15. Droplet Evaporation in the Non-continuum Regime

At low pressures, the evaporation rate of a droplet is not adequately described by the equations of continuum mechanics, that is, by mass diffusion and conduction heat transfer. When the mean free path of the evaporated molecules is large compared with the droplet radius, the kinetic theory of gases can be applied to determine the evaporation rate. In this limit, the free-molecule regime, it is assumed that the molecules have a Maxwell-Boltzmann distribution of molecular velocities. In the intermediate regime, the Knudsen regime, molecular collisions distort the Maxwell-Boltzmann distribution and reduce the rate of transport of the molecules leaving and arriving at the droplet surface. This chapter reviews the theory and measurements of droplet evaporation in the free-molecule and Knudsen regimes.