Flow And Boundary Slip Properties Of Gas Mixture In Parallel-plate Nanochannels

At present,the development of science and technology is constantly extending towards two directions – macro scale and micro scale.While human explores the unknown outer space,micro/nano scale field has attracted considerable attention.Since the end of 20 th century,micro/nano electromechanical system(MEMS/NEMS)and microfluidic devices was born and has got a rapid development,which induces a trend of device in the development at micro/nano scale.The research of micro/nano gas flow greatly promotes the theory development of the fluid dynamics.With the decrease of the channel feature sizes,the basic theory of macroscopic fluid dynamics is not available to describe the phenomenon of micro/nano gas flow.Hence,developing the research of the micro/nano gas flow that is of important significance to match the relevant basic theory and further provide important theoretical basis for the device design and production.In this thesis,the flow and boundary slip properties of gas mixture in parallel – plate nanochannels are explores by molecular dynamics simulations,the research may offer some insight and clues to further explore the mechanism of gas flows in nanochanels.Molecular dynamics simulations have been carried out to study the Couette flow of Nitrogen-Oxygen mixture confined in parallel-plate nanochannels with the channel width ranging from 3.155 to 9.465 nm.The simulation results show that the shear stress linearly increases with the increase of the shear rate when the channel width is a constant,implying that the shear viscosity of the gas flow is constant in our simulations,which increases with the channel width.However,as the channel width reaches a threshold,the effect of channel width on fluid viscosity decreases gradually.And the simulation results also found that the slip velocity in the gas-solid interface has a decreasing trend within various channel width.When the size of channel width is smaller,a significant reduction in slip velocity is detected,and followed by a linear and slow decrease in slip velocity at a large size of channel width.Moreover,the influences of the gas-solid coupling strength on the velocity slip at the fluid boundary are investigated.The results show that the slip velocity decreases with increasing the solid-gas coupling strength.Interestingly,the shear viscosity of fluid confined in the nanochannels increases with enlarging the component ratio of nitrogen,which is different from the continuum theory that the viscosity of oxygen is always higher than that of nitrogen at the same temperature at macroscale.The main reason is that the solid-like structure easily occurs for the nitrogen molecules confined in the platinum parallel-plate nanochannels at the gas flow boundaries.Then,the model of Nitrogen-Oxygen mixture with different proportions of carbon dioxide has been studied by molecular dynamics simulations.Simulation results show that the slip velocity of gas mixture confined in nanochannels increases with enlarging the component ratio of carbon dioxide.And the velocity of gas mixture linearly increases across channel width when the proportion of carbon dioxide is less than 20%.However,the velocity of gas mixture is no longer linearly increasing across channel width when the proportion of carbon dioxide is more than 40%.The velocity profiles of gas mixture in nanochannels can be divided into two regions,the bulk region and the near wall region.The velocity of gas mixture is still linearly increasing across channel width in the bulk region,whereas it is nonlinear in the near wall regions.Simulation results also indicated that the variation trend of slip velocity of the gas mixture with 20% carbon dioxide is similar to that of the nitrogen-oxygen mixture with increasing the channel width.In addition,the slip velocity of the former is always larger than that of the latter for different channel width while the difference slip velocity between the two cases is not sensitive to the channel width.