Jetting Dynamics of Viscous Droplets on Superhydrophobic Surfaces

We investigated the dynamics of liquid jets engendered by the impact of droplets on a fractal superhydrophobic surface. Depending on the impact conditions, jets emanate from the free liquid surface with several different shapes and velocities, sometimes accompanied by droplet ejection. Experimental outcomes exhibit two different regimes: the singular jet and columnar jet. We found that droplet impacts at a lower impact velocity and low viscosity result in singular jets, attaining a maximum velocity nearly 20-fold higher than the impact velocity. The high-speed video frames reveal that the formation and subsequent collapse of the cylindrical air cavities within the droplet favor the formation of these high-speed singular jets. In contrast, the capillary wave focusing engenders columnar jets at a moderate to high impact velocity. With an increase in viscosity, singular jets are suppressed at lower impact velocities, whereas columnar jets are seen regularly. The columnar jets ascend and grow over time, feeding a bulbous mass, and subsequently the bulb separates itself from the parent jet due to capillary pinch-off phenomena. The quantitative analysis shows that columnar jets' top jet drop size varies nonmonotonically and is influenced by preceding jetting dynamics. At moderate viscosity, the drop size varies with jet velocity, following a power-law scaling. At very high viscosities, both singular and columnar jetting events are inhibited. The results are relevant to several recent technologies, including microdispensing, thermal management, and disease transmission.

Automated summary

The dynamics of liquid jets generated by droplet impact on a fractal superhydrophobic surface were investigated. Two different regimes of jets, singular and columnar, were observed depending on the impact conditions. Singular jets were formed at lower impact velocities and low viscosity, with a maximum velocity up to 20 times higher than the impact velocity. Cylindrical air cavities within the droplet played a crucial role in the formation of high-speed singular jets. In contrast, columnar jets were generated through capillary wave focusing at moderate to high impact velocities. The size of columnar jet drops varied nonmonotonically and was influenced by the preceding jetting dynamics. Both singular and columnar jetting events were suppressed at very high viscosities. These findings have implications for various technologies such as microdispensing, thermal management, and disease transmission.