Effect of various surface conditions on nanochannel flows past permeable walls

Abstract

In this paper, effects of surface conditions on the gas flow in nanochannels with permeable walls have been investigated by molecular dynamics simulations. The hydrodynamic characteristics of the gas flow in nanochannels, including the density distributions, slip length and boundary friction coefficient, have been significantly influenced by the molecular interactions between gas molecules and wall atoms. The density layering phenomena are observed at the fluid–wall interface under a strong fluid–wall interaction (FWI). Particularly, there is a peak of the gas density on the permeable surface where the gas density is increased and about 3 times larger than the bulk one under the strong FWI. It indicates a strong non-continuum density distribution on the permeable surface and the step down in density from nanopores to the bulk region. On the other hand, the nanoscale vortices are produced in the nanopores. Moreover, the mass flux of gas flow in nanopores is reduced, and the hydrodynamic boundary has been shifted above the permeable surface further under the weak FWI. Slip characteristics on the permeable surface under various conditions are explored. It has been found that the slip length on the permeable surface may vary as a logarithmic function with the molecular mean free path. Apparently, the skin friction on the permeable surface is affected by the velocity slip. These results are significant in the understanding of nanoscale hydrodynamics.