title: Enhanced Jumping-Droplet Departure.
abstract: Water vapor condensation on superhydrophobic surfaces has received much attention in recent years because of its ability to shed water droplets at length scales 3 decades smaller than the capillary length (∼1 mm) 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 (Uexp) is fundamentally limited by the internal fluid dynamics during the coalescence process (Uexp < 0.23U). Here, we theoretically and experimentally demonstrate multidroplet (>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 as a result of three fundamentally different mechanisms: (1) coalescence between two droplets, (2) coalescence among more than two droplets (multidroplet), and (3) coalescence between one or more droplets on the surface and a returning droplet that has already departed (multihop). 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 multidroplet jumping, speeds above the theoretical two-droplet limit (>0.23U) can be achieved. However, we discovered that multihop 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 multidroplet jumping can enhance performance by up to 40%. Our results provide a starting point for the design of enhanced-performance jumping-droplet surfaces for industrial applications.
authors.affiliation: Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign , Champaign, Illinois 61801, United States.
authors.forename: Moon-Kyung
authors.initials: MK
authors.lastname: Kim
authors.affiliation: Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign , Champaign, Illinois 61801, United States.
authors.forename: Hyeongyun
authors.initials: H
authors.lastname: Cha
authors.affiliation: Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign , Champaign, Illinois 61801, United States.
authors.forename: Patrick
authors.initials: P
authors.lastname: Birbarah
authors.affiliation: Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign , Champaign, Illinois 61801, United States.
authors.forename: Shreyas
authors.initials: S
authors.lastname: Chavan
authors.affiliation: Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign , Champaign, Illinois 61801, United States.
authors.forename: Chen
authors.initials: C
authors.lastname: Zhong
authors.affiliation: Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign , Champaign, Illinois 61801, United States.
authors.forename: Yuehan
authors.initials: Y
authors.lastname: Xu
authors.affiliation: Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign , Champaign, Illinois 61801, United States.
authors.forename: Nenad
authors.initials: N
authors.lastname: Miljkovic