Effects of evaporation and thermocapillary convection on volatile liquid droplets

Results of an experimental investigation of evaporating sessille drops on a glass-slide surface for three volatile liquids show that both evaporation and thermocapillary convection in the sessile drop strongly affect the drop spreading and contact angle. The evolution of contact diameter or the drops can be divided into four stages: 1) initial spreading, 2) spreading-evaporation balance, 3) evaporation-dominating contraction, and 4) final rapid contraction. Molecular-kinetic spreading always occurs in the early first stage and is rapidly restrained and then taken over by the effects of evaporation. Thermocapillary convection, induced by the evaporation, promotes the competition of evaporation over the spreading and shortens the spreading-evaporation balance stage to become undetectable. Evaporation may increase or decrease the contact angle of the evaporating sessile drops, depending on the evaporation rate.