Enhanced Water Vapor Flow in Silica Microchannels: The Effect of Adsorbed Water on Tangential Momentum Accommodation

Maxwell introduced the tangential momentum accommodation coefficient (TMAC) in 1879 to describe boundary conditions in fluid flow. Recent findings of anomalously high gas flow rates in carbon nanotubes have been attributed to smooth hydrophobic walls that reduce TMAC. Here we report the first measurements of water vapor flows in microchannels over a wide humidity range and show that for silica there is a range of humidity over which an adsorbed water layer reduces the TMAC in a way that depends on the surface conditions. We develop a model that unifies Maxwell’s ideas on TMAC with Langmuir’s ideas on adsorption. We fit TMAC data with the model using two stages of Langmuir adsorption and derive total adsorption isotherms for water on silica. The first adsorption stage reduces TMAC by smoothing the surface. The second stage is bulk liquid that imparts large TMAC. We find that leak testing of moisture barriers with an ideal gas such as helium may not be accurate for critical applications and recommend direct measurements of the water leak rate