Study On Non-equilibrium Kinetic Behavior Of Active Particles In Obstacle Environment

The self-driven movement is ubiquitous in the biological world,from the micro world,the direct transport of molecular motility proteins along the microtubules,the transmission of microorganisms such as bacteria in the soil to the migration of animal groups in the macro world,etc.In recent years,inspired by bioactive substances,people have designed and manufactured a variety of artificial self-propulsion materials,because of their good controllability,they can be used as a good platform to study the non-equilibrium peculiar behavior in the active system.Boundaries and“obstacle”are inevitable in the actual habitat or application environment of the active system,such as intracellular motility proteins,bacteria in the gastrointestinal tract or soil,artificial self-propelled particles in microfluidic devices or blood vessels(self-propelled particles,SPP),etc.Therefore,the emergence behavior of active substances in complex environments has recently attracted a lot of research interest The research object of this thesis is spherical(2D circular)active particle system,the kinetic behavior of the system in a single and multiple circular obstacle environment was systematically studied,the specific sections are arranged as followsIn the first chapter,at first,the active system and its non-equilibrium kinetic behavior are summarized,and then the research progress in recent years is briefly introduced,finally,the molecular dynamics simulation method used in this thesis is briefly introduced.The second chapter studies the movement behavior of active particles around a single circular obstacle,the circular obstacle is densely arranged as a smooth surface by non-active particles and fixed in the active particle bath during the whole simulation.The effect of the size of the circular obstacle,the activity of the active particles,and the density of the active particles on the motion behavior of the system were discussed in detail.Through the flow distribution of active particles,the distribution of driving velocity flow,the radial distribution of the absolute value of tangential velocity,the probability distribution of the absolute value of tangential velocity,the cumulative angle of system rotation,the probability distribution of the absolute value of angular velocity,etc.,it is determined that the active particles will spontaneously form three kinds of rotational forms under the influence of circular obstacles:random rotational state(undirectional rotation),transition state,directional rotation state(vortex).The basic structural conditions and driving mechanism of active particles for directional rotation are analyzed.The reason why directional rotation cannot occur is explored,and the relation between rotational state and circular obstacle size is revealedIn the third chapter,based on the study of a single circular obstacle,we changed the obstacle to two circular obstacles of the same size to study the motion behavior of the active particle system in this environment.the effects of the distance between the two circular obstacles,the active particle density,the magnitude of the active force,and the rotational diffusion coefficient on the motion behavior of the active particles system are discussed in detail.Studies have shown that the radius of the two obstructions is R=15,1)When the active particles density ? is small(?<0.1),regardless of how the distance between the two obstacles changes,the active particles only exhibit a state of aggregation and random rotation;2)When the density of the active particles is moderate(0.15 ???0.25)and if the distance between the two obstacles is zero(dis=0,i.e.the two obstacles are connected),the active particles rotate around the two obstacles in a direction to form a whole vortex;when the distance between the two obstacles is moderate(?=0.2,9?dis?11),the active particles form two cooperative directional rotation vortices between the two circular obstacles;when the distance between the two obstacles continues to increase,the rotation of the system becomes unstable and the vortex disappears;when the distance between the two obstacles is very large,the cooperative rotation of the active particles around the two obstacles does not affect each other,forming two independent vortices;3)When the density of active particles is high(?>0.25),the vortex probability of cooperative directional rotation is very small because of the thick aggregation layer of active particles,the overall rotation fluctuation of the particles is large,and the direction of rotation is difficult to maintain for a long timeIn the fourth chapter,the thesis reviews and summarizes the research work as a whole,and puts forward the prospect of the follow-up research.