Thermal atomization during droplet impingement on superhydrophobic surfaces: Influence of Weber number and micropost array configuration

Abstract An experimental study of thermal atomization intensity during droplet impingement on superheated hydrophobic and superhydrophobic surfaces of varying microstructure was performed. Thermal atomization in these scenarios is the result of droplet boiling, where vapor bubbles burst upwards through the droplet lamella, causing a fine spray of secondary droplets. A smooth hydrophobic surface and three post-patterned superhydrophobic surfaces of similar solid fraction but differing post size were investigated over a range of surface temperatures from 120 ∘C to 320 ∘C and Weber numbers from 20 to 200. Trends in atomization intensity were characterized using a high-speed image processing technique. Changes in surface temperature, Weber number, and microstructure configuration were shown to significantly influence atomization intensity, and these parameters are thought to be directly linked to three main mechanisms accounting for atomization dynamics in impingement scenarios. These mechanisms are vapor generation at the liquid-solid interface of the impinging droplet, vapor bursting through the spreading lamella, and vapor escape laterally beneath the droplet. Vapor generation increases with an increase in heat transfer to the droplet, which may be produced by increasing surface temperature or increasing liquid-solid contact through droplet wetting. Vapor bursting upwards through the lamella depends mainly on lamella thickness which decreases with increasing Weber number. Finally, vapor escape beneath the droplet may occur as vapor flows laterally through the micropost arrays. This is found to be enhanced by increasing the spacing between structures. These competing mechanisms result in thermal atomization, which generally increases with increasing Weber number and decreasing pitch. Additionally, the Leidenfrost point was also found to increase with increasing Weber number and decreasing pitch. A scale analysis was performed to explore the effect of resistance to vapor escape through micropost arrays on thermal atomization, and the resulting scaling describes the experimental findings well.