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Non-Maxwell slippage induced by surface roughness for microscale gas flow: a molecular dynamics simulation

Rarefied gas flows in rough microchannels are investigated by non-equilibrium molecular dynamics simulations. The surface roughness is modelled by an array of triangular modules. The Maxwell slip model is found to break down due to the surface roughness for gas flows in microchannels with large surface roughness. Non-Maxwell slippage shows that the slip length is smaller than that predicted by the Maxwell model and is nonlinearly related to the mean free path. For larger surface roughness and smaller Knudsen number, the non-Maxwell effect becomes more pronounced. The boundary conditions, generally including velocity slip, no-slip and negative slip, depend not only on the Knudsen number but also on the surface roughness. Simulation results show that A/λ ≈ 1 is a good criterion to validate the no-slip boundary condition and A/λ > 0.3 can be a criterion to judge the occurrence of non-Maxwell slippage, where A is the surface roughness size and λ is the mean free path of gas molecules. The permeability enhanced by the surface roughness may be responsible for the roughness-induced non-Maxwell slippage.