We have studied the translational and rotational motion of a sphere in a viscous Lennard-Jones liquid using molecular dynamics simulations. The drag and torque on a sphere in an effectively unbounded fluid are found to agree with continuum hydrodynamics results even when the size of the sphere is comparable to that of the fluid molecules. The diffusivity of a spherical tracer particle is in accord with the Stokes-Einstein relation, and the corresponding Brownian motion is determined by its interaction with the layers formed by fluid molecules around it. When a sphere moves near a solid wall, we find that the drag and torque agree with lubrication theory down to molecular scales, but the predicted divergence is regularized at very short distances due to depletion of fluid molecules near the wall and the appearance of slip at high shear stress.

• Received 4 December 1995

DOI:https://doi.org/10.1103/PhysRevE.53.4852