Bursting bubble aerosols

Abstract

AbstractWe depict and analyse the complete evolution of an air bubble formed in a water bulk, from the time it emerges at the liquid surface, up to its fragmentation into dispersed drops. To this end, experiments describing the drainage of the bubble cap film, its puncture and the resulting bursting dynamics determining the aerosol formation are conducted on tapwater bubbles. We discover that the mechanism of marginal pinching at the bubble foot and associated convection motions in the bubble cap, known as marginal regeneration, both drive the bubble cap drainage rate, and are responsible for its puncture. The resulting original film thickness $h$ evolution law in time, supplemented with considerations about the nucleation of holes piercing the film together culminate in a determination of the cap film thickness at bursting ${h}_{b} \propto {R}^{2} / \mathscr{L}$, where $R$ is the bubble cap radius of curvature, and $\mathscr{L}$ a length which we determine. Subsequent to a hole nucleation event, the cap bursting dynamics conditions the resulting spray. The latter depends both on the bubble shape prescribed by $R/ a$, where $a$ is the capillary length based on gravity, and on ${h}_{b} $. The mean drop size $\langle d\rangle \ensuremath{\sim} {R}^{3/ 8} \hspace{0.167em} { h}_{b}^{5/ 8} $, the number of drops generated per bubble $N\ensuremath{\sim} \mathop{ (R/ a)}\nolimits ^{2} \mathop{ (R/ {h}_{b} )}\nolimits ^{7/ 8} $ and the drop size distribution $P(d)$ are derived, comparing well with measurements. Combined with known bubble production rates over the ocean, our findings offer an adjustable parameter-free prediction for the aerosol flux and spray structure caused by bubble bursting in this precise context.