Delayed droplet coalescence during droplet shedding from superhydrophobic surfaces

Abstract

Droplets shedding from inclined functional surfaces has important implications in dropwise condensation and anti-icing applications because it allows for fresh nucleation sites, effectively enhancing the heat transport. It is particularly important for dropwise condensation, which has been showed to be highly effective in terms of heat transfer when compared to filmwise condensation. Here, we study the shedding behavior of water droplets from inclined textured superhydrophobic substrates. Our experimental setup, which consists of high speed imaging and a piezoelectric dispenser, is specifically tailored to image the shedding droplets from the side and the top, while depositing microdroplets to a larger droplet at a predefined rate. Our results show that at the onset of droplet shedding, the coalescence of the microdroplets, which was instantaneous before the shedding occurs, starts to be delayed significantly, as evidenced by the presence of a number of satellite droplets on the surface of the larger droplet. Such a delay would cause the heat transfer enhancements from dropwise condensation to plummet and needs to be studied in detail. For that purpose, we studied three different explanations; namely antibubble formation, cloaking, and instabilities. Our investigation eliminated antibubble formation and cloaking as possible explanations, and determined the instabilities caused by the rapid rotation of the droplet while shedding generated an outward acceleration which limited the capacity to coalesce. This rotation was caused by a combination of external forces applied by the micro/nanostructures on the surface, and internal forces applied by the rapidly changing Laplace pressure. Our study presents a fundamental understanding of a unique fluid dynamics phenomenon with many implications to condenser surface design with the potential to be further generalized into the whole area of interfacial physics.