When larger drops evaporate faster

In an ensemble of volatile liquid drops, either in suspension or on a partially wetted substrate, large drops grow at the expense of smaller ones when the pressure is just above the saturated-vapour pressure. Below this critical pressure, all drops evaporate, with the smallest drops evaporating fastest. But an array of water drops of different sizes on a mica substrate cleaved in a vacuum behave differently: below the equilibrium vapour pressure, the largest drops evaporate fastest and the smallest ones more slowly. Here we show that this behaviour is related to the coexistence of thick and thin water films during evaporation.