Experimental measurements of thermal and tangential momentum accommodation coefficients on solid surfaces: Water vapor in comparison with noble gases

In the high Knudsen number flows, like micro gaseous flows and flows in vacuum, the gas-surface interaction plays an important role. Among many gas species, water is a common and useful fluid in wide area of engineering applications, and it is one of the main residual gas components in the high vacuum environment. However, experimental measurements on the gas-surface interaction of water molecules are still limited. In this study, we focus on the accommodation coefficient, which represents the mean behavior of gas molecules on the scattering process from a surface. We measure the thermal accommodation coefficient (TAC) and the tangential momentum accommodation coefficient (TMAC) of water molecules. TAC is measured from the heat flux between two surfaces with different temperatures in vacuum. TMAC is derived through the viscous slip coefficient from the measurement of the mass flow rate through a microtube. To investigate the characteristics of water molecules clearly, the measured accommodation coefficients are compared with those of noble gases, helium, neon, and argon, measured with the same apparatus and procedures. For the accommodation coefficients of noble gases, TAC increases with increasing the molecular weight, while TMAC decreases, which are in good agreement with the previous studies. The TAC and TMAC of water are slightly larger than those estimated from the corresponding molecular weight of noble gases.