The thermodynamic stability of noble-gas bulk nanobubbles in a closed and finite volume of liquid metal

The existence and persistence of gas-filled bulk nanobubbles (bNBs) is usually puzzling, whether in nature or computer experiments using Molecular Dynamics (MD) methods, since the mechanical and chemical equilibrium conditions justifying their thermodynamic stability are rather vague. In the spirit of theories and experiments concerning microbubbles stability in closed and finite-volume systems, a theoretical perturbation approach of bNBs thermodynamic stability is revisited. Also, analogies with earlier theories are highlighted, and a novel counterpart computational framework is established. Under similar conditions and constraints of the system as theoretically hypothesized, a kinetic approach using classical MD to realize a proposed thermodynamically stable (or irreversible) state of bNBs is reviewed. Helium bNBs in liquid Pb17Li alloy is selected as case study, which fulfills the assumptions and approximations required by the theoretical approach that is mainly intended for noble-gas/liquid-metal systems. Several important static and dynamic properties of the system (and its components) are consistently estimated to facilitate analyses and discussions. Estimated pressure–volume relations of bNBs are used to analytically reconstruct the thermodynamic stable-state of bNBs, and agreement with results from the kinetic approach is shown. The influence of relevant properties and factors on the probed state are thoroughly discussed. It is concluded that while the theoretical perturbation is ideal, it is conservative and valid as long as the theoretical constraints and assumptions are fulfilled. Ultimately, this study resolves ambiguities about the influence of the simulation domain and time on the observed bNBs stability in computer experiments, and it relates Ward’s theory to CMD simulations.