Determination of diffusion coefficient-vapor pressure product of some liquids from hanging drop evaporation

Abstract

Drop evaporation experiments were performed in still air for 11 liquid drops, and the results were expressed by using the drop half-life time method. This method was found to be useful to judge the importance of the related factors such as vapor diffusion coefficient and vapor pressure of the liquid and the temperature decrease on the drop surface. (Drop weight)(2/3) versus time plots were drawn and clearly linear plots over the drop half-life were obtained. The product of diffusion coefficient and drop surface vapor pressure DPv* was calculated from the experimental drop mass decrease data and a proportionality was found between the DPv* product and the vapor pressure of the drops at the ambient temperature. DPv* product is found to be a useful parameter to determine the peripheral contact angle during following the evaporation of sessile or pendant drops on solid surfaces. The surface temperature of drops was calculated using DPv* product and by assuming the heat transfer is only by conduction. Considerable surface cooling was found for liquid drops having high vapor pressures. A new procedure is also proposed to calculate the partial water vapor pressure during evaporation in humid air and the corresponding decrease in water drop surface temperature.