Multi-stage heterogeneous bubble nucleation under negative pressures

In this paper, we investigate homogeneous and heterogeneous bubble nucleation processes in systems under tension using molecular dynamics simulations. The maximum pressure (nucleation pressure, $\sigma_{min}$) sustained by the system is used a as measure of the system's propensity to nucleate a vapor bubble. In the presence of a planar gold substrate, nucleation pressure is essentially the same as homogeneous values when strong interaction exists between the gold atoms and water molecules; at weaker interactions, a significant lowering of nucleation pressure is observed, signifying that nucleation from such surfaces is easier. Reduction in nucleation pressure with decreasing gold-water surface interaction strength obtained from our simulations shows a good qualitative agreement with classical heterogeneous nucleation theory. As compared to planar surfaces, surfaces with grooves show a further reduction in nucleation barrier only for weak interfacial interactions. Furthermore, the groove dimensions also influence $\sigma_{min}$ -- an optimal groove geometry exists for which $\sigma_{min}$ is minimized; our results indicate that this occurs when the length scale of the defect is comparable to that of the critical (homogeneous) bubble nucleation radius. Moreover, in the presence of defects, multiple barriers to nucleation exist. Our findings provide design guidelines for surface grooves for controllable generation of vapor bubbles; such hydrophobic grooves should be avoided for maximizing overheat or can be used for spatially controlled boiling.