The Molecular Dynamics Simulation For The Capillary Flow In Nanoscale

As the development of the micro and nano electro mechanical system, the research on capillary rise in nanoscale appears to be very important. It faces a lot of difficulties for the experimental method to research the fluid flow in nanoscale. Gradually deepening in this thesis, the microstructure and the dynamic behavior of the fluid near the solid-liquid interface, the calculation of the surface tension in the gas-liquid interface, the calculation of the viscosity and the capillary rise law in nanoscale are numerically investigated. At last, the applicability of the Lucas-Washburn equation is discussed.Firstly, the microstructure and the dynamic behavior of the fluid near the wall in nano slit are investigated. It is found that the density of the fluid near the wall oscillates. With the increase of the slit width, the number of the peaks increases firstly and reachs the stable status finally. The oscillation amplitude decreases firstly instead and reachs the stable status finally. In the oscillation structure, the distance between the first peak and the wall is approximate to the distance of the neighbor peaks, in which the potential of atoms in the neighbor peaks is equal to zero.The density of the fluid in the middle of the slit with a lager width is uniform, which means that the wall can only impact the fluid near it. With the increase of the wall wettability, the amplitude of the oscillation increases. But the position and the number of the peaks keep stable. The number of peaks and the amplitude of the oscillation will decrease as the temperature of the fluid increases. But the position of the peaks keeps stable.Secondly, the potential energy analysis of the liquid finds that the potential energy near the wall oscillates. The peaks’ position of the potential energy distribution and the troughs’ positon of the density distribution are one to one correspondence. The single atom potential energy near the wall is lower than that in the stable region. The quantitave calculation shows that the potential energy reduction in the solid-liquid interface is the source of the gravitational potential energy and the heat of the wetting in the macro capillary flow.Further more, the surface tension and the viscosity, which are two key physical quantities in the capillary flow, are investigated by molecular dynamics simulation. When the number of the atoms in the gas-liquid interface model is larger than 2000 and the length of the model in the direction vertical to the interface is more than three times of the length in the direction paralleled to the interface, the calculation of the surface tension will not be influenced by the model. However, the calculation of the surface tension will be influenced by the truncation radius. As the truncation radius increases, the surface tension increases. Among the three methods of viscosity calculation, the periodic poiseuille flow method has a comparable convergent speed with the stress autocorrelation method. And the shear flow method has the slowest convergent speed. In terms of the accuracy, the periodic poiseuille flow method gives a more accurate result.At last, the capillary flow in nanoscale is investigated. When the nanometer capillary are completely wetted, the capillary rise speed decreases instead as the strength of the interaction between the wall and the fluid increases and the speed is faster than that from the prediction of the Lucas-Washburn equation. Slip near the wall is the reason resulting in the anomalism. A corrected Lucas-Washburn equation which is obtained by bringing the slip length to the Lucas-Washburn equation describes the simulation results better.