The Dynamic Characteristics Of 2D Magnetized Colloids Driven By Dc And Ac Forces

Colloid widely exists in our life, and it closely affects our daily lives. Colloid is stable because the sizes of colloidal particles are between manometers and microns, and at this scale, colloid is not easy to coagula but presents a Brownian motion to keep its stable. Especially, colloid is able to present various phase and phase transition, such as gas, liquid, solid and liquid crystalline phases and their transitions. Therefore, colloid has attracted much attention. At the same time, colloid has become an ideal model system for physics, material science, physical chemistry and so forth.With the development of microscopy, the movement of colloidal particles can be observed directly under a microscope. This is a great breakthrough in experiment. Under this case, the theoretical results of colloid dynamics could be supported by experiment with the microscopic technology. Because the external driving force will lead the equilibrium system into a non-equilibrium moving state, which will cause the problem to be complex, at present, the research of the colloid dynamics mainly focuses on the low-dimensional system.This thesis mainly investigates the dynamic features of two-dimensional magnetic colloid system under the external field. The direct-current field responses of the charged and the magnetic colloidal systems have been studied extensively. However, the research of the dynamics of colloidal system driven by direct- and alternating-current field is rare. This thesis aims at the dynamics of two-dimensional magnetic colloidal system driven by the direct- and alternating-current external fields.The dynamic features of the two-dimensional magnetic colloid system on the substrates with periodically and randomly distributed pinning centers have been investigated respectively. With the alternating-current driving force, obvious steps in the characteristics of the averaged colloid velocity versus the direct-current driving force were observed within a certain ranges of alternating-current amplitude and frequency. At the steps the colloidal particles move in parallel channels along the external direct-current force, the channels are stable, and the dynamic structure factors show evidently six Bragg peaks. These reveal that colloidal particles move in a typical moving crystalline phase. The formation of steps is due to that the colloidal particles move coherently along the direction of the direct-current drive, and when the alternating-current drive is applied, mode-locking behavior occurs when the frequency of coherent movement of colloidal particles is close to that of alternating-current drive, causing the occurrence of interference mode-locking steps in the characteristics of the averaged velocity versus the direct-current force.Furthermore, the variations of the step width and the averaged velocity at the step have been studied systematically. The step width was found to vary in a Bessel-function oscillation with the amplitude of the alternating-current drive. This agrees with the results of the magnetic flux dynamics. But it was observed to change in a inverted parabola form with the frequency of the alternating-current drive and the substrate pinning strength as well as the interaction strength between colloidal particles. The averaged velocity of colloidal particles at the step was shown to increase linearly with the frequency of the alternating force and the interaction strength between colloidal particles.The obtained results in this thesis are helpful for the continuous fractionation of the mesoscopic particles and the cell as well as the biological macromolecules.