NANOSURF

Nanostructured surfaces are characterized by elementary structures whose size varies between a few to several hundreds of nanometers. Important researches have been recently done on small scale surface structuration owing to its important possible applications to micro-thermo-fluidics. According to the situation which is considered, this structuration can notably reduce the friction factor (for super-hydrophobic nanostructured surfaces) or can increase the heat transfer (local hydrodynamic modification under the influence of microstructures). In recent years, boiling on nanostructured surfaces has been the subject of many efforts from the scientific community. As a matter of fact, the strong influence of the structuration on the wetting or non-wetting properties is the main reason of this interest: highly hydrophilic surfaces increase the boiling critical heat flux owing to the decrease of the contact angle in the vicinity of the triple line; on the contrary, hydrophobic surfaces are characterized by a low activation energy of the nucleation sites but, unfortunately, favour coalescence of the nucleated bubbles in such a way that they create an unwanted insulating vapour blanket. This project aims at studying the impact of nanostructuration on the boiling and evaporation heat transfer. These problems will be tackled both experimentally and theoretically. Several scales will be considered. Although various techniques will be used for the nanocoating of the studied surfaces, the study of these techniques by themselves will not be part of this project. Nevertheless, the detailed characterization of the surfaces which have been coated for our studies (by SEM or AFM) will be carefully done as it is a key point of the experiments which will be done. As regards experimental work, various prototypes will be built according to which aspect of flow boiling is studied. At the microscale, a particular attention will be paid to the behaviour of the triple line under static or dynamic conditions. A Hele-Shaw cell will be built for studying the effect of a nanostructured surface on the growth of an insulated bubble submitted to a shearing cross-flow. At the macroscale, the heat transfer coefficient and the critical heat flux will be studied for both pool boiling and flow boiling. Wires and plain surfaces will be used. A high speed camera will allow obtaining information on the link between heat transfer and hydrodynamics. Experimental results will be the subject of a theoretical analysis. In this respect, two different models, each corresponding to a specific length scale, will be used: Molecular Dynamic model will be used for the smallest scales whereas Phase Field models will be devoted to intermediate length scales. The ultimate goal of these developments will be to propose Millimetric Continuous Models. Each model will provide adequate boundary conditions to the other with the objective that no intermediate scale will be missing and that informations could circulate freely from the smallest to the largest scale and vice-versa. Evaporation in the vicinity of the triple line will be described by Molecular Dynamic. Phase Field models are known to be efficient for the prediction of the effect of the surface structuration on the wetting properties but they have never been used in convective heat transfer problems. Extending the range of application of these models is one of our major objectives in order to help understanding the influence of surface structuration on hydrodynamic and heat transfer during convective boiling.