The baroeffect and an appropriate momentum boundary condition

As disparate molecular weight gases isothermally diffuse between two ends of a capillary tube, they can support a pressure gradient. The magnitude of this pressure gradient depends critically on viscous wall stress and becomes a measure of the boundary condition. This baroeffect has been used to test the state of a gas surface layer. Specifically, it allows one to quantify whether a binary gas has a finite wall velocity (diffusive slip). Here, a one-dimensional analytical model is proposed that allows for specular gas reflection from the wall. It explains anomalous (4/3) correction factors required previously to match experiment to baroeffect models. It predicts a new physical phenomenon, a surface-driven baroeffect for equal molecular weight gases. Diffusive slip contributions exceed the order of convective diffusion for Peclet number Pe<1 and approximately equal convective diffusion for 1<Pe<4. For binary gases, this model further extends the baroeffect experiment as a means to find momentum accommodation coefficients (MAC), slip lengths, and external friction coefficients. A corrected binary diffusion coefficient is defined. Analogous arguments can be made to explain corrections to thermal and viscous slip.