Note on evaporation in porous media

Factors are discussed which govern evaporation of liquid in the small capabilities of a porous medium. Attention is directed to sheet-like aggregates from which the vapor can escape with little obstruction. Marked temperature gradients are then found to be confined to close neighborhoods of the menisci and evaporation is shown to proceed in statistically quite unstable configurations under a dynamic balance of surface tension, local evaporation rate and viscous shear. Estimates of evaporation rates and fluid velocities are given. The results discourage constitutive theories for porous media because mere size of capabilities, independently of shape and chemistry, is found to change the physical processes underlying macroscopic behavior.