Droplet trampolining on heated surfaces in the transitional boiling regime

Droplet dynamics on a heated surface plays an important role in applications ranging from spray cooling to solidification and pharmaceuticals. Here, we show that in the transitional boiling regime, an unconstrained droplet exhibits trampolining where the droplet bounces to higher heights with each consecutive contact with the substrate. A water droplet of volume 1 mu L bounces to a height as high as 6-7 times of its diameter and the height of bounce increases to 18-20 times the diameter as the volume reduces to ~& nbsp; 0.1 mu L . The dynamics of the droplet in the transitional boiling regime results in a non-monotonic variation in the total evaporation time of the droplet with an increase in substrate temperature - an observation that is absent for a constrained droplet. We develop a force-based model to explain the trampolining behavior of the droplet and propose that the gain in momentum of the droplet that results in trampolining is caused by bubble nucleation and escape at the liquid-substrate interface prior to droplet takeoff. (C) 2022 Elsevier Ltd. All rights reserved.