and should be properly taken into account.