Electric Charging Effects on Condensed Droplet Jumping

When condensed droplets coalesce on a superhydrophobic surface, the resulting droplet can jump due to the conversion of surface energy into kinetic energy. This frequent out-of-plane droplet jumping has the potential to enhance condensation heat transfer. Furthermore, for more than a century, researchers have shown that droplet-surface interactions can be dominated by electrostatic charge buildup. In this work, we studied droplet jumping dynamics on nanostructured copper oxide and carbon nanotube surfaces coated with tri-chloro silane and PFDA hydrophobic coatings, respectively. Through analysis of droplet trajectories and terminal velocities under various electric fields (0 -- 50 V/cm), we show that condensation on these surfaces having both conducting and insulating substrates results in a buildup of positive surface charge (H$^{\mathrm{+}})$ due to dissociated water ion adsorption on the superhydrophobic coating. Consequently, an accumulation of the opposite charge (OH$^{\mathrm{-}})$ occurs on the condensing droplet interface, which creates an attractive force between the jumping droplet and the condensing surface. Using this knowledge, we demonstrate a novel condensation mechanism whereby an external electric field is used to oppose the droplet-surface attraction, further enhancing the coalescing droplet jumping frequency and overall surface heat transfer.