Dynamics Of Colloidal Particles In An Active Bath

Active matter systems consist of self-propelling units that can convert ambient or stored energy into persistent motion in systems.Typical active systems range from shoals of fish,flocks of birds,swarming of bacteria in nature and artificial colloidal particles propelled by chemicals or light.Due to the intrinsic nonequilibrium property of active systems,passive particles suspended in an active bath often exhibit unusual behaviors not found in equilibrium systems.The research on dynamics of colloidal particles in active baths would be helpful to explore the micro mechanism of nonequilibrium physics,and provide a new idea for the fabrication of artificial intelligence devices.To investigate the dynamic behaviors of colloidal particles in active baths,in this paper we experimentally track the motion of colloidal particles dispersed in E.coli solutions by microscope video,with the optical trap as a constraint method,and combined with Brown dynamics simulation,we study the generalization of three important physical concepts or relations in the active bath,including the depletion force,the energy equipartition theorem and noise.In the first part,we study the depletion force between colloidal particles in the active bath.Firstly,we propose a direct method in simulation to measure the active depletion force,and the method is verified in equilibrium states.Then the active depletion force under various constraint strength is measured in experiment and simulation,and is found to be affected by the external constraint.Particularly,the measured effective interaction between two free passive particles is qualitatively different from the one between two fixed particles.The micro mechanism of the effect of constraint strength on the active depletion force is discussed and analyzed,which is attributed to the active accumulation of the active spheres in the concave area between the passive microspheres.We find that it is not feasible to try to calculate the active depletion force using the pair correlation function even by introducing the effective temperature.Finally,we find that the friction experienced by the passive particles(a kinematic constraint)similarly influences the effective interaction force between the colloidal particles.In the second part,we study the operation of energy equipartition theorem in the active bath from the perspective of potential energy.Firstly,we experimentally measure the position distribution of colloidal particles constrained in an optical trap in the active bath,which is found to exhibit non-Gaussian characteristics with a wider range than the equilibrium case.This is due to the fact that the passive particle can be pushed further away from the trap center due to the persistent collisions with the bacteria,compared to the case of the thermal bath.Then,we measure the average potential energy of colloidal particles with different constraint strength in experiment and simulation,and find that the average potential energy of a passive particle clearly deviates from the energy equipartition theorem in the equilibrium case.We qualitatively believe that the inverse relationship between the potential energy and the optical trap stiffness coefficient is due to the force balance between the continuous self-propelled force of the active particles and the recovery force from the optical trap.In the third part,we study the correlation between the active noise,the effective temperatures and the external conditions.By theoretical analysis,we extract the active noise from the simulation results,and find that magnitude and correlation time of the active noise experienced by passive particles in the active bath sensitively depend on constraint imposed on the particles.Our results show that three widely used temperatures(separately qualified as the ratio of fluctuation to dissipation,the average potential and kinetic energies of the passive particle)all change significantly with the constraint strength in different manners.The constraint dependence of active noise and effective temperatures arises from the fact that the kinetics of the passive particle is greatly impacted by the constraint during the persistent collisions between the passive and active particles.