JUMP, SLIP, AND CREEP BOUNDARY-CONDITIONS AT NONEQUILIBRIUM GAS SOLIDINTERFACES/

The notion of local (dynamical, thermal, and chemical) equilibrium at fluid/solid interfaces which are the site of interesting nonequilibriu m processes has proven useful to engineers for nearly a century and pr ovides the basis for widely used methods described in textbooks on Tra nsport Processes. Indeed, continuity of tangential velocity (''no-slip ''), temperature, and species chemical potential are usually treated a s ''commandments'', rather than often-useful approximations! However, in many current and emerging applications this class of approximations becomes unacceptable for easily understood reasons. We illustrate thi s here for ideal gas/solid interfaces across which, or tangent to, the re are nonzero molecular fluxes of momentum, energy, and/or species ma ss. We make use of the concept of a Knudsen sublayer, at most several gaseous mean-free paths thick, inevitably present adjacent to the soli d surface. While many scientific aspects of these phenomena have been known since the earliest studies of J. C. Maxwell (1879), we show that their engineering importance is now such that their understanding sho uld be part of the education of all chemical engineers. Moreover, mole cular-level numerical techniques can now be brought to bear to illumin ate the nature of these near-interfacial regions, under more realistic nonequilibrium circumstances. Analogous phenomena occur in dense vapo r/solid and liquid/solid cases. Such systems, far less well understood theoretically, are characterized by effects which are smaller numeric ally but which may still be quite exploitable (as for separations (Gid dings, 1991; Caldwell, 1988)).