De. Rosner et Dh. Papadopoulos, JUMP, SLIP, AND CREEP BOUNDARY-CONDITIONS AT NONEQUILIBRIUM GAS SOLIDINTERFACES/, Industrial & engineering chemistry research, 35(9), 1996, pp. 3210-3222
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)).