Surface topography controls bubble nucleation at rough water/silicon interfaces for different initial wetting states

The etching process of the silicon die often produces nanoscale roughness, wherein nanostructures alter the solidliquid heat transfer process and thus influence the bubble nucleation. The difference in the initial wetting state further complicates the effect of nanostructures on the nucleation process. To fully understand the nucleation process at a rough water/silicon interface for different initial wetting states, we employ molecular dynamics simulations to study and characterize this phenomenon. The nucleation processes for the nanostructured surface with different cavity widths, depths and numbers are systematically analyzed in terms of the position, time and temperature of nucleation. The relevant effect mechanisms of surface topography for different initial wetting states are elucidated in depth. The results manifest that regardless of the initial wetting state, the cavity always controls the nucleation site by the original vapor nucleus for the Cassie state or a local heat accumulation effect for the Wenzel state. Moreover, in general, the cavity delays the onset of bubble nucleation for the Cassie state yet promotes the nucleation event for the Wenzel state compared with the smooth surface because of the change of the local solid-liquid contact area. Importantly, the nucleation time for the two wetting states shows a complex and different dependence on the cavity size. Furthermore, the nucleation temperature (529.8-550.3 K) for the Cassie state is significantly lower than that (576.7-592.1 K) for the Wenzel state because the original vapor nucleus can significantly lower the energy barrier of nucleation for the Cassie state. This study is of importance to promote further insights into the onset of water film boiling on rough silicon surfaces and provide guidance for the nanostructure design of the silicon die for enhancing nucleate boiling.