@article{travis_gubbins_2001, title={Computer simulation of isothermal mass transport in graphite slit pores}, volume={27}, ISSN={["1029-0435"]}, DOI={10.1080/08927020108031361}, abstractNote={Abstract Results are presented from a simulation study of the mass transport of oxygen and nitrogen through graphite slit pores. The work is motivated by an attempt to understand the molecular origins of the kinetic selectivity displayed when air is separated into its major components using pressure swing adsorption. A combination of non-equilibrium molecular dynamics (NEMD), equilibrium molecular dynamics (EMD) and grand canonical Monte Carlo methods has been employed in our study to extract the maximum information. Transport diffusivities, self-diffusivities, permeabilities and Darken thermodynamic factors have been calculated as a function of pore width and temperature for pure component oxygen and nitrogen. In addition, new EMD simulation data for an 80:20 mixture of nitrogen and oxygen is reported, including a direct calculation of the Stefan-Maxwell coefficients. The results are discussed in terms of the oxygen selectivity and the possible mechanisms, which increase or decrease this quantity. We find that the pore width behaviour of the diffusion coefficients consists of three distinct regimes: a regime at larger pore widths in which single component diffusion coefficients are largely independent of pore width, an optimum pore width at which both diffusivities increase substantially but the slit pore is selective towards nitrogen, and a regime at very low pore widths at which the diffusivities decrease sharply, but the slits are selective towards oxygen. The mechanism behind each of these regimes is discussed in terms of “entropic” effects and potential barrier heights. We have also found that permeability selectivity is substantially reduced in a mixture of the two gases with a composition similar to that of air. Cross diffusion coefficients in the mixture have been calculated and shown to be non-negligible.}, number={5-6}, journal={MOLECULAR SIMULATION}, author={Travis, KP and Gubbins, KE}, year={2001}, pages={405–439} }
@article{travis_gubbins_2000, title={Combined diffusive and viscous transport of methane in a carbon slit pore}, volume={25}, ISSN={["0892-7022"]}, DOI={10.1080/08927020008044126}, abstractNote={Abstract The transport of mass through porous materials can occur by essentially two different mechanisms: (1) diffusion and (2) viscous flow. The former occurs when there is a gradient in chemical potential of the pore fluid, while the latter occurs in the presence of a pressure gradient. In general, fluid transport occurs by both of these mechanisms and their respective contributions to the total intra-pore flux are approximately additive. Experimentally, there is no unambiguous way of determining the individual contributions to the total flux of these two modes of transport. Fortunately, molecular simulations does provide a solution. We present a novel simulation method in which the separate contributions to the total flux are determined. The method involves the use of two non-equilibrium molecular dynamics techniques: dual control volume grand canonical molecular dynamics (DCV GCMD) and an algorithm for simulating planar Poiseuille flow. We apply this technique to study the combined (viscous and diffusive) transport of methane through single slit-shaped graphite pores of width 2.5, 5.0 and 10.0 methane diameters. We find that the viscous contribution to the total intrapore flux through each of these pores is 10%, 15% and 34%, respectively.}, number={3-4}, journal={MOLECULAR SIMULATION}, author={Travis, KP and Gubbins, KE}, year={2000}, pages={209–227} }
@article{travis_gubbins_2000, title={Poiseuille flow of Lennard-Jones fluids in narrow slit pores}, volume={112}, ISSN={["0021-9606"]}, DOI={10.1063/1.480758}, abstractNote={We present results from nonequilibrium molecular dynamics (NEMD) simulations of simple fluids undergoing planar Poiseuille flow in a slit pore only a few molecular diameters in width. The calculations reported in this publication build on previous results by including the effects of attractive forces and studying the flow at narrower pore widths. Our aims are: (1) to examine the role of attractive forces in determining hydrodynamic properties, (2) to provide clearer evidence for the existence of a non-Markovian generalization of Newtons law, (3) to examine the slip-stick boundary conditions in more detail by using a high spatial resolution of the streaming velocity profiles, (4) to investigate the significance of the recently proposed cross-coupling coefficient on the temperature profiles. The presence of attractive interactions gives rise to interesting packing effects, but otherwise, does not significantly alter the spatial dependence of hydrodynamic quantities. We find the strongest evidence to date that Newton’s Law breaks down for very narrow pores; the shear viscosity exhibits singularities. We suggest a method to test the validity of the non-Markovian generalization of Newton’s Law. No-slip boundary conditions are found to apply, even at these microscopic length scales, provided one takes into account the finite size of the wall atoms. The effects of any strain rate induced coupling to the heat flow are found to be insignificant.}, number={4}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Travis, KP and Gubbins, KE}, year={2000}, month={Jan}, pages={1984–1994} }
@article{travis_searles_evans_1999, title={On the wavevector dependent shear viscosity of a simple fluid}, volume={97}, ISSN={["0026-8976"]}, DOI={10.1080/00268979909482841}, abstractNote={Non-equilibrium molecular dynamics (NEMD) simulations employing the sinusoidal transverse force (STF) method are used to calculate the wavevector and strain rate dependent shear viscosity of a WCA fluid at a zero wavevector number density of 0.8442 and zero wavevector temperature of 0.722. The functional form of the dependence of shear viscosity on the strain rate and wavevector are examined, in the zero field limit. The ramifications of these results for the possible existence of the Burnett coefficients are discussed. The shear rate dependent viscosity for a range of wavevectors can be fitted to both a Cross analytical form and a formula derived from Quentrec's local order theory. Also it can be fitted to two square-root regimes. The extrapolated zero strain rate viscosities are consistent with a Lorentzian functional form, a Taylor series expansion in q 2 and also to a q 3/2 variation at low wavevector. This study reveals that with current computational resources it is still not possible to establish the validity of the Burnett coefficients.}, number={3}, journal={MOLECULAR PHYSICS}, author={Travis, KP and Searles, DJ and Evans, DJ}, year={1999}, month={Aug}, pages={415–422} }