Colloidal Particles In Electric Field Under The Physical Properties

Colloid is a type of mixture where some tiny particles are dispersed throughout the medium.Both the energy and the entropy play an important role in the micro structure formation of colloid.As a result,colloids have many particular properties such as selforganizing. Moreover,colloids can easily be modulated by electric field,temperature, PH scale,etc.The rapid and remarkable response is the reason why colloids raise so many interests.The dissertation is devoted to the study of micro behavior of colloidal particles under applied electric field theoretically which includes the interparticle forces between particles and the dynamic effects of nonlinear responses.The theoretic models are compared with experiment results in order to explain the experiments and give advices. The dissertation is organized as follows.In Chapter One,we present the background introduction,which is made up of two parts:electrorheological(ER) fluids and dynamic behaviors of colloidal particles.The basic concepts,the recent research development,and the problems we faced are introduced.In Chapter Two,we deeply study the interparticle forces between colloidal particles. Firstly,we introduce the theory methods to calculate the interparticle forces between two dielectric particles.These methods are Green's function approach method,spectral representation method,and multi-mirror method.Considering the effect of conductivity, we introduce a complex dielectric constant to our theory.Secondly,we find that when the particles are rotating the interparticle forces will reduce because of the relaxation process of the surface charge on the particles.The force reduction corresponds to experiment results.Furthermore,we suggest that we can obtain the value of relaxation time from the force reduction which can be detected experimentally.Thirdly,we extend the theory to graded particles.And we find that the interparticle forces change from attractive to repulsive and again to attractive with increasing frequency of applied field.The magnitude and the crossover frequency are quite related to the electric profile of particles.Finally,for the sake of reveal the mechanism of giant electrorheological (GER) fluids,we take the orientation polarization of coated material into account.The relation between the number of effective orientated dipoles and the local field fits the Langevin function.In Chapter Three,we study the nonlinear frequency response of colloids.We find that the coupling between particle rotation and applied field enriches frequencies of the higher-order harmonics.The amplitude of each harmonic response can be calculated in our theory.Specially,the triple frequency generation under a dc field will have a maximum when the rotating particle has a finite conductivity.This phenomenon is also related to the relaxation process.Then,we detected the duple and triple harmonic generation of porcine erythrocyte.In the fitting,the form factor and the volume fraction are considered.In Chapter Four,we study two application cases.In the first case,we further discuss how to import the influence of the coated shell for the GER material.And we discuss the feasibility of ion-driven ER fluids.The scaling behaviors in settling process of fractal aggregates are investigated by hydrodynamics in the second case.In Chapter Five,we make a conclusion,and look forward for the future work.