Directional Droplet Coalescence-Induced Jumping Regulated by Laplace Pressure

Coalescence-induced droplet jumping, a spontaneous droplet transport phenomenon, holds significant potential in anti-icing, anti-fogging, self-cleaning, and enhancing condensation heat transfer. However, droplet jumping has a low energy efficiency and an uncontrolled jumping orientation, severely restricting its practical use. We demonstrate experimentally that a pillar superhydrophobic surface may achieve dimensionless jumping velocity <i>v</i>_j^* = 0.72 and outstanding energy efficiency <i>η</i> = 56%. Compared to a flat superhydrophobic surface, the energy efficiency is raised by about 860%. The improvement in jumping efficiency is due to the pillar limitations and regularization of the internal droplet flow by restricting droplet deformation. For the first time, we have accomplished controlled droplet directional jumping within the 45-130° range by adjusting the magnitude and direction of the Laplace pressure. In addition, we thoroughly investigate how directional droplet jumping is affected by pillar geometric dimensions, droplet radius, and droplet size mismatch. This work introduces a new avenue for increasing the jumping velocity while managing the direction, resulting in better droplet jumping performance in applications.