Electrostatic-induced coalescing-jumping droplets on nanostructured superhydrophobic surfaces

Coalescing-jumping droplets from the condensation of water vapor on a non-wetting surface return to the substrate due to resistance forces. While some can coalesce with neighboring droplets and jump again, some adhere to the surface and become larger, leading to progressive flooding, limiting heat transfer performance. To address these issues, an electric field is utilized. This study investigates the jumping height, the droplet charge, the jumping angle, the gravitational force, the drag force, the inertia force and the electrostatic force of coalescing-jumping droplets in electric fields through experiment and mathematical models. The results show that an electric field can enhance the jumping height due to a significant increase in the electrostatic force. With the applied electric field, the maximum jumping height is over three times higher than those without. Additionally, the study reports the intersection point at the jumping droplet radius of 35 μm separating jumping droplet motion into two regimes; the drag-force-dominated regime where the small-sized droplets can jump and reach the top plate, and the gravitational-force-dominated regime where the larger droplets can jump, but return to the substrate. The other intersection point is between the gravitational force and the inertia force showing a decrease in the influence of the inertia force with a greater applied electric field. Moreover, it is also found that the average charge of the droplets is relatively constant in all pressure conditions and applied electric fields. The results of these findings can further advance knowledge on the enhancement of heat transfer and can be applied to several applications including self-cleaning, smart windows, thermal diodes and condensation heat transfer enhancement.