Study On Microscope Characteristics Of Interface By Molecular Dynamics Simulation

The main purpose of the present dissertation is to systematically study the interfacial phenomena and phase transition in microscopic level by means of molecular dynamics simulation (MDS).It is well known that phase transition is a physical phenomenon which appears frequently in engineering and the interface is a face between different phases. Generally speaking, the thickness of the interface is about several diameters of a molecule. The interface is so thin that it could be considered as a geometric surface without thickness. But this thin interface has an important influence on flow, transport properties and chemical process of the fluids nearby it. In recent years, there are many researches on properties of the interface and transportation phenomena among phases. The exploration of the interfacial phenomena is a remarkable field of scientific study for a long time. By using molecular dynamics simulation, the relation between the microscopic properties and the macroscopic properties of a system is investigated on the microscopic scale. The microscopic picture of macroscopic phenomenon is clearly unfolded. It makes a better grasp of the essence of a phenomenon, so that to enrich the theory and direct the practice.The main contributions of the present dissertation are as follows:â‘ The saturated properties of fluids were calculated to determine the correlation between saturated temperature and saturated pressure. In which, argon, methane and water with strong polarity were chosen as the working substances. Then, the simulated results were compared with the calculated data by using the state equation of real gases and the corresponding experimental values. It was proved that the molecular dynamics simulation method was very successful and efficient for real gas to determine its properties. Meanwhile, the validity of the existing state equations of real gas was contrasted and verified using MD. In addition, the microscopic information was withdrawn during the simulation according to the theory proposed by Prof. Danling Zeng, in which the fractional Brownian function was adopted to describe the probability density function of the molecule's random motion. This could be indicated quantitively the derivation degree of a perfect gas from a real gas.â‘¡The phase equilibrium between two phases of liquid and vapor were performed by MD. It is found that except bulk liquid and bulk vapor, there exists an anisotropic interfacial phase. In this section, the profiles of number density, temperature, normal and tangential pressure tensors were studied in detailed for the fluids in the interfacial phase. Because of the anisotropic characteristics in the interface, the effect of the potential cutoff radius was estimated particularly on the densities of saturated vapor and saturated liquid in the homogenous phases and on the surface tension in heterogeneous phase. Then, the new algorithm of choosing the cutoff radius, so called"variable cutoff radius method", was proposed, which satisfies both the calculation of the densities of saturated vapor and saturated liquid in the homogenous phase as well as the surface tension in heterogeneous phase, so as to overcome the disadvantages of the uniform cutoff radius and increase the calculation precision. Furthermore, the influence of fluctuation phenomena on heterogeneous phase was investigated by the new method proposed.â‘¢From the plane interface extended to spherical one, the microscopic process of the evaporation and solidification for liquid droplets was analyzed and investigated on the basis of the theory of radial distribution functions. So that the mechanisms of interface phenomena in the phase transition would be explored in detail. Then the corresponding macroscopic properties were derived during the simulation of microscale liquid droplets, which will lay a solid foundation for predicting the unknown or undetermined properties of macroscopic system.â‘£The effect of solid wall on the fluid molecules was investigated by MD. In the study of interfacial properties of solid-liquid system, a modified and semi-experimental potential function was brought out in the dissertation. In which, two parameters coupling the intermolecular force were introduced. Firstly, the effect of coupling parameters on the interaction potential function was analyzed. Moreover, the mathematical model was established for the interaction between solid wall and liquid molecules. It is found that there exists a layer with properties of both bulk solid and bulk liquid at the same time in the vicinity of the solid wall, named"quasi-ordered layer"by us. In the dissertation, the effect of physical parameters of liquid and solid, respectively, on the quasi-ordered liquid layer was investigated. In addition, the distribution of molecules, the motion of a molecule and pair distribution functions in the quasi-ordered layer were shown in the paper, respectively, which provided a necessary basis of microscopical analysis for the corresponding research. In order to make a whole sight into the properties of fluids constrained in the micro and nanochannel, the transport properties of fluids and Poiseuille flow in a nanochannel were investigated and compared with the corresponding macroscopic properties. In the paper, the distribution of molecules, the change of pair distribution functions, the motion of molecules and its trace images and frequency spectrum chart for fluid constrained in the nanochannel were investigated in detail. Some interesting and significant results were obtained, which provided the basis for studying the properties of the fluids in a nanochannel and investigating the special properties, such as the size effect etc.. Based on the foregoing study, the transport coefficient (diffusivity) of fluids in the nanochannel and the velocity profiles were plotted for the Poeseuille flow. It was seen evidently that the simulated results under some conditions would deviated from the N-S solution. On one hand it shows that there exists the unnegligible difference of fluid properties between microscopic and macroscopic scale from the viewpoint of theory, on the other hand it could provided some references for the practical applications.