Thermal Response of Nanobubble Suspensions under Freeze–Thaw Cycles through Experiment and Molecular Dynamics Simulations

Thermal response and solutal stability of bulk nanobubbles are critical for advancing their applications in food preservation and environmental remediation; herein, we investigated these properties through a combination of experiments and molecular dynamics simulations. Characteristics of nanobubbles on size distribution, number density, and zeta potential were examined over two cycles of freeze-thaw treatment. Nanobubble suspensions were generated through ultrasonic cavitation and subsequently subjected to freezing using either a conventional freezer or liquid nitrogen. To clarify how thermal and solutal factors affect nanobubbles, the ethanol-water mixtures with several ethanol volume fractions of 1 vol %, 3 vol %, 5 vol %, and 10 vol % and saline water with 3.5 wt % salt concentration were prepared. Both pure water and saline water exhibited a decrease in number density and zeta potential after two cycles, while nanobubbles considerably remained stable in ethanol suspensions. Moreover, rapid cooling significantly slowed the decline in bubble concentration over two cycles compared with slow cooling in pure and saline water. During the freezing and thawing of electrolyte solutions, concentration gradients develop in the bulk phase, inducing fluctuations in the formation and stability of nanobubbles. Experimental measurements, combined with molecular dynamics simulations, revealed that enhanced zeta potential and hydrogen-bonding networks of ethanol at the gas-liquid interface contribute to the stability of nanobubbles in the ethanol-water mixture. These findings provide new insights into the complex interplay between solvent composition, thermal treatment, and interfacial structure, offering guidance for the design and control of nanobubbles in practical applications.