Chapter 8 - Pure Diffusion

Abstract

The evaporation phenomenon is a phase change transition from a liquid to its own vapor. The molecular scale description is a transfer of molecules from the liquid to the surrounding gas, while the macroscopic scale description is a mass flux crossing the liquid/vapor interface. Evaporation occurs as soon as the vapor partial pressure in the gas is lower than the saturation pressure. This basic mechanism is called diffusive evaporation, and it always exists until the saturated conditions are not reached. Later, other supplementary heat and mass transfer mechanisms can appear, such as convections (thermo-gravitational, thermo-capillary, and thermo-solutal) or radiation. This chapter discusses these additional mechanisms and examines the theory of purely diffusive evaporation from analytical solution to approximated models. Indeed, droplet evaporation is usually considered a process that is controlled by the vapor diffusion into the environment. This is considered the basic modeling of droplet evaporation. This description appears to be very satisfactory for water evaporating into air at room temperature.