Enhanced Evaporation of Small, Freely Falling Water Drops Due to Surface Contamination

Abstract

Experimental evidence for enhanced evaporation of small (3–9 μ radius) water droplets is presented. Both “pure” droplets and droplets contaminated with surface active materials that were dropped into air of known relative humidity (96–99%) at an ambient temperature of 29.9C evaporated. The rates of evaporation were determined by photographing the evaporating drops, measuring the distance they fell during a fixed time interval, and applying Stokes law. The contaminated droplets exhibited a rate of evaporation approximately 1.8 μ² sec⁻¹ faster than the “pure” drops. This increase can be compared with rates for “pure” drops of 1.9 and 5.7 μ² sec⁻¹ at 0.1 and 0.3C dew point depressions, respectively. It is suggested that this phenomenon is caused by the ejection of tiny spurts of water from the drop, this action being a result of the presence of foreign molecules adsorbed on the surface.