(6gf) Measurement and Control of Slip-Flow Boundary Conditions at Solid-Gas Interfaces

In traditional Chemical Engineering applications one usually assumes that the no-slip boundary condition for flow applies at the solid-gas interface.  However, it has been known since the time of Maxwell that, even for moderate Knudsen numbers, partial slip could occur, of order the mean free path of gases.  With the growing number of applications for micro- and nano-scale devices and flow systems, it is interesting to quantify more precisely the flow boundary condition.

In this work I describe unambiguous measurements where I show that partial slip does occur, that the slip length is a function of both the gas and solid, and that the solid can be altered in-situ to change the slip length.  In-situ changes in slip length could in principal be used to control flow in a variety of important applications.

The main theme of the thesis was the effect of solid surface properties on the flow boundary condition. The effect of water films, organic films, electric fields and the gas species was studied. Water films had a large, but complex effect. On bare hydrophobilic glass, the tangential momentum accommodation coefficient (TMAC) for nitrogen on hydroxyl-terminated silica changed from 0.25 to 0.88 when the humidity changed from 0 to 98 %. On hydrophobized glass, TMAC changed from 0.20 to 0.56 in the same range. The effect of humidity was interpreted with the formation of water film, verified with a quartz crystal microbalance. TMAC on  octadecyltrichlorosilane-coated glass surfaces was examined for five different gases (helium, nitrogen, argon, carbon dioxide, hexafluoride sulfur). A lower TMAC occurred for greater molar mass.

I also developed methods for controlling the flow boundary condition using external stimuli – temperature and electric fields. Using Atomic Force Miscroscopy (AFM), each of these stimuli altered the roughness of the film, so control of the boundary condition was attributed to surface roughness change.  The temperature was used to change the roughness of an octadecyltrichlorosilane film on glass, and the electrical field was used to change the roughness of a 16-mercaptohexadecanethiol film on gold. Considering collisions of simple spheres that conserved momentum and kinetic energy, a surface with periodic triangular grooves had higher TMAC with steeper slopes.

A novel method of measuring gas pressure using an AFM cantilever was presented.  The frequency spectrum of the cantilever vibration in the gas was measured, which was then fit to a continuum description that depends on the density.  Use of the continuum approximation restricts calculation of the pressure to the continuum regime. In the range 0.1 to 2.2 atm, the gauge deviated by less than 5 % compared to a commercial gauge.