Enhanced coalescence-induced jumping droplet departure

Water vapor condensation on superhydrophobic surfaces has received much attention in recent years due to its ability to shed water droplets at length scales three decades smaller than the capillary length (~ 1mm) via coalescence induced droplet jumping. Jumping-droplet condensation has been demonstrated to enhance heat transfer, anti-icing, and self-cleaning efficiency, and is governed by the theoretical inertial-capillary scaled jumping speed (U). When two droplets coalesce, the experimentally measured jumping speed (U_exp) is fundamentally limited by the internal fluid dynamics during the coalescence process (U_exp 2) coalescence as an avenue to break the two-droplet speed limit. Using side-view and top-view high-speed imaging to study more than 1000 jumping events on a copper oxide nanostructured superhydrophobic surface, we verify that droplet jumping occurs due to three fundamentally different mechanisms: 1) coalescence between 2 droplets, 2) coalescence between more than 2 droplets (multi-drop), and 3) coalescence between 1 or more droplets on the surface and a returning droplet that has already departed (multi-hop). We measured droplet-jumping speeds for a wide range of droplet radii (5-50 µm), and demonstrated that while the two-droplet capillary-to-inertial energy conversion mechanism is not identical to that of multi-drop jumping, speeds above the theoretical two-droplet limit (> 0.23U) can be achieved. However, we discovered that multi-hop coalescence resulted in drastically reduced jumping speeds (<< 0.23U) due to adverse momentum contributions from returning droplets. To quantify the impact of enhanced jumping speed on heat transfer performance, we developed a condensation critical heat flux model to show that modest jumping speed enhancements of 50% using multi-drop jumping can enhance performance by up to 40%