Molecular Dynamics Simulation Of Velocity Slip And Its Effect On Micro-and Nanoscale Flow

With the rapid development of micro/nano-electro-mechanical systems (MEMS/NEMS) and nanotechnology in recent years, it is quite necessary to have deep insight into micro- and nanoscale fluidics. By reason of the decreased fluid system size, velocity slip affects both the microscale gas flow and the nanoscale liquid flow greatly. The present dissertation investigates the slip phenomenon and its effect on micro- and nanoscale flows by molecular dynamics simulation method.Based on analyses of the equilibrium state characteristics, the nonequilibrium momentum transports and the slip boundary condition of a two-dimensional ideal gas system, the relationship between the two-dimensional and the three-dimensional flow systems is determined quantitatively. Thus, the two-dimensional molecular dynamics (2DMD) method can be applied to simulate microscale gas flows with characteristic lengths of submicron scale.The temperature dependence of the tangential momentum accommodation coefficient (TMAC) is investigated by examining gas flows in a submicron channel by using molecular dynamics simulations. The results show that the TMAC decreases with the increasing temperature following an exponential decay law, and is more sensitive to lower temperatures than to higher ones. The molecular trapping-desorption behaviors near the channel surface are found to be responsible for this dependence.The effect of velocity slip on microscale gas flows is found to depend on three characteristic lengths, including the roughness size A, the mean free path of gas molecules X, and the flow system's characteristic length H. Though the Knudsen number indicates that the flows are in the slip regime (0.001 1. Only with A/λ<.0.2, the Maxwell slip model works well to characterize the slip length of gas microflows.Simulations of the liquid flow in smooth nano-channels show that wettabilitybetween the liquid and the channel surface dominates the boundary velocity slip and the flow friction. Large slip is observed for liquid flow over hydrophobic surfaces. It implies that surface nano-structures may be applied to control the surface wettability and the nanoflow friction. This idea is confirmed by present molecular dynamics simulation. The molecular dynamics simulations of contact angles of argon liquid droplets on structured model solids indicate that the nano-structure surface may show superhydrophobicity if the potential interaction between the liquid and the surface is meanwhile weak. The superhydrophobicity leads to large velocity slip of liquid flow over nano-structure surfaces and reduces the nanoflow friction.