The Kinetic Behavior And Mechanism Of Passive Particles In Active Baths

Active matter can utilize energy from itself or its surrounding environment to drive continuous directed motion.Due to constant energy input,active systems exhibit complex collective motion,long-time density oscillations,and anomalous rheological properties,among other rich and novel non-equilibrium phenomena.In addition,passive particles immersed in active baths exhibit peculiar non-equilibrium physical characteristics,such as enhanced diffusion,self-assembly,phase separation,and directed motion.Recent studies suggest that the broken spatial symmetry due to the ratchet effect is an effective way to extract energy for passive particle directional motion in an active bath.However,the current understanding of the methods for extracting energy from an active bath using a single symmetric passive particle and the interactions among passive particles in active baths are still limited.Therefore,this paper utilizes an active Brownian particle model to numerically simulate active baths and thoroughly investigate symmetric passive particles’ dynamic behavior and inherent mechanism in active baths.The main results and conclusions of this paper are as follows:(1)This article employs a numerical model to investigate the motion behavior of passive particles in an active bath under different strengths of harmonic potential.A novel method is proposed to extract energy from the active bath using symmetric passive particles,and its underlying mechanism is elucidated.The results show that the dynamical behavior of a rod-shaped passive particle fixed on the x-axis in the active bath depends on the strength of the harmonic potential,the length of the passive particle,and the active velocity.Free rod-shaped passive particles can maintain almost constant directional motion for a long time under the push of neighboring active clusters with spontaneously broken symmetry.If the passive particle is constrained by a suitable harmonic potential,it can spontaneously maintain periodic oscillations at a larger active velocity and passive particle length.The study reveals that the core of periodic oscillations lies in the fact that the harmonic potential’s restoring force triggers the jumping-off of the speed of the passive particle.In addition,the functional relationship between external force and velocity of the passive particle indicates that the effective viscosity of the active bath is negative within a certain parameter range.Finally,this article further elucidates the underlying mechanisms of directed motion and periodic oscillations of passive particles through a simplified one-dimensional theoretical model.(2)This article uses simulation to investigate the effects of various factors such as shape,confinement,and size of passive particles on active depletion forces and elucidates the underlying mechanisms.Previous studies have shown that the depletion forces between large particles in a thermal bath are shape-and confinement-independent,and exhibit short-range attraction.In contrast,our study demonstrates that the form of active depletion forces depends on the shape and confinement of passive particles,while the size of passive particles only regulates the magnitude of the forces.Furthermore,we find that the formation of sharp corners among fixed spherical passive particles facilitates the accumulation of active particles,leading to a dominant repulsion effect in the depletion forces.In contrast,the pipe-like regions formed between stationary rodshaped passive particles intermittently unclog active particles,causing their depletion forces to oscillate between repulsion and attraction.However,passive particles of freedom experience slight displacements upon collision with active particles,causing the inner side to be less prone to gathering active particles,resulting in a significant decrease in the depletion force repulsion peak.This paper examines the dynamics of a single rod-shaped passive particle,offering new insights into the influence of moving boundaries on active baths and presenting a novel method for extracting energy from such baths using passive particles.Additionally,the results of this paper on active depletion contribute to a better understanding and development of active systems,and at the same time can provide some theoretical guidance for the self-assembly of passive particles.