Nonequilibrium Molecular Dynamics Simulations of Steady-State Heat and Mass Transport in Condensation. II. Transfer Coefficients.

We present coefficients for transfer of heat and mass across the liquid-vapor interface of a one-component fluid. The coefficients are defined for the Gibbs surface from nonequilibrium thermodynamics and determined by nonequilibrium molecular dynamics simulations. The main conductivity coefficients are found to become large near the critical point, consistent with the disappearance of the surface in this limit. The resistivities of transfer found by molecular dynamics simulations are compared to the values predicted by kinetic theory. The main resistivity to heat transfer is found to agree from the triple point to about halfway to the critical point. The resistivity to mass transfer was used to determine the condensation coefficient, which was found to be practically constant with a value of about 0.82. The resistivity coupling coefficient predicted by simulations also agrees with values predicted by kinetic theory from the triple point until about halfway to the critical point. Copyright 2001 Academic Press.

Nonequilibrium Molecular Dynamics Simulations of Steady-State Heat and Mass Transport in Condensation.

We present evidence for the hypothesis of local equilibrium for a liquid-vapor interface in a one-component fluid, using molecular dynamics simulations. Lennard-Jones/spline particles are studied in a two-phase system that is out of global equilibrium. Equilibrium simulations are first used to establish the equation of state for the vapor and interface. A procedure is developed to define the boundaries of the interface. Finally it is shown that the equation of state for the interface applies also when there is heat and mass transport through the interface. The temperature gradient in the vapor was 10(8) K/m in these studies. Copyright 2000 Academic Press.

Diffusion measurements at long observation times in the presence of spatially variable internal magnetic field gradients

Diffusion measurements in heterogeneous media may contain a significant source of error, the influence of the coupling between the applied and internal magnetic field gradients on the attenuation of the NMR signal. The application of bipolar magnetic field gradients has been introduced to suppress this error. The basic assumption for the successful removal of the coupling is that the diffusing molecules are experiencing a constant internal gradient during the experiment. We will provide theoretical and experimental evidence that the application of bipolar magnetic field gradients may fail to suppress the effect from all the cross terms between internal and applied gradients effectively at long observation times. It will be shown experimentally that a successful suppression of the cross terms is strongly dependent on the observation time, and on the tau value in the bipolar pulsed field gradient stimulated echo experiment. Copyright 2000 Academic Press.

Measurements of diffusion in porous polyethylene powder using PFGSTE NMR.

Pulsed field gradient stimulated echo (PFGSTE) nuclear magnetic resonance (NMR) has been applied to study the diffusion of toluene in porous semicrystalline polyethylene powder. A pulse sequence with unequal bipolar gradients that reduces effects from internal gradients was used. The effects of different diffusion regimens and restricted diffusion in this system were indicated by use of a simple two-component analysis of the diffusion data.