Evaporation in Porous Media

Abstract

The physical factors are discussed which govern evaporation in a porous medium. If the escape of the vapour is relatively unobstructed, marked temperature gradients are found only close to menisci. Even in the simplest cases, evaporation is shown to proceed in statically unstable configurations under a dynamic balance of surface tension, local evaporation rate and viscous shear. It turns out that a porous medium in the shape of a sheet may have different “constitutive” laws from the same medium formed into the shape of a ball. Most of all, it is demonstrated that porous media cannot be thought of as continua in the sense of mechanics: useful macroscopic laws can be obtained only from considerations of microscopic physics and statistical information on the distribution of passage sizes. This helps to explain why repeatable experiments can be difficult in soil mechanics.