In this article we study systematically the effect of wettability on the onset of boiling on the same nanometrically smooth surface. By grafting different monolayers of molecules, we were able to explore the wettability from the equilibrium static contact angle, θ0 = 0° to θ0 = 110°, without changing the surface topography. The superheat temperature at the onset of pool boiling was measured and eventually a non-classical trend of TONB as a function of wettability was observed. The nucleation site densities for the different grafting cases were also measured by image analysis. Moreover, we propose a novel theoretical interpretation to this phenomenon linking nucleation and the molecular diffusion coefficient. MD simulation results support this approach.

Pool boiling experiments were performed with degassed water on stainless steel substrates with different surface topographies and wettabilities. Boiling curves and visual observations of the boiling process have been carried out. The onset of nucleate boiling (ONB) has been measured and the influence of roughness and wettability has been quantified. Boiling curve shape is different between the hydrophilic and the superhydrophobic cases; superhydrophobic surfaces reaching the ONB heat flux at a lower superheat and presenting a "quasi-Leidenfrost" regime, without showing the typical boiling curve. Bubbles are easier to form on superhydrophobic surfaces, therefore the nucleation temperature is smaller, and bubbles are larger and stable. The ONB appears after less than 5K of superheat on superhydrophobic surfaces, while on hydrophilic surfaces, with the same surface roughness, the superheat is above 7K. Furthermore, superhydrophobic samples with a different roughness present the same boiling curve, meaning that, when the contact angle exceeds a certain value, the wettability has a predominant role on the surface roughness.

Pool boiling experiments are performed with degassed water on highly smooth surfaces of two different wettabilities: hydrophilic and hydrophobic cases. Boiling curves and visual observations on the boiling have been performed. The onset of nucleate boiling (ONB) has been measured and the influence of the wettability has been quantified. As the inherent mean roughness of the glass substrates was lower than one nanometer it was possible to show the sole effect of the wettability. No hysteresis in the boiling curve was observed for both cases. The ONB was observed after 3.5 °C superheat on the hydrophobic case and the heat transfer coefficient (HTC) changed suddenly from the one of a convection regime (1.5 kW/m2 K) to the one of a nucleate boiling regime (4 kW/m2 K). On the contrary for the hydrophilic case, despite superheat above 37 °C and presence of boiling, the HTC was kept as the one of the convection regime.

Experimental investigation of pool boiling is conducted in stationary conditions over very smooth bronze surfaces covered by a very thin layer of gold presenting various surface treatments to isolate the role of wettability. We show that even with surfaces presenting mean roughness amplitudes below 10 nm the role of surface topography is of importance. The study shows also that wettability alone can trigger the boiling and that the boiling position on the surface can be controlled by chemical grafting using for instance alkanethiol. Moreover, boiling curves, that is, heat flux versus the surface superheat (which is the difference between the solid surface temperature and the liquid saturation temperature), are recorded and enabled to quantify, for this case, the significant reduction of the superheat at the onset of incipient boiling due to wettability.