On the Dynamics of Wetting of Water-Methanol Volatile Sessile Drops on Smooth Substrates

Abstract

The wetting and evaporation behaviour of methanol-water droplets deposited on a smooth silicon substrate were investigated experimentally. Contact angle and droplet shape kinetics were studied using an optical technique. Drops were deposited onto a silicon substrate and enclosed in a cell with nitrogen as the ambient gas. Besides the case of pure water and pure methanol, three different volume fractions of methanol in water were investigated: 10%, 50% and 80%. Using a Kruss DSA100 contact angle analyser, the behaviour of the contact angle, droplet volume, and base width was determined as a function of time. Results show that evaporation of the droplet takes place in successive stages for mixtures. The more volatile component seems to evaporate principally in the first stage, during which the contact angle of the binary drop is closer to that of pure methanol. Because the wetting behaviour is partly dictated by the surface tension of the liquid-vapour interface, methanol is believed to be concentrated at the interface during this first stage. After complete evaporation of the methanol, the wetting behaviour of the droplet tends towards that of pure water. The mechanisms that dictate the evaporation and wetting behaviour of such binary droplets include many effects: diffusion of methanol in water in the liquid phase; accumulation of one of the component near the interface and preferential evaporation followed by diffusion of one component in another in the vapour phase. In order to model the phenomenon, the above effects must be taken into account. Solutal Marangoni stress as well as interfacial instabilities may also play an important role in the behaviour of theses systems.