Abstract
Impact velocity plays a crucial role in the control of droplet ejection when producing micro-droplets with directed diameters and velocities from millimetre-sized droplets. In this study, high-speed photography is used to experimentally investigate the characteristics of the ejection of micro-droplets after impacting a super-hydrophobic surface with different impact velocities ranging from 0.16 to 2.34 m/s. The results indicate that droplet ejection only occurs at impact velocities ranging from 0.26 to 1.31 m/s. Furthermore, it is found that the droplet ejection process can be classified into five stages based on the Weber number. In Stages 1–3, the ratio of the ejected to initial droplet diameters changes only slightly with the Weber number. In Stage 4, the ratio first increases, then decreases, and finally increases again. In Stage 5, the ratio decreases linearly. An equation describing the decrease in the ratio during Stage 5 is obtained and the correlation between the ratio of the ejection to impact velocities and the Weber number for the same stage is investigated. It is concluded that the formation of the central liquid column and the final characteristics of droplet ejection are mainly the result of the competition among inertial force, reaction force from the surface, gravity, and surface tension; and the formation of the jet is determined by the collapse of the air cavity.
Published Version
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