Collective Motions In Self-propelled Colloidal System

Collective motion is a universal phenomenon in nature,such as the flock of bird,the school of fish and bacterial colonies.In physics,these collective motions are in the scope of non-equilibrium self-organization.Therefore,exploring the principles behind these collective motions is of fundamental importance in the study of non-equilibrium phenomenon.Nevertheless,in-situ observation at the individual level is difficult in biosystems.In this case,self-propelled colloid systems offer a good model for the study of non-equilibrium self-organizing.So far,a plenty of active colloidal particles have been developed and employed in the study of collective motions.In this thesis,we explore the basic principles of collective motions in a two-dimensional active colloidal system,in which particles are driven by an electric field via the mechanism of Quincke rotation.We found that the mobility of Quincke rollers are density-dependent: the speed of particles in high-density regions is significantly faster than that of isolated particles in the system.Observations show that this enhanced mobility of particles arises from the enhanced rotation torque as particles pair and align in velocity.More interestingly,we found that freestanding vortex can be formed at high-density region as the electric field is below the critical value to activate isolated particles.It follows that the formation of vortex is completely a consequence of collective motion.Upon increasing the electric field,vortex transforms to donut,swirl subsequently and disappear as the electric field is beyond the critical value.This observation reveals a distinct mechanism for vortex formation and shed new light on our understanding on the possible models of collective motions.