Karl P. Travis

Poiseuille flow of Lennard-Jones fluids in narrow slit pores

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 th...

THERMOSTATS FOR MOLECULAR FLUIDS UNDERGOING SHEAR FLOW: APPLICATION TO LIQUID CHLORINE

In this article we show that atomic thermostats which have been used in the past for nonequilibrium molecular dynamics (NEMD) simulations of molecular fluids were incorrectly formulated. The error stems from an incorrect assumption made about the form of the streaming angular velocity. This assumption is incorrect even in the linear regime. One spurious effect of this atomic thermostat is the generation of a nonsymmetric pressure tensor. We outline a general method, based on a variational principle, for calculating the position and orientation dependent streaming velocity. Using this streaming velocity we develop an atomic thermostat for molecular fluids which does not bias the positional or orientational distribution functions for the fluid. The new atomic thermostat is validated in NEMD simulations of liquid chlorine undergoing planar Couette flow.

Computer simulation algorithms for molecules undergoing planar Couette flow: A nonequilibrium molecular dynamics study

Results from extensive nonequilibrium molecular dynamics (NEMD) simulations are presented for liquid chlorine subject to planar Couette flow. Comparisons are made between the so‐called atomic and molecular SLLOD algorithms [R. Edberg, G. P. Morriss, and D. J. Evans, J. Chem. Phys. 86, 4555 (1987)] with atomic and molecular thermostats, respectively. These two thermostats differ in the assumptions that are made regarding the streaming velocity. Both thermostats are responsible for the production of string phases characterized by a translational ordering at very high strain rates. In addition, the atomic thermostat is responsible for the existence of a nonvanishing antisymmetric stress and enhanced orientational ordering.

Poiseuille flow of molecular fluids

We examine a generalised Navier-Stokes theory applicable to fluids composed of non-spherical molecules. We compare the theoretical predictions for flow velocity and viscosity with results obtained from nonequilibrium molecular dynamics (NEMD) simulations of a fluid undergoing gravity fed flow down a rectangular channel.

Combined Diffusive and Viscous Transport of Methane in a Carbon Slit Pore

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.

Strain rate dependent properties of a simple fluid

The strain rate dependence of a range of rheological properties of the Weeks-Chandler-Andersen fluid at the Lennard-Jones triple point is investigated by non-equilibrium molecular dynamics simulation. A moderately large system size and very long simulation times have yielded data in the very low strain rate regime with relatively high statistical precision. The strain rate variation of the shear viscosity cannot be accounted for by a single square-root functional form. Some functional forms consistent with the data are discussed.

A technique for the calculation of mass, energy, and momentum densities at planes in molecular dynamics simulations

We present a new technique for the evaluation of hydrodynamic densities (for example mass, energy or momentum densities) at planes in molecular dynamics simulations. This technique employs an easily computed expression for the density at a plane that is formally exact, unlike other expressions such as histogram approximations. We present simple examples of applications of this procedure to the calculation of mass and momentum densities, and hence the streaming velocity, at planes in a fluid undergoing planar Poiseuille flow, and show how the temperature profile can be obtained by the same procedure.

On the Rheology of n-Eicosane

Abstract We carry out non-equilibrium molecular dynamics (NEMD) simulations of liquid n-eicosane (C20H42) undergoing planar Couette flow. We show that a different rheological picture emerges if one uses the standard (but incorrectly formulated) ‘atomic’ thermostat rather than a thermostat which only couples to the centre of mass translational degrees of freedom. In the latter case eicosane shows shear thinning at low to moderate shear rates and then shear thickening at higher shear rates [G. P. Morriss, P. J. Daivis and Denis J. Evans, J. Chem. Phys., 94, 7420 (1991)]. However in the atomic thermostatted simulations, eicosane shows only shear thinning across the range of shear rates studied. Moreover in the atomic case, the eicosane molecules are more elongated and spin at a lower rate in the shear plane.

Computer simulation investigation of diffusion selectivity in graphite slit pores

Equilibrium molecular dynamics simulations are reported of oxygen and nitrogen molecules confined in graphite slit pores. Self- and collective diffusion coefficients have been calculated as a function of pore width, temperature and density for each pure component in the pore space. The aim of this study was to elucidate the mechanism by which oxygen and nitrogen are kinetically separated when air is passed over an adsorbent bed consisting of molecular sieving carbon in the commercial production of oxygen. It was found that a critical pore width exists for each species at which there is a sharp drop in the rate of diffusion (both self- and collective diffusion) of each fluid. The critical pore width is one for which the individual molecules are prevented from rotating freely about one of their axes. The greater length of a nitrogen molecule means that the critical pore width is higher for this species than for oxygen. Consequently, oxygen molecules diffuse substantially faster than nitrogen molecules in the vicinity of the nitrogen critical pore width. From an analysis of correlation functions and their corresponding power spectra it is shown that the restricted rotations, which occur at or below the critical pore width, cause a decoupling of translational and rotational modes, with the net result being a lowering of translational diffusion. The nitrogen critical pore width lies within the range of the mean pore size of most commercial molecular sieving carbons, and so this mechanism may help to explain the high oxygen selectivities reported in the literature.

Computer Simulation of Isothermal Mass Transport in Graphite Slit Pores

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.