ICLASS 2012, 12 th From single droplet impact to micrometric droplet chains: scaling effect surf. topography 1 From single droplet impact to micrometric droplet chains: scaling the effect of surface topography A. S. Moita 1* , A. L. N. Moreira 1 Instituto Superior Técnico – TU Lisbon, Portugal anamoita@dem.ist.utl.pt and moreira@dem.ist.utl.pt Abstract Tuning the wettability based on surface structuring is becoming a hot topic, within the last years, due to sev- eral micro-scale applications. Many researchers consider an empirical trial-error approach, which may lead to contradictory and not always efficient results. In this context, the present work proceeds with the experimental identification of the topographical parameters which should lead to the optimum design of micro-patterns, to maximize the cooling performance of impinging droplets and streams. The surfaces made of silicon wafers are micro-textured with regular patterns of square pillars. The results show that for the patterns leading to homoge- neous wetting, optimization of the parameter r f =(2l+ λ R ) 2 /[(2l+ λ R ) 2 +8lh]was found to be a good path to design patterns leading to an improved cooling performance of the micro-textured surfaces in contact with both milli- metric and micrometric droplet streams, within the nucleate boiling regime. Within the film boiling regime, the effect of the topography is clearly more evident. However, with the current surfaces we are on the limits of ap- plicability of Wenzel’s theory, so a further scaling down is required. Introduction Droplet/wall interactions have been widely explored within the last decade to develop smart interfaces and particularly enhanced surfaces to obtain an accurate control of heat and mass transfer, which is vital, for instance for cooling applications (e.g. [1]). Tuning the wettability based on surface structuring is becoming a hot topic, within the last years, due to several micro-scale applications. Many strategies have been recently explored to create custom made hydrophobic and/or hydrophilic surfaces applied to diverse situations, covering both micro and macro-scale systems (e.g. [2]). Among these, altering the surface wettability by changing its topography is probably one of the most recurrent approaches [3], probably due to its high ratio of simplicity vs. efficacy. In this context, “roughening” the surface has been argued for many years, as a good strategy to enhance heat transfer between liquids and solids, by increasing the liquid-solid contact area and therefore the convective heat transfer coefficients. However, recent studies [4] have shown that stochastic coatings are quite difficult to relate with the hydrodynamic phenomena, particularly when phase change is involved. Even when regular patterns are considered in the surface micro-structuring, an empirical trial-error approach is often followed exception made to few authors, e.g. [5]. Consequently, contradictory results can be found in the literature when reporting the effect of surface topography in the heat and momentum transfer phenomena. The crucial link that is missing is an accurate mapping of the wetting regimes, related to the heat transfer processes, using a systematic approach, as proposed, for instance by Moita et al. [6, 7]. The present work proceeds with the experimental identification of the topographical parameters which should lead to the optimum design, to maximize the cooling performance of impinging droplets and streams. Indeed, cooling of hot spots can be assessed based on single droplet impact, but in many cooling applications, the liquid delivery is made using one or various droplet chains, which alters significantly the boundary conditions of the problem. So, the study proposed here addresses a step by step approach to scale surface micro-structuring, from individual droplets on a dry surface to a wet condition. The scaling starts with millimetric droplets (2.5<D 0 <3.3mm). Then, a small final sub-section will introduce the scaling down to micrometric droplets (80µm<D 0 <340µm). The impact velocity is varied between 0.88<U 0 <7ms - 1 . The surfaces are made from silicon wafers and are micro-patterned using square and round structural elements, ranging from 5µm up to 200µm. Such wide range of the size of the structural elements is required for the accurate scaling, which depends on the size of the droplet and of the boundary conditions (e.g. dry vs. wet surface). Experimental Methods The experimental setup mainly consists on a droplet generator (Model MDG100) producing single droplets and monodisperse streams, which was directed onto a heated surface placed at a distance of 100mm. The test sur- faces are coupled on a copper block heated by 264W cartridge heaters. The basic set-up is schematically repre- * Corresponding author: anamoita@dem.ist.utl.pt