Drop evaporation on solid surfaces: constant contact angle mode

There are two pure modes of evaporation of liquid drops on surfaces:  one at constant contact area and one at constant contact angle. Constant contact area mode is the dominating evaporation mode for water and many other drops on solids when the initial contact angle is less than 90°. However, the constant contact angle mode has been reported in a few instances, such as water drop evaporation on poly(tetrafluoroethylene) where the initial contact angle is greater than 90°. In this work, we report the evaporation of n-butanol, toluene, n-nonane, and n-octane drops on a poly(tetrafluoroethylene) surface, which occurs with constant contact angle mode and an initial angle of less than 90°. Video microscopy and digital image analysis techniques were applied to monitor the drop evaporation. The decrease of the square of contact radius of these drops was found to be linear with time for most of the cases. This paper discusses the theoretical background and compares the experimental data with results from the previous models.