Dynamics Of Two-dimensional Active Colloids

There has been a rapid development in the study of soft matter since last century, because soft matter has rich scientific connotation. Now, soft matter has become a focus in physics and other scientific fields.Colloid is a typical type of soft matter. It has rich phases and phase transitions, and therefore becomes an ideal model system for condensed matter physics and physical chemistry and so forth. With the development of computer technology, computer simulation has been widely applied to study the microscopic properties of colloids. Through numerical simulation, we can obtain a large number of physical properties that could not be achieved in the practical experiments.Although amount of experiments and theoretical studies have been devoted to the equilibrium properties of colloids, there has been a systematical studies of the non-equilibrium dynamics of colloids in recent 20 years. Firstly we have reviewed the developments on the non-equilibrium dynamics of charged and magnetic colloids, particularly the computer simulated results.Most recently, researchers have devoted their attention to the active colloids. Active colloid particles have self-propelled interactions, and thereby there will appear clusters with limited sizes in the system. These are closely related to the formation of living organisms. However, most of living organisms are usually formed in the non-equilibrium conditions. Therefore, investigations of non-equilibrium dynamics of active colloids are necessary. More than that, living organisms are widely distributed in the space with electromagnetic fields, exploring the influences of external fields is important. Through investigating the non-equilibrium dynamics of active colloids under the external fields, we can probe the mechanisms of activity and self-organism of biological molecules, even control and tune the activity and self-organism of biological molecules by the external fields.In this thesis, using Langevin molecular dynamics, we have systematically investigated the non-equilibrium dynamics of two-dimensional active colloids on the substrate with randomly distributed point-like pinning centers, and found much rich dynamical phenomena.When the magnetic field is exerted perpendicularly to the system plane, we found that with an increase in the pinning strength, there are dynamical transitions from the elastic crystal to the elastic smectic motions and then to the plastic flows near depinning. Moving clusters appear in the plastic flow when we increase the pinning strength further, accompanied by appearance of a peak in the critical pinning force, i.e., there is an increase and then decrease in the curve of critical pinning force versus the pinning strength. On increasing the magnetic field strength, i.e., the strength of interaction between colloidal particles, the critical pinning force decreases, and there are dynamical transitions from the plastic flows to the elastic smectic motion and then to the elastic crystal motion near depinning. Moving clusters are also observed to occur in the plastic flow. Increasing the strength of Lenard-Jones potential leads to periodic appearances of dynamic phase transitions and the moving clusters.When the magnetic field is exerted parallel to the system plane, we found that colloids move in chains, and with an increase in the magnetic field strength, short chains grow, beam chain becomes to form; the critical pinning force decreases with an increase in the magnetic field strength. Further increasing the magnetic field strength, we found that chains become more apparent and the number of beam chains increase. This indicates that the coherence between colloidal particles increases with the magnetic field strength. Pinning disorder will destroy the chain structure, and the competition between the disorder and the temperature will melt the cluster or chain aggregations.The above research results will be helpful for revealing the mechanism of the activity and self-organism of biological molecules, even for controlling and tuning the activity and self-organism of biological molecules by the external fields.