HYDRODYNAMIC BEHAVIOR OF A SOLUTE PARTICLE BY MOLECULAR-DYNAMICS

The hydrodynamic behaviour of a single solute particle immersed in a s olvent system of (N - 1) particles, interacting through a repulsive Le nnard-Jones potential, has been studied for several system sizes, rang ing from N = 108 to 2048 particles using isothermal-isochoric molecula r dynamics computer simulation. The solute particle was projected at a fixed relative velocity with respect to the host fluid. The computati ons show that the linear resistance force versus velocity behaviour, p ostulated by the Stokes law, is obeyed quite well even up to relativel y large (i.e. near thermal) solute velocities. However, two finite siz e effects have been found for large solute particles (ca. sigma(B)/sig ma(S) > 1, where sigma(B) and sigma(S) are the diameters of the solute and solvent particles, respectively) when they are confined in such s mall periodic systems. At high drift velocity, there is a breakdown in this linear relationship for the smaller systems leading to a maximum in the opposing force on the solute particle. Also there is a slow sy stem size convergence to the thermodynamic (N --> infinity), Stokes la w limit, as measured by an effective Stokes c parameter, and when comp ared with the value given by the Stokes-Einstein relationship.