Energy transfer during inertial-capillary-driven droplet jumping on superhydrophobic surfaces

With the accelerating development of hydrophobic structured surfaces for a variety of applications including self-cleaning, condensation heat transfer enhancement, and anti-icing, the need for more detailed insights of dynamic droplet interactions on these surfaces has emerged. Specifically, when two or more droplets coalesce on a low adhesion surface, they spontaneously jump away from the surface due to the conversion of excess surface energy to kinetic energy. However, this phenomenon is not well understood beyond the limited insight provided by simple energy state analyses. From both a fundamental and practical perspective, there is a need to examine this energy conversion process in greater detail. Here, we used experiments and simulations to develop mechanistic understanding of how the excess surface energy released during coalescence is transferred to the translational kinetic energy of the jumping droplet. To isolate the energy transfer process we condensed water under both ambient and environmentally controlled conditions on nanostructured superhydrophobic surfaces that demonstrated minimal surface adhesion. On these surfaces, jumping velocities were approximately 6× larger than previously reported. Meanwhile, using numerical simulations to calculate the flow momentum generated within the droplet during coalescence and relating this to the momentum of the jumping droplet, we found good agreement with our experimental results and confirmed that only a small fraction of the excess surface energy (≤ 6%) is converted into the translational kinetic energy of the jumping droplet. Our findings take a step beyond simple energy state analyses by providing insights into the role of internal fluid dynamics during coalescence that drives the jumping process.