Simulations of porous gaseous flows are routinely used to investigate membrane permeation in catalytic adsorption and separation problems. Although basic continuum equations are supposed to breakdown in these nanoscale pores, many studies of force/flow relations assume flow to be linear in chemical potential or pressure differences. We present atomistic simulations of the flow of a Lennard-Jones gas through a cylindrical pore with distant, Langevin forcing at densities stretching through the transition and free flow regimes. We find that even at the scale of 10 molecular diameters, local equilibrium reasoning can be applied to the steady-state. However, we also identify two finite-size effects that prevent application of the original Joule-Thomson macroscopic reasoning. First, there is a steady flow of heat from the high-pressure reservoir backward to the thermostat region, and second, a significant proportion of the channel flow originates from the monolayer adsorbed to the flat outer wall. Despite these effects, the difference in pressure building up between the two sides of the pore serves as a clear order parameter for analysis of the system.

Finite size effects in simulations of molecular Joule-Thomson flow

255th National Meeting (2018)

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