  It is argued that the van der Waals force exerted by the liquid and vapor/air
on the molecules escaping from one phase into the other strongly affects the
characteristics of evaporation. This is shown using two distinct descriptions
of the van der Waals force: the Vlasov and diffuse-interface models, each of
which is applied to two distinct settings: a liquid evaporating into its vapor
and a liquid evaporating into air (in all cases, the vapor-to-liquid density
ratio is small). For the former setting, the results are consistent with the
Hertz--Knudsen Law (HKL), but the evaporation/condensation probability is very
small (in the classical HKL, it is order one). For the latter setting, the
dependence of the evaporation rate on the difference between the saturated
vapor pressure and its actual value is shown to be nonlinear (whereas the
classical HKL predicts a linear dependence). The difference between the two
settings indicates that the van der Waals force exerted by the air strongly
affects evaporation (contrary to the general assumption that the ambient gas is
unimportant). Finally, the diffuse interface model is shown to be inapplicable
in a narrow region at the outskirts of the interface -- as a result, it
noticeably underestimates the evaporative flux by comparison with the (more
accurate) Vlasov model.
