A VARIATION CALCULATION OF THE RATE OF EVAPORATION OF SMALL DROPLETS

A model for slowly evaporating drops is set up and approximated numerically by a variation calculation based on a theorem of Cercignani. The calculations were carried out for an extended range of Knudsen number and indicated that the popular Fuchs-Sutugin formula is almost a universally valid approximation for a wide range of pressures, condensation coefficients, mass ratios and intermolecular potentials. Discrepancies were found, however, when the method was used to interpret an old set of data on the rate of evaporation of diamyl sebacate droplets suspended in air. Since the present calculation agrees well with several other extended range calculations, it is suggested that a more sophisticated model is required for the dynamics of molecules adsorbing on and departing from a liquid surface in contact with an inert ambient gas.