Multicomponent Droplet Crystallization: Latent Heat Release and Bubble Formation

Bubble formation in evaporating saline droplets at subboiling temperatures is a striking and nonintuitive phenomenon that challenges conventional understanding of phase change processes. In this study, the mechanisms of bubble formation are systematically investigated in ternary saline droplets (water + two salts) with different combinations of high- and low-latent heat of solid formation (LHSF) salts, which can potentially be extended to multicomponent systems encompassing all possible combinations. The results reveal that bubble formation is not governed solely by the LHSF but also critically by the rate at which this latent heat is released. Real-time imaging is employed to capture the onset of nucleation, crystal growth, and bubble evolution, while in situ thermocouple measurements provide direct evidence of transient thermal fluctuations. A one-dimensional energy balance at the solid-liquid interface demonstrates that the governing parameter is not LHSF alone but the combined influence of latent heat release and the kinetics of crystal growth. Bubble formation is consistently observed in droplets containing high-LHSF salts as well as in high-high and high-low LHSF combinations, the latter mainly due to the presence of high-LHSF salts. Remarkably, certain low-low LHSF systems also produce bubbles when crystallization proceeds at sufficiently high growth rates. These findings establish that bubble formation arises from the interplay between the thermodynamic magnitude (LHSF) and the kinetic rate (growth), thereby proposing a generalized mechanism for this unusual phenomenon.