POISEUILLE FLOW OF MOLECULAR FLUIDS

We examine a generalised Navier-Stokes theory applicable to fluids com posed of non-spherical molecules. We compare the theoretical predictio ns for flow velocity and viscosity with results obtained from nonequil ibrium molecular dynamics (NEMD) simulations of a fluid undergoing gra vity fed flow down a rectangular channel. We study two different fluid s: one composed of spherical particles and the other composed of uniax ial molecules at two different channel widths, W=5.1 and 10.2 molecula r diameters. Our results show that aside from boundary effects due to the roughness of the atomistic walls, the generalised Navier-Stokes th eory gives a reasonable qualitative account of a fluid composed of mol ecules that possess spin, even in a channel that is only 10.2 molecula r diameters wide. In the simple fluid case, we find that classical beh aviour is approached at this same channel width (W=10.2) but in the W= 5.1 channel, Navier-Stokes theory begins to break down. For both chann el widths we find that the assumption of a constant shear viscosity is incorrect and, further, that the viscosity in the narrow channel of 5 .1 molecular diameters is probably non-local.