Smart surface in flow boiling: Spontaneous change of wettability

We examined the heat transfer coefficients (HTC) and critical heat fluxes (CHF) in flow boiling on titanium dioxide (TiO2) and zinc oxide (ZnO) coated surfaces that are suggested as smart surfaces in this study. These surfaces were initially hydrophobic; they induced early onset of nucleate boiling (ONB), and many nucleation sites were observed at the initial stage of boiling compared with the SiO2 reference surface. As a result, high HTCs were identified on the smart surfaces. Additionally, under high wall superheat conditions, the surfaces became hydrophilic; there was no degradation of CHF by promoting water supply to dry areas. The results were attributed to spontaneous change of wettability of the smart surfaces. We investigated the contact angles of the surfaces after they underwent heat treatment. The contact angles decreased on the smart surfaces as the temperature increased. In conclusion, we postulated that the change in the wettability characteristics was a key factor in explaining the improvement of the HTC without degradation of the CHF, and suggest the smart surfaces.