The contact angle of droplets evaporating at ambient temperature

Experiments on sessile drops evaporating in normal atmosphere in the situation of complete wetting are reported and compared with a theoretical model by Ben Amar and Boudaoud. For alkanes and water, the model accounts very well for the behavior of radius and contact angle, which scale like power laws of the time interval to drop vanishing. However, the range where these power laws are obeyed is significantly wider in experiments, especially for small contact angles, as it is the case for polydimethylsiloxane oligomers. The reason why is the need of an intermediate length scale between the macroscopic wedge and the microscopic contact line which leads us to reconsider the mere definition of the contact angle in that case.