Oscillating state transition in pinned nanobubbles with coupled fluctuations

Analogous to other porous solids, pinned nanobubbles serve as a zero-dimensional stable nanoscale chamber with controllable thermodynamic parameters, whereas they can respond to state change of guest molecule. Here we analyzed peculiarities of phase transitions in pinned nanobubbles, which were experimentally proved to be superstable. By combining molecular dynamics simulation and thermodynamic analysis, we reveal that guest molecules encapsulated inside a nanobubble exhibit distinct state behaviors: a state in vapor phase, a reversible two-state oscillation, and a stable nanodroplet@nanobubble state, depending on the number of guest molecules and the external pressure. The free-energy landscape shows how state metastability gradually develops with external stimuli and leads to the specific bistable state of two-state oscillation. The existence of strong coupling between nanobubble breathing and two-state oscillation is identified. Our simulation results demonstrate that the flexibility of pinned nanobubbles plays at least the same important role as space confinement in determining the states of guest molecules. Our findings indicate that pinned nanobubbles, serving as soft porous media that possess high stability and reversible transformability, show a wealth of properties that are not found in bulk solutions and in porous solids.