Surprisingly high probability of evaporation for a molecule passing through the Knudsen layer.

The most important characteristics of the Knudsen layer are the temperature jump ΔT between the liquid and vapor and the probability θ_{K} that a molecule, emitted by the liquid into the vapor, reaches infinity (as opposed to bouncing back due to collisions). In the present paper, the linearized Bhatnagar-Gross-Krook model is used to calculate ΔT and θ_{K} and show that, in the absence of macroscopic gradients of temperature and density, θ_{K}≈96.3%. It is then examined how ΔT and θ_{K} are affected by a combination of a temperature gradient (heat flux) and the matching density gradient, such that the resulting pressure field is uniform.