A passive technique for coalescence-induced jumping of droplets (with high viscosity or low surface tension) at Ohnesorge number > 1*

When two liquid droplets coalesce on a super-repellent surface, the excess surface energy is partly converted to upward kinetic energy and the coalesced droplet jumps away from the surface. However, the efficiency of this energy conversion is very low. In this work, we employed a simple and passive technique consisting of superomniphobic surfaces with a macrotexture to experimentally demonstrate coalescence-induced jumping with an energy conversion efficiency of ≈ 19% (i.e., ≈ 600% increase compared to superomniphobic surfaces without a macrotexture). The high energy conversion efficiency arises primarily from the effective redirection of in-plane velocity vectors to out-of-plane velocity vectors by the macrotexture. Utilizing the increased energy conversion efficiency on our superomniphobic surfaces with a macrotexture, we demonstrate coalescence-induced jumping of droplets with low surface tension droplets (26.6 mN m^-1) and very high viscosity (220 mPa s). These results constitute the first-ever demonstration of coalescence-induced jumping of droplets at Ohnesorge number > 1.