Bubble Dynamics and Atomization Mechanisms in Burning Stable Emulsion Droplets

We delineate and analyze bubble growth regimes and breakup mechanisms in burning stable emulsion droplets. Emulsions were prepared using different base oils and altering the dispersed phase volume fraction (ϕ). Four distinct regimes of bubble growth were identified during droplet combustion. Regime I corresponds to the growth of a single bubble through micro-bubble coalescence. Regime II represents the formation of individual vapor bubbles through micro-bubble coalescence, the merger of these individual bubbles, and their subsequent growth. Regime III exhibits the formation and growth of vapor bubbles through micro-bubble coalescence, uniquely separated by a thick film, preventing merger. Regime IV depicts the formation and breakup of a single large bubble, where the propagation of multiple capillary waves on the droplet surface is revealed. Finally, the breakup of the vapor bubble leads to the formation of a ligament that pinches off into secondary droplets. The ligament pinch-off mechanism is observed to determine the diameter and velocity of the secondary droplets, as well as subsequent shape oscillations in the parent droplet. We show that microemulsion bubble dynamics and, eventually, the breakup behavior can be accurately controlled by optimizing the base oil properties and ϕ.