EVAPORATION KINETICS OF VENTILATED WATERDROPS COATED WITH OCTADECANOL MONOLAYERS

We have used remote thermometry and video microscopy to monitor simultaneously the temperature and size of evaporating waterdrops coated with monolayers of 1-octadecanol (stearyl alcohol). The experiments are conducted by acoustically levitating individual, 1-3-mm-diameter waterdrops in a 1.5 m/s nitrogen flow conditioned at 0, 35, and 64% relative humidity. We observe a continuous increase in monolayer resistivity as evaporation compresses the monolayer. The change in monolayer resistivity is independent of relative humidity. However, the rate of the resistivity increase is inversely proportional to the relative humidity. Remote drop temperature measurements show that the drop remains in thermal steady state with its surroundings during the monolayer phase change. We show that the additive Eu(EDTA), necessary for remote thermometry, accelerates the onset of the monolayer phase transition and reduces the final resistivity of the monolayer without effecting the monolayer during the phase change. We also discuss the appropriate use of accommodation coefficients and report them when possible.