DIRECT SIMULATION OF HYDRODYNAMIC RELAXATION IN MICROCHANNELS

Simulations were performed on a fluid confined between two parallel wa lls. The fluid is modeled by a Lennard-Jones potential and the walls b y a simple cubic lattice of harmonically bonded sites. A Lennard-Jones potential is also used to model the interactions between the wall and the fluid. The simulation consisted of over 30 000 sites arranged to form a liquid film approximately 35 Lennard-Jones diameters in thickne ss. This is large enough to begin approximating the range where classi cal hydrodynamics is expected to be applicable. Both equilibrium simul ations and simulations of velocity transients were performed on the sy stem. Two values of the wall-fluid interaction strength were examined, which appear to correspond to a wetting and nonwetting surface. Resul ts from equilibrium simulations show that both the density and the str ess tensor relax to their bulk values within a short distance of the w all. Furthermore, examination of the relaxation of spontaneous momentu m fluctuations indicates that there is little change in the value of t ransport coefficients near the boundary compared to the bulk fluid. No nequilibrium simulations on the decay of a parabolic velocity profile, however, suggest that the decay of the profile is faster than would b e predicted from classical hydrodynamics and that the type of boundary conditions that should be used in a hydrodynamic analysis may depend on the details of the wall-fluid interaction. (C) 1993 American Instit ute of Physics. [S0021-9606(98)50725-8].