How Surface Orientation Affects Jumping-Droplet Condensation

The out-of-plane jumping of coalescing droplets on superhydrophobic surfaces is capillary-inertial in nature; therefore, the effects of gravity or surface orientation are generally ignored. Here, we show that surface orientation actually has a profound effect on jumping-droplet condensation. For a horizontally oriented superhydrophobic surface, jumping droplets return to the surface and eventually become stuck. In contrast, vertical orientations can remove stuck droplets by gravitational shedding, which also serves to sweep away neighboring condensate. These differences affect the resulting droplet size distribution, which was captured using time-lapse photography and image analysis for 3 h of jumping-droplet condensation on horizontal, 45 degrees tilted, or vertically oriented surfaces. The maximum droplet size was an order of magnitude smaller for the inclined surfaces compared to the horizontal orientation, which would enhance the theoretical heat transfer coefficient by 40% for the 45 degrees tilt and by 100% for the vertical orientation.