The classical model of evaporation of liquids hinges on Maxwell’s assumption that the vapour near the liquid’s surface is saturated. It allows one to find the evaporative flux without any knowledge of the physics of the interface (separating liquid and air). Maxwell’s hypothesis is based on an implicit assumption that the vapour emission rate through the interface exceeds the throughput of air, i.e., its ability to pass the vapour on to infinity. If indeed so, the air adjacent to the liquid would get quickly saturated, making the interfacial flux decrease and adjust to that in the air.
In this work, I use the so-called diffuse-interface model to account for the interfacial physics and, thus, derive a generalised version of Maxwell’s boundary condition for the near-interface vapour density. It is then applied to a spherical drop floating in air. It turns out that the vapour emission rate of the interface exceeds the throughput of air only if the drop’s radius is rd ≳ 10μm, but for r d ∼ 1μm, the two are comparable. If r d ≲ 0.1μm, evaporation is interface-driven, and the resulting evaporation rate is much smaller than the prediction of the classical model.