The Collective Diffusion Characteristics And Control Of Fluid Particles

The collective diffusion coefficient(CDC)is the main parameter that characterizes the transport and diffusion properties of fluid particles.There are two approaches to obtain the collective diffusion coefficient.The first one is to calculate the CDC indirectly through the self-diffusion coefficient,and the other is to get it directly from Fick's law.In general,the diffusion space of the particles is finite,so in many cases,the particles are regarded as restricted diffusion.However,some previous studies use the method of dealing with free diffusion to deal with restricted diffusion,which maybe lead to large error and mislead the experimental verification.Therefore,it is necessary to investigate the method of the collective diffusion coefficient in a finite space as well as to compare the advantages and disadvantages of different methods.Asymmetric effects often lead to the appearance of special phenomena such as asymmetry of mass flow and molecular orientation.It is expected to perform many potential applications in the separation of particles.In this thesis,using molecular dynamics and Monte Carlo method I will establish different models to study the calculation method of the collective diffusion coefficient in the finite space as well as the asymmetric effects of particle transport.The study includes three parts:First,to study the transport diffusion coefficients of molecules in finite space.It is found that the diffusion coefficient is related to the sampling time in the finite space.This thesis answers some doubts about the two calculation methods of the collective diffusion coefficient.I also compare the advantages and disadvantages of two calculation methods and propose an appropriate method to calculate the self-diffusion coefficient and the collective diffusion coefficient in the finite space.Second,to study the collective diffusion behavior of asymmetric molecules(diatomic molecules).It is revealed that when asymmetric diatomic molecules transport in two-dimensional model,the molecular orientation is not uniform.The factors,such as the asymmetry of the molecule itself,the ambient temperature,the concentration gradient and temperature gradient,will affect the molecular orientation.The physical mechanism can be explained by the Onsager formula.Third,to study the collective diffusion in asymmetric fluid environment.It is found that it can produce directional molecular transport in asymmetric micro-channels under the alternating electric field.Moreover,the direction of sodium and potassium ionic flow can be different.The study provide a potential method to separate sodium and potassium ions by asymmetric nano-channels and alternating current fields.