The leading edge of evaporating droplets

New experiments on drops evaporating in normal atmosphere from smooth substrates in the situation of complete wetting are reported and compared with the available theoretical model. They are the continuation of previous work with alkane or water sessile drops, which is first briefly summarized. The model accounts very well for the dynamics of the drop radius, but the predictions are only qualitative for the contact angle, especially for small angles. Experiments with hanging drops allow us first to discard any influence of convection in the gas phase on the drops dynamics. Then the main part of the paper concerns new experiments with polydimethylsiloxane oligomers. These silicone oils are similar to alkanes as far as evaporation rate is concerned, but have lower surface tensions, and therefore smaller dynamic contact angles. The purity of the oils appears to be critical for the experiments, and requires a preliminary investigation. Then a systematic study of the drops dynamics is presented, as a basis for forthcoming theoretical work. (c) 2006 Elsevier Inc. All rights reserved.