Study of the effect of the topographical length scales on the wettability as strategy to control the interfacial phenomena at droplet-wall interactions

The present paper addresses the study of the effect of the topographical length scales on the wetting behaviour and on its consequences in the hydrodynamic and thermal behaviour of impinging droplets. This work is part of a broader investigation to establish a systematic approach to design of micro-textured surfaces for cooling appli-cations based on spray and droplet impingement. The phenomena related to the wetting regime are strongly re-lated to the boiling mechanisms of the lamella but are not directly related to the heat transfer calculations. How-ever, controlling the droplet motion can be used to modify the contact area and residence time of the liquid over the surface, thus controlling the heat transfer mechanisms. This is the strategy followed here. For the set of sur-faces tested so far, the results show that the main topographical characteristics, namely the amplitude of the rough peaks, h, the distance between them, λR and the characteristic size of the peaks a seem to govern the wet-ting regimes and therefore the spreading and receding motion of the droplet, thus indirectly affecting the heat transfer to the surface. To cseparate the wetting from the heat transfer and boiling phenomena, complete wetting regime was promoted for all the surfaces tested here. So, while the characteristic dimensions of h, λR and a lead to a homogeneous wetting regime, (in this case the order of magnitude of h is much smaller than that of a and λR) increasing the ratio h/λR decreases the spreading diameter but also reduces the receding velocity, thus keep-ing the liquid for longer periods in contact with the surface. The averaged contact area of the lamella during the entire droplet/surface interaction is also larger. Consequently, the surface temperature is kept stabilized within lower values, for longer periods of time. However, these characteristic dimensions of the surface roughness will also interact with the boiling mechanisms of the lamella. The increase of h/λR performed at the expenses of a significant increase of h or a significant decrease of λR in may cause a strong effect in the nucleation mecha-nisms which becomes dominant regarding the cooling performance of the droplet. So, if the ratio h/λR ap-proaches a critical region for which, at sufficiently high heat fluxes, the cavities promote the formation of large vapour bubbles and/or strong interaction between them, a vapour blanket can be entrapped generating a dynamic droplet behaviour resembling that of the heterogeneous wetting and degrading the cooling performance. The characteristic size of a will act with the same trend of λR. In line with this, the cooling performance deterioration was correctly interpreted in previous works to be the result of the entrapment of a vapour blanket. However, this phenomenon is not a direct result of a heterogeneous wetting caused by surface topography, but instead is the consequence of the interaction of the roughness structures on the boiling mechanisms occurring within the la-mella.