Experimental study of evaporating water-ethanol mixture sessile drop: influence of concentration

In this paper we study the evaporation of a drop on a rough polytetrafluoroethylene substrate. A water-ethanol binary drop of few millimetres size is evaporating in a controlled pressure environment. An experimental set up is built to investigate the influence of ethanol concentration and drop profile on the evaporation rate. The measurements were performed using an optical technique. This latter allows measurements of the dynamic contact angle, the drop volume and the base width as function of time. For pure substances (water, ethanol) the evaporation rate and the drop profile are found to have a monotonous evolution with time. For binary water-ethanol mixtures, three stages corresponding to different wetting behaviours are identified. The evaporation rate measurement indicates that the more volatile component evaporates entirely in the first stage while in the last stage the less volatile component is dominantly evaporating. The behaviour of the wetting angle is correlated with the volume of the drop and the ethanol concentration. It was clearly demonstrated that at high ethanol concentration (75%) the wetting contact angle of the drop matches the behaviour of pure ethanol during the first stage and tends to follow the behaviour of pure water during the third stage. This suggests that, as the ethanol evaporates in the first stage it diffuses to the interface where it dictates the surface tension properties and hence the wetting contact angle. Towards the end of the droplet lifetime, the wetting contact angle jumps to join the behaviour of pure water. (C) 2003 Elsevier Ltd. All rights reserved.