On the fate of a drop jumping over a gap

Abstract

Droplets impinging on solid surfaces, as well as the countless varieties of the resulting possible dynamics, are found frequently in both natural and industrial environments. Among such dynamics, the contact time with the surface, the amount of deformation and the occurrence of breaking induced by geometrical singularities of the solid surface are key aspects in a wide range of applications. We report the results of an extensive experimental activity investigating the capability of liquid droplets to jump over a gap while sliding/rolling over a hydrophobic solid plane. These drops impact on the downstream sharp edge of the gap and undergo the amount of deformation that allows them to climb the edge. We ascribe this unique behaviour to the transformation of rotational momentum into linear momentum. Such conversion can take place only if the right amount of deformation occurs upon impact. Indeed, within the explored range of Weber number (0.5 ≲ We ≲ 40), we show the existence of a sub-range for which the drops show a significantly higher probability of jumping over the gap if compared to solid spheres, whose behaviour is predicted accurately by a purely ballistic and elastic impact model. We formulate a minimal energy balance in order to show that such peculiar drops are indeed the ones featuring high rotational energy. The results of this study also contribute to shed light on the debate about the amount of rotational speed characterizing liquid droplets running over hydrophobic surfaces.