When Do Microdroplets Bounce?

The interactions between microdroplets and surfaces result in a diverse range of phenomena. We observe 30 to 50 μm diameter droplets sticking and partially rebounding off surfaces with static contact angles as low as 110° using a 100,000 frames per second camera. We show that bouncing off a hydrophobic surface like Teflon is velocity-dependent due to surface adhesion, and contact angle hysteresis gives rise to a partial rebound mechanism. This experimental work is backed up by numerical phase field simulations using COMSOL and a simple ball, spring, and dashpot model. All these elements combine to give rise to simple stick-to-bounce transition criteria dependent on the droplet and surface properties. This criterion establishes that for low viscous forces (small Ohnesorge number), only slowly incoming (small Weber number) droplets stick to the surface, whereas at larger viscosities and for superhydrophobic surfaces, the transition becomes velocity-independent. This stick-to-bounce transition has broad implications for understanding aerosol behaviour near surfaces, from bioaerosols to industrial processes.