INFLUENCE OF EVAPORATION ON THE CONTACT-ANGLE OF WATER DROPS

The contact angle of a sessile drop evolves significantly if the surrounding atmosphere is not saturated with the liquid vapour. Evaporation follows three stages. Initially, drop height h and contact angle theta diminish whilst contact radius r remains constant. This continues until a limiting value theta is reached, apparently less then the receding angle obtained by direct methods. If the solid is smooth enough, the next stage corresponds to a reduction of h and r, at constant theta. Finally, h, r and theta tend to zero with the drop disappearing. This stage is probably influenced by the surface state of the solid. A simple model is proposed for evaporation at constant r.