Investigation of droplet jumping on superhydrophobic coatings during dew condensation by the observation from two directions

Superhydrophobic coatings with nanoscale random roughness structure were prepared onto a Si substrate using boehmite (AlOOH) particles and hydrophobic silanes. The samples were cooled by a Peltier cooling element. Then spontaneous jumping behavior of the water droplets that had condensed on the coatings was observed from two directions (top-view and side-view) using high-speed camera systems. Spontaneous jumping of water droplets occurred subsequent to the coalescence of more than two water droplets, deformation, and shape recovery. Small droplets exhibited high initial jumping velocity, which decreased concomitantly with increasing difference in droplet size before coalescence. The actual jumping velocity was lower than the theoretical one, suggesting the existence of energy dissipation. When the sample was declined at 30, the jumping frequency of water droplets fluctuated against the cooling time with repetition of the increasing-decreasing cycle, and sustained a certain value. The water droplet jumping height was increased remarkably under an external electric field. The droplet possessed positive charge. Coulombic force was expected to contribute to this phenomenon. (C) 2014 Elsevier B.V. All rights reserved.