The Influence of Concentration-Dependent Diffusion on Rate of Evaporation

It has been suggested that evaporation through a membrane or from a polymer film has a characteristic behaviour different from that commonly associated with a free liquid surface. In particular it is claimed that an increase in surface humidity produces an increased rate of evaporation through a membrane, though the opposite effect is well known for a liquid surface. This behaviour is said to be observed when diffusion within the membrane or film is the rate controlling process in systems for which the diffusion coefficient increases with increasing concentration In this paper the behaviour of such diffusion-controlled evaporation processes is examined from the point of view of the diffusion equations and it is found that there is no theoretical reason to expect such processes to show anomalous behaviour It is seen that when the experimental data for evaporation through a membrane are correctly interpreted they are in agreement with theoretical prediction. The so-called ` skin effect ' in the dry spinning of fibres is examined and appears to be due to complicating features other than diffusion.