Liquid redistribution and the Kelvin effect in evaporating breath figures

Multiple droplets are abundant in nature and industry, for example: condensing from warm breath on a cold day, on the underside of a coffee cup lid and as raindrops on windows.

Recent research has studied their collective evaporation rates and focused on classifying the interactions between the droplets. It has been shown that small arrays of millimetre sized droplets evaporate according to existing diffusive theory.

In this work, we experimentally study the collective evaporation of confined micron sized droplets, like those in exhaled breath, sprays and clouds. We show the significance of small differences in vapour pressure (0.08%), caused by the Kelvin effect, which are often dismissed as insignificant at this length scale. The Kelvin effect causes the big droplets grow as the expense of their smaller neighbours, whilst the whole array is evaporating. The experimental growing and shrinking rates of each droplet are compared to modified diffusive theory showing good agreement. The dynamics of the arrays are directly connected to the size distribution of the droplets.

These findings could have epidemiological, industrial and meteorological applications.