Abstract
Self-propelled jumping drops are continuously removed from a condensing superhydrophobic
surface to enable a micrometric steady-state drop size. Here, we report that subcooled
condensate on a chilled superhydrophobic surface are able to repeatedly jump off the surface
before heterogeneous ice nucleation occurs. Frost still forms on the superhydrophobic surface
due to ice nucleation at neighboring edge defects, which eventually spreads over the entire
surface via an inter-drop frost wave. The growth of this inter-drop frost front is shown to be
up to three times slower on the superhydrophobic surface compared to a control hydrophobic
surface, due to the jumping-drop effect dynamically minimizing the average drop size and
surface coverage of the condensate. A simple scaling model is developed to relate the success
and speed of inter-drop ice bridging to the drop size distribution. While other reports of
condensation frosting on superhydrophobic surfaces have focused exclusively on liquid-solid
ice nucleation for isolated drops, these findings reveal that the growth of frost is an inter-drop
phenomenon that is strongly coupled to the wettability and drop size distribution of the surface.
A jumping-drop superhydrophobic condenser was found to be superior to a conventional dropwise condenser in two respects: preventing heterogeneous ice nucleation by continuously
removing subcooled condensate, and delaying frost growth by minimizing the success of interdrop
ice bridge formation.
