Gas Flows near Solids Coated with Thin Water Films

We determined the tangential momentum accommodation coefficient (TMAC) of nitrogen gas at glass surfaces that were coated in thin water films produced by controlling the humidity. The accommodation coefficients were determined by measuring the damping of a spherical particle which was separated from a flat plate by a distance of 10 nm to 10 mu m. We examined two solids: hydrophilic glass terminated in hydroxyl groups and hydrophobic glass terminated in methyl groups. At the same relative humidity, thicker films of water form on the hydroxyl-terminated films than on the methyl-terminated films, consistent with formation of hydrogen bonds on the hydroxyl-terminated solid and not on the methyl-terminated solid. The TMACs are surprisingly interesting functions of the humidity. For the hydroxyl-terminated silica, the TMAC is 0.5 at 0% relative humidity and increases almost to 1 when the humidity increases to a few percent. The TMAC decreases to 0.25 at around 87% humidity before rising to 0.9 at 100% humidity. We rationalize the increase in TMAC at low humidity in terms of interactions with a very thin layer of water with significant lateral mobility and vibrational and rotational modes to receive momentum; the increase near saturation is explained by nitrogen interacting with near-bulk water. The minimum at intermediate humidity is totally unexpected. The TMAC on methyl-terminated silica follows roughly the same trend, but with more variability between samples.