Physics of Fluids at Low Reynolds Numbers—A Molecular Approach

techniques has opened a new window on diverse problems in physics. Our focus, in this article, is on a set of issues pertaining to the behavior of fluids, at low Reynolds numbers, for which the inertial forces are tiny compared to viscous forces. We shall discuss classes of long-standing problems for which a computational approach yields qualitatively new information that is inaccessible using conventional methods. Broadly speaking, these problems involve questions of behavior at the subcontinuum level for which experiments are unable to provide the requisite answers. In these cases, computers provide a bridge between microscopic and macroscopic scales and are beginning to yield new insights into technologically important issues. The computational method is molecular-dynamics (MD) simulation, which entails the integration of Newton’s laws of motion for a set of interacting molecules (see the flow diagram in Fig. 1). At the simplest level, molecular simulations are important for the interpretation of nanoscale experiments. A detailed understanding of the behavior of materials at very small scales is necessary for the fabrication of miniature devices and the manipulation of materials at the molecular scale. The behavior of materials at the nanoscale is often quite different from that in the bulk. For example, it has been found recently that very small systems may exhibit solid-liquid coexistence over a range of temperatures, quite distinct from the macroscopic behavior described by equilibrium statistical mechanics.1 Another example occurs in studies of friction in narrow systems,2 in which a combination of experiment and simulation has elucidated the relationship between the stick-slip behavior observed in experiments and the local freezing and melting first discovered in MD simulations by Thompson and Robbins.3 Microscopic studies are needed to assess the range of