Ballistics of self-jumping microdroplets

Water-repellent materials ideally operate at very different liquid scales: from centimetersize for bugs living on ponds through millimeter-size for antirain functions to micrometersize for antifogging solids. In the last situation, it was recently evidenced that microdrops condensing on a highly nonadhesive substrate can take advantage from coalescence to jump off the material, even if the dynamical characteristics of the jump were not established at such microscales. We demonstrate in this paper that the jumping speed of drops is nonmonotonic with the drop size, showing a maximum around 5 mu m (a size commonly observed in dew), below and above which viscous and inertial effects, respectively, impede the takeoff. We quantitatively describe this optimum in antifogging. We also studied the ballistics of the jumping microdrops, from the height they reached to their behavior at landing; a situation where retakeoff is surprisingly found to be nearly unachievable despite the extreme nonwettability of the material.