Waterbowls: Reducing Impacting Droplet Interactions by Momentum Redirection.

Superhydrophobic surfaces can dramatically reduce the transport of mass or energy from impacting droplets by making them bounce off. Such transport processes are dependent on both the contact time and the contact area between the drop and the surface. To reduce transport, recent studies have focused on reducing the contact time. However, the contact area, which is dictated by the conformal spreading of the droplet, remains large, thereby limiting the reduction in the overall transport. Here, we show how surfaces can be designed to redirect the momentum of the spreading lamella causing it to lift off into 3-dimensional shapes, thereby dramatically reducing the contact area. We design superhydrophobic surfaces with an in-plane discontinuity leading to the accumulation of vertical momentum resulting in the out-of-plane ejection of the lamella into water bowls. We demonstrate a 2-fold reduction in the heat transfer between a cold rain and a warm surface. These insights can be broadly applied to other transport phenomena involving mass and energy exchange to limit heat loss under precipitation, icing of surfaces, reduce salt deposition on a surface exposed to ocean spray, or inhibit the formation of a water film on wings or wind turbine blades.