Electrostatic Charging of Jumping-Droplets on Superhydrophobic Surfaces

Exactly one hundred years ago in 1913, Millikan analyzed the motion of droplets electrified by a cathode ray tube in a uniform electric field to quantify the fundamental charge of an electron. Since then, researchers have extensively studied the nature and mechanism of charge accumulation on atomized droplets, sessile droplets, and the hydrophobic coatings beneath them, sometimes using a modification of Millikan's approach to measure the charge, and often arriving at conflicting results between positive and negative charging, in what still remains a topic of significant debate. Recently, with the broad interest and development of superhydrophobic surfaces for a variety of applications including self-cleaning, condensation heat transfer enhancement, thermal diodes, and anti-icing, more detailed insights on droplet interactions on these surfaces have emerged. Specifically, when two or more small (≈10-100 µm) droplets coalesce, they can spontaneously jump away from a superhydrophobic surface due to the reduced droplet-surface adhesion and release of excess surface energy, which promises enhanced system performance by passively shedding water droplets. While this droplet jumping phenomenon has been studied on a range of surfaces, research has focused on the role of surface structure on the jumping mechanism, viscous dissipation during coalescence, and modeling of the jumping process, while assuming no electrostatic interactions and charge neutrality of the coalescing and departing droplets. Here, we show that jumping droplets on a variety of superhydrophobic surfaces, including copper oxide, zinc oxide, silicon nanopillars, and carbon nanotubes, gain a net positive charge that causes them to repel each other mid-flight. In a modified experiment inspired by that of Millikan, we used a uniform transverse electric field to quantify the charge on the droplets and showed that the charge is dependent on the surface area of the departing droplet as well as the hydrophobic coating and independent of the structure beneath the coating. Accordingly, we explained the mechanism for the charge accumulation, which is associated with the formation of the electric double-layer at the droplet-coating interface, and subsequent charge separation during droplet jumping governed by the fast time scales of droplet coalescence. Our results demonstrate the important role of surface charge interactions on jumping droplet dynamics and also provide insight into jumping droplet physics, in addition to sessile droplet charging on hydrophobic surfaces. We anticipate this work to be a starting point for more advanced approaches for enhancing jumping droplet surface performance. For example, an external electric field could be used to control the jumping efficiency from the surface to enhance condensation heat transfer, anti-icing, and self-cleaning performance. Furthermore, the charge separation phenomenon may offer an advantageous metrology to characterize the electrokinetic properties, such as zeta-potential, of hydrophobic coatings on large scale superhydrophobic surfaces.