Slip due to Rarefaction, Surface Roughness, and Intermolecular Interactions for Gases and Liquids

In the present study, slip phenomena are investigated in two different sets of experiments conducted in gases and one Newtonian liquid. Overall, differences in near-surface slip behavior are illustrated for these two different fluid mediums, where the slip is induced surface roughness and rarefaction in the gases, and by surface roughness and intermolecular interactions in the liquid. Within both sets of experiments, flows are induced within micro-fluidic passages by rotation within C-shaped fluid chambers formed between a rotating disk and a stationary surface. When gases are employed, accommodation coefficients are determined in a unique manner from experimental results and analysis based on the Navier-Stokes equations. In all cases, roughness size is large compared to molecular mean free path. When channel height is defined at the tops of the roughness elements, slip is believed to be a result of rarefaction as well as fluid shear. With this arrangement, tangential accommodation coefficients decrease and slip velocity magnitudes increase, at a particular value of Knudsen number, as the level of surface roughness increases. With Newtonian water as the working fluid, hydrophobic roughness is used to induce near-wall slip in the fluid chamber. The magnitudes of slip length and slip velocities increase as the average size of the surface roughness becomes larger. The resulting slip length data show a high degree of organization when normalized using the fluid chamber height, such that experimental data obtained using different chamber heights and different disk roughness magnitudes collapse along a single line, illustrating strong linear dependence of the slip length on the normalized radial-line-averaged shear stress.