EFFECTS OF NANO- TO MICROPORE DIAMETER ON WATER VAPOR TRANSPORT DIFFUSIVITIES WITHIN POROUS POLYCARBONATE BARRIERS

摘要

Effective pore diffusivities for dilute water vapor in air were measured at atmospheric pressure and 30?C by continuously weighing an evaporator capped with a porous polycarbonate barrier and correcting steady-state water vapor fluxes for apparatus mass transfer resistances. The Knudsen number was increased from 0.018 to 2.4 at constant pressure by decreasing pore diameter from 5,400 to 39 nm, thus increasing the influence of vapor molecule collisions with pore walls on mass transport. Correspondingly, diffusivities decreased in qualitative accord with a model equating total pore diffusion resistance to the sum of the continuum diffusion (Kn < 0.01) and Knudsen diffusion (Kn > 10) resistances. Correction for irregular pore geometry with a best-fit tortuosity (～4) significantly improved agreement between measured and predicted diffusivities.