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Spray Production by Collective Bubble Bursting

Séminaire Axe Environnement, Gestion des Ressources et des Risques

Baptiste Néel – Princeton University

Mercredi 22 décembre Amphithéâtre Nougaro

The bursting of bubbles at the surface of water is a key feature of mass transfers between ocean and atmosphere, as it is a major production mechanism of sea spray aerosols. While recent progress has been made to understand the role of water physico-chemistry on the aerosols, experimental works linking collective effects in the bubbles assembly to the spray production remain elusive. Our experimental approach fully characterizes the dynamics and statistics of bubbles assemblies for increasing surface contaminations, by generating a nearly monodisperse bubble plume underwater, and following its evolution at the water surface. The decay of a raft of bubbles gives access to the dynamical properties, namely the bubbles merging and bursting rates, which control the overall statistics of the bubble rafts. At low contamination, we observe a complex interplay between bursting and merging time scales, leading to broad distribution of surface bubble size, while at high concentration, we observe a regime of raft with quasi-monodisperse sizes. A simple automaton model successfully confirms the primary role of merging and bursting, and reproduces the surface statistics with respect to the surface contamination. The control of surface contamination on the surface bubble population has direct implications on the resulting spray production, in terms of size and total number. We show that bubble bursting is most efficient for intermediate levels of contaminations, when coalescence is inhibited and surface bubbles are relatively short-lived. This study finally makes a strong and documented case for the introduction of a « surface bubbles transfer function » when attempting to predict the features of a spray directly from the bulk bubbles population, prediction that we perform and discuss carefully.