Jumping droplets

When microdroplets with quasi-spherical contact angles coalesce together on a low-adhesion substrate, the capillary-inertial expansion of the liquid bridge induces a dramatic out-of-plane jumping event due to symmetry breaking. From the onset of merging, droplet jumping initiates after a capillary-inertial time scale of $t_{\text{ci}}\sim 1–100$ μs with characteristic jumping speeds of order $v_j\sim 0.1$ m/s. This coalescence-induced jumping-droplet effect is most commonly observed among a population of growing dew droplets on a superhydrophobic condenser, but can also occur by colliding deposited droplets together or during droplet sliding on fog harvesters. In this review, we cover the historical development of capillary-inertial jumping droplets, summarize the decade-long effort to rationalize the ultra-low energy conversion efficiency and critical droplet size of the phenomenon, and then present 15 variations on a theme of jumping. Capillary-inertial jumping droplets are not only a visceral illustration of the surprising power of surface tension at the microscale but they also have the potential to enhance phase-change heat transfer, enable self-cleaning surfaces, combat frost formation, harvest energy, and govern the rate of disease spread for wheat crops.