Study On The Mode-locking Of Two-dimensional Active Magnetized Colloids

There are many kinds of substances,which can be divided into macroscopic,microscopic and mesoscopic substances according to their sizes.The study of macroscopic and microscopic substances has been relatively mature.However,the study of mesoscopic substances is just emerging.Soft matter,such as human blood,cells,DNA carrying genetic information and some life-related substances and so forth,are all in mesoscopic scale.Therefore,the 21 st century is called as a century of soft matter or life science.Since 1991,when the Nobel prize in physics was awarded to the field of soft matter,more and more people began to explore this field.Colloid is a typical soft matter with a scale between nanometer and micrometer,which is much larger than that of atom and molecule.They are easy to fabricate in experiments.In addition,colloids can form many different kinds of phases,which are rich in changes and easy to control.Thus,they are a good model material.Colloid research involves a wide range of disciplines,including physics,chemistry,biology,materials and condensed matter.However,most of the research work focuses on the equilibrium physical and chemical properties of colloids.In recent years,the dynamic characteristics of colloids under non-equilibrium conditions have attracted extensive attention of experimental and theoretical researchers,especially the formation of self-organization of active colloids is closely related to the formation of biological cells,and the formation of living organisms is mostly carried out under non-equilibrium conditions.Therefore,it is of great significance to study the dynamic characteristics of colloids in non-equilibrium state for revealing the origin of life.In the first chapter of this thesis,the concept of soft matter,the influence of entropy and the formation of self-organization are summarized.In the second chapter,as typical representatives of soft matter,colloids are reviewed,including charged and magnetized colloids,active colloids,Janus colloidal particles and the latest research progress of colloidal dynamics.In the third chapter of this thesis,the research progress of mode-lockingproperties of colloids is summarized,including directional mode-locking properties and interference mode-locking properties under AC and DC external fields.The directional mode-locking and interference mode-locking properties of two-dimensional active magnetized colloids on the substrate with periodically distributed pinning centers are systematically studied by Langevin molecular dynamics.The research results of directional mode-lockig characteristics show that the motions of active colloidal particles are collectively locked in some specific symmetrical directions(30° and 60°)of the substrate,and the directional mode-locking is more likely to occur at low temperature and small angle field.This is consistent with the results of passive colloids.However,the activity leads to the aggregation of particles into clusters,and the direction of coherent motion of some particles in clusters deviates from the mode-locking direction,so that not all colloidal particles are strictly locked in the symmetrical direction of the substrate,which is different from the passive colloidal system.The results of the study on the interference mode-locking characteristics show that in a certain range of AC amplitude and frequency,there are mode-locking steps on the curve of the average velocity of colloidal particles varying with DC external field force.The step width oscillates with AC amplitude as a like first-order Bessel function,varies in inverse parabola with AC frequency,pinning strength and repulsive interaction strength between particles.The average velocity of colloidal particles at the step increases linearly with the AC frequency and the repulsive interaction strength between particles,but does not change with the AC amplitude and frequency.The above rules are consistent with passive systems,but more obvious than passive systems.The reason is that the activity improves the coherence between moving particles,which leads to more obvious interference mode-locking.The results of this study are important for the separation of different kinds of active mesoscopic particles using DC or AC/DC external fields.