Transient geometrical features of a dense spray puff

A spray puff is, from a fluid dynamics point of view, an impulsive source of mass and momentum diluting in a quiescent, dry environment. We address the question of its lifetime, namely the time it takes for all the droplets it is composed of, to fully evaporate. This lifetime is much larger than the one of the individual droplets taken in isolation (typically seconds versus milliseconds). The contrast is attributed to the markedly non-Poissonian repartition of the droplets within the puff. Intermittent clusters of concentrated droplets are separated by large voids. Using liquids with very different volatilities, we document the dynamics of the clusters (through the evolution of their `coarse grained scale’, Phys. Rev. Letters (2006) 97, 144506), and of the voids (through the evolution of droplets field Voronoi tessellations). From these measurements emerges a unified picture explaining the substantial evaporation delay of the puff in spite of its initial division into fine droplets. This scenario is furthermore compatible with the evolution of the transient droplets size content of the spray, of which we give a full description up to the complete evaporation.