Life and death of a dense spray puff

An exhalation is, from a fluid dynamics point of view, an impulsive source of mass and momentum diluting in a quiescent environment. The mean, coarse-grained expansion rate of the momentum carrying cloud is well known and its connexion to the turbulent cascade dynamics obvious. We are interested here in the fine-grained aspect of the phenomenon, namely in situations where the puff is initially concentrated in a liquid form liable to fragment and evaporate in an outer gas phase.

Puffs formed from liquids with very different volatilities demonstrate that the liquid soon fragments into close-by droplets spatially concentrated in dense stretched regions separated by large voids. The lifetime of an individual droplet embedded in such a spray is much larger than expected from the usual {\it d-squared} law describing the fate of a single drop evaporating in a quiescent environment. By analogy with the way mixing times of scalars are understood from the coupling between stretching and diffusion, we show, thanks to unique in-situ measurements, that the spray boundary with the diluting environment is slaved to the dynamics of its saturating vapor concentration field, thus explaining the very substantial evaporation delay of the liquid in spite of its division into fine droplets. This scenario is furthermore compatible with the evolution of the transient droplets size spray content, of which we give a full description up to the complete liquid evaporation.