A Molecular Dynamics Study on Nanobubble Coalescence Events in Heterogeneous Nucleation on a Hydrophilic Surface

The bubble nucleation process has practical applicationsand hasgarnered a significant amount of attention. In the literature, researchershave commonly accepted that the incipient of the bubble nuclei isinitiated by vapor trapping in cervices on the heated substrate. However,experimental observations have found that nucleation can occur withmuch lower superheat temperatures on ultrasmooth surfaces where vaportrapping is not applicable. To investigate the nucleation mechanism,molecular dynamics (MD) simulations were carried out to study nanobubblenucleation behavior on smooth and grooved hydrophilic substrates.The force field of argon atoms heated by solid substrates was describedby using the Lennard-Jones (LJ) 12-6 potential field. The MDresults revealed that the nanobubbles that emerged on the two- andthree-groove surfaces could merge, forming a metastable nucleus viathe coalescence event. The coalescence event lowered the requiredenergy cost and accelerateed the nucleation process. Energy analysesalso showed the bifurcation of the energy rise between the right andleft regions of grooved surfaces. Furthermore, the mean first-passagetime method was used to evaluate the corresponding critical nucleusvolume and nucleation rate for all grooved substrates. The resultssuggest that nanobubble coalescence could be an alternative pathwayin the nucleation process that could reduce the critical nucleus sizeand its associated energy cost.