Evaporating droplets

The evaporation of droplets on a substrate that is wetting to the liquid is studied. The radius R(t) of the droplet is followed in time until it reaches zero. If the evaporation is purely diffusive, R proportional to root t(0) - t is expected, where t(0) is the time at which the droplet vanishes; this is found for organic liquids, but water has a different exponent. We show here that the difference is likely to be due to the fact that water vapour is lighter than air, and the vapour of other liquids more dense. If we carefully confine the water so that a diffusive boundary layer may develop, we retrieve R(t) proportional to root t(0) - t. On the other hand, if we force convection for an organic liquid, we retrieve the anomalous exponent for water.