Aging dynamics of bulk nanobubbles under pressure oscillations

The formation and unexpected longevity of bulk nanobubbles remain a fundamental puzzle in gas-liquid systems. Herein, we exploit molecular dynamics simulations, experiments, and theoretical modeling to elucidate the mechanism of nanobubble nucleation and stabilization under cyclic pressure oscillations. Both simulations and experiments reveal that alternating compression-rarefaction drives the aggregation of dissolved gas into stable bulk nanobubbles even in gas-undersaturated solutions. The process is thermodynamically favored by a negative variation in the partial molar Gibbs energy of gas, sustained through a hysteresis loop during the cyclic oscillation. Successive oscillations substantially reshape the interfacial structure; namely, water molecules accumulate at the gas-liquid interface while their order of dipole orientation diminishes, leading to a conspicuous reduction in interfacial tension. This interfacial reshaping, accompanied by a decrease in gas diffusivity, results in an exponential decline in the gas transport rate, thereby enhancing the diffusion stability. Incorporating these effects into a modified Epstein-Plesset theory predicts that bulk nanobubble lifespan can increase by up to three orders of magnitude, irrespective of collective stabilization from clusters. Our results elaborate that bulk nanobubble stability arises from the coupling modulation of interfacial energy and molecular transport within the framework of thermodynamics. This unified thermodynamic-kinetic mechanism reconciles long-standing discrepancies between experimental longevity and classical theory, providing a fundamental insight into nanobubble stability and offering new strategies for controlling interfacial gas dynamics in diverse chemical, biological and energy systems.