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.

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The kinetic hydrogen exodiffusion and its temperature dependence in amorphous silicon prepared by glow discharge of silane has been studied using conductivity, electron paramagnetic resonance, "B~n uclear reaction, and infrared absorption measurements. Comparison of the results obtained with these techniques shows the existence of two principal stages in the H exodiffusion. The hydrogen evolution for T g 500'C is controlled by a diffusion process with a diffusion coefficient D. D is thermally activated; D = D, e 8 "&T with D"=4.7g10 ' cm'/s ' and ED = 1.5 eV (where k~is the Boltzmann constant). The hydrogen evolution above 500 'C is controlled by a first-order process. The activation enthalpy and entropy are, respectively, 8H, = 3.4 eV and BS, = 7;8k~. The fact that the EPR signal appears during the second stage and that 3H, is equal to the Si-H bound energy is a direct evidence that EPR signal is associated with the breaking of this bond. %'e then deduce an exodiffusion model assuming that hydrogen atoms can be bound in two sorts of centers, A possible configuration of H occupying such sites in the amorphous network is proposed.

Pool boiling experiments were performed with degassed water on stainless steel substrates with different surface topography and wettability. Boiling curves were determined and high-speed visualizations of the boiling processes were performed. 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 hydrophilic and superhydrophobic case ( Fig. 1): on superhydrophobic surfaces the ONB is reached at lower superheat and boiling presents a quasiLeidenfrost regime without showing the typical "S" shape of the boiling curve, i.e. without passing through a CHF point. 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 5°C of superheat on superhydrophobic surfaces, while on hydrophilic surfaces with the same surface roughness the required superheat is above 8°C. Furthermore, superhydrophobic samples, although presenting different roughness, present the same boiling curve, meaning that the wettability has a predominant role on the surface roughness when the contact angle exceeds a certain value. quasi-Leidenfrost regime