Scaling the effects of surface topography in the secondary atomization resulting from droplet/wall interactions

The impact of droplets onto micro-structured surfaces has been the focus of numerous recent studies, under the perspective of many different applications. However, much is still to be known about the effects of surface patterning in order to devise realistic physical models to accurately predict interfacial transfer rates. In this context, the present paper addresses the question of how to scale the effects of the surface topography to find adequate parameters, which can be easily obtained a priori. The approach is based on the characterization of the hydrodynamic and thermal behaviors of individual droplets impacting onto smooth and micro-structured heated surfaces, with the objective of quantifying the effects of the modified wettability associated with the topography of the surface. The focus is put on the thermal-induced mechanisms of secondary atomization as these are of particular interest for spray-cooling applications. The analysis suggests that different wetting properties lead to particular characteristics of the thermal-induced atomization, which can be related with the ratio between the roughness amplitude and the fundamental wavelength of the surface topography R _a/λ_R. This hypothesis is consistent with the theoretical prediction of the wetting behavior of the surfaces. The results also show a good correlation between the mean sizes of the secondary droplets generated by thermal-induced atomization and the ratio R _a/λ_R.