Symmetric Structure In Active Bath

Active matter is one of the hot research subjects in the field of non-equilibrium and complex systems in recent years.The rich and unique properties of active matter both attract many research interests and show great potential in application,e.g.in the design and fabrication of functional micro-/nano-devices.Exploring the novel phenomena and understanding the corresponding underlying physics in active matter system with immersed micro-/nano-objects become the research frontier.In the first chapter,we give a brief overview of current research on the behavior of active Brownian particles in the presence of obstacles or boundaries and the anomalous motion of micro-/nano-objects immersed in the active-particle bath.And,we summarize current understandings on the spontaneous rotation of micro-gears based on the temporal and spatial symmetry breaking.In chapter two,we introduce molecular dynamics simulation method and models.In chapter three,we present our study on the motion of a symmetric micro-gear in active-particle bath.We find that such symmetric gear can also rotate spontaneously,which opposes the conclusions based on the experiments of micro-gears in bacterial suspension.We discuss the reasons for such discrepancy.Symmetric gear does not contribute to the spatial symmetry breaking required for rectified motion.We reveal that the spontaneous rotation of the gear originates from the time inversion symmetry breaking of the motion of the active particles and the self-sustained dynamical spatial symmetry breaking.We modify the activity of the active-particle bath by tuning the rotational diffusion coefficient Dr and the number density φ of the active particles.We find that there are three different rotational states:Spontaneous rotation,spontaneous rotation with flips,and random rotation.We draw phase diagram in the Dr-φ space by analyzing the probability distribution of the rotational velocity.Based on the statistics on the particle and force distributions,we reveal a positive feedback mechanism for sustaining the spontaneous rotation of the gear.In chapter 4,we show the further study on the gear/active particle system.Now,we check an extreme situation,i.e.the symmetric gear is fixed in the active-particle bath.We find that the active particles aggregated around the gear can distribute asymmetrically and form directional circular flow in the situation of small rotational diffusion coefficient and high number density of active particles.Such directional circular flow has the same origin of the spontaneous rotation of the movable gear in chapter 3.It also gives sustained directional torque on the fixed gear,which may cause the gear to rotate spontaneously if the gear is released.We also explored the formation process of the circular flow based on the preliminary simulation results,and further research will be carried out in the future.In the above two chapters,we make clear the mechanisms and the conditions for the directional rotation of a symmetric gear in the active bath.In chapter 5,we further study the interplay between active particles and the gear by setting different rotational speeds of the gear.We explore how the gear with different rotational speeds affects the density distribution and flow pattern of the active particles,and also the reaction of the particles on the rotating gear.We get to the conclusion that there is a stable rotational speed for the gear.Below the speed,the active particles would generate an effective push to accelerate the rotation of the gear.On the contrary,above the speed,the active particles would impede the rotation.By this end,we have a comprehensive understanding on the dynamic process and mechanism of the spontatneous rotation of the gear in active-particle bath.In chapter 6,we discuss the influence of structure size R on rotational state and rotational speed by a cross structure model in active particle bath,and obtain the Dr-R phase diagram.It is found that there is a non-monotonic relationship between size R and rotational speed ω of the cross structure.In chapter 7,we study the shape transformation of a two-dimensional vesicle induced by active particles both inside and outside.We find that the shape of the vesicle changes from circle,to capsule,and eventually to dumbbell with the enhancement of the particle activity.This work provides new ideas to the control of the vesicle morphology and new insights into the dynamics in the vesicle’s shape transformation.In chapter 8,we summarize the conclusions of our work and look into future research plans.