Study On The Characteristics Of Liquid-Vapor Interface By Using Fractal Theory And Molecular Dynamics Simulation

The study on liquid-vapor interface is important not only in the fundamental research but also in engineering applications. From macroscopic point of view, the layer of interface is regarded as a geometric surface. Gibbs assigned certain equivalent energy and surface tension to the interface by using "surplus parameter" method. But in so doing, some difficulties will occur in microscopic space and under some special conditions. Therefore, a systematic study on the characteristics of liquid-vapor interface was conducted in the dissertation by using fractal theory and molecular dynamics simulation.In molecular dynamics simulation, distributions of temperature and density of the system are computed statistically. It was found from computation that for the same fluid, the higher the temperature, the thicker the liquid-vapor interface layer, and the temperatures both in vapor and in liquid region are fluctuating around the average, but the fluctuation in the vapor region is larger than that in the liquid. Also, there exist a valley and a peak in the temperature profile. Explanations on above phenomena are given in the dissertation from microscopic point of view.The dissertation asserts that the interface of liquid-vapor is not a regular geometric surface, but a fractal one fluctuating with time and changing with the conditions (such as the pressure, temperature, and so on). It is known that the concept of continuous space was proposed by Hausdorff in 1919. It is asserted that the spatial dimension may vary continuously, i.e., the dimension may either be an integer or a fraction. Fractal theory based on such a concept was well developed in recent years, but its application in engineering was not so much, especially in the study of the characteristics of liquid-vapor interface. By the aid of computer molecular dynamics simulation, this study applies the fractal theory to investigate the characteristics of liquid-vapor interface.On the basis of the study on distributions of the parameters in interface layer, fractal theory is introduced to study the geometric characteristics of the interface, and the method for computing the fractal dimension of liquid-vapor interface was advanced. By using molecular dynamics simulation, fractal representation and dimension of the interface are obtained. It is demonstrated that the liquid-vapor interface is really fractal. Calculation also shows that the fractal dimension of the interface changes with temperature and pressure, the values of it are between 2 to 3.During the study of the fractal interface, based on the statistics and analysis on a vast amount of data, a rule for making the judgment of the molecules locating on the interface was proposed which is described as follows: in the interface area, we define 1.2 times of the averaged distance between the molecules of liquid as a reference distance, to a certain molecule, if the number of molecules is 2 ~ 5 inside the region within the reference distance to it, we regard it as a surface molecule, if the number is less than 2, we regard it as a vapor molecule, and if the number is more than 5, we regard it as a liquid molecule. Such a rule is called (2 ~ 5)/1.2rm rule.In order to determine the fractal dimension of the interface, structures with different rotation radii are set in the transition region. By using above regulation, the number of surface molecules of each rotation structure with certain radius can be obtained, then correlate these molecule numbers with the corresponding radii, in a logarithmic coordinate system the asymptotic slope of the correlated curve is the fractal dimension of the liquid-vapor interface. Also the fractal representation of interface is obtained by using this regulation.According to thermodynamics, the surface tension of interface is the surplus free energy on a unit area of Gibbs surface. Fractal interface has no concept of area, but it is obvious that different fractal dimension of surface should have different molecular surplus free energy. In this dissertation, a method of deter...