Molecular dynamics study on heterogeneous nucleation mechanism: Plausible nanobubble coalescence pathway

The Bubble Nucleation process has been attracted research attention due to its wide practical applications. In literature, researchers commonly accepted vapour trapping in cervices on the heated substrate as the mechanism initialising the incipient of bubble nuclei. However, experimental observations found that the onset of nucleation could occur on ultra-smooth surfaces where vapour trapping is not applicable. Aiming to investigate the nucleation mechanism, molecular dynamics (MD) simulations have been carried out to study nanobubble nucleation behaviour on smooth and grooved substrates. The Lennard-Jones (LJ) 12-6 potential field is adopted to describe the force field of argon atoms which are heated by solid substrates. Instantaneous snapshots of MD results show nanobubbles pinned on the two-grooved surfaces could merge, forming a metastable nucleus via the coalescence event. The coalescence pathways effectively lower the required nucleation energy cost. Energy analyses also clearly show a consistent energy level between the right and left regions of the two grooves. The results confirm that nanobubble coalescence could be an alternative pathway in the nucleation process, which could reduce the critical nucleus size and its associated energy cost.