Non-wettability propulsion mediated by the gas film after the impact of droplets

Droplet propulsion is important in numerous applications, such as microfluidic chips, biomimetic micro-robots, and drug delivery. A droplet on a liquid film always hovers and wets the surface throughout the impact process. In this study, we report a phenomenon in which a droplet on a non-uniformly heated surface exhibited self-propulsion from a cold to a hot surface without wetting. There was a micrometer-sized, or even thinner, gas film beneath the droplet that continuously prevented it from wetting the surface of the liquid film. An experimental investigation on droplet propulsion by color interferometry and high-speed photography showed a gas film beneath the droplet, and the thickness of the gas film was measured during droplet self-propulsion. The propulsion acceleration of the droplet increased linearly with the temperature gradient of the liquid film surface, and a theoretical model was developed to explain the observed self-propulsion of the droplet based on the dynamics of the thin gas film. When the temperature gradient of the liquid film surface was 0, the droplet stayed at the impact point without propulsion, and thus the droplet acceleration was 0. However, when the temperature gradient of the liquid film surface was 10.73 K/mm, the droplet acceleration could even reach 129.3 mm/s². The diameter of the gas film increased with the droplet size because more gas was entrapped by the larger droplet. As the droplet diameter increased from 1.5 to 2 mm, the diameter of the gas film almost doubled. The effects of the droplet viscosity on the droplet self-propulsion were negligible, but the extremely low-viscosity droplet propelled slowly because of the energy loss in the bouncing process. This self-propulsion can be exploited to manipulate a droplet mediated by thin gas film in specific directions through well-defined temperature gradients.