Abstract
A high-speed jet is formed when a pulse force acts on a concave interface of the liquid, which is due to the focus of the momentum on the center of curvature of the interface. We also found that there are fascinating jets in the experiments with a pulse acceleration imposed on the convex liquid interface while the arrangement is limited between two parallel panels. The mechanism of the convex interface jets is analyzed experimentally and numerically in this paper. A droplet with a diameter of approximately 12 mm is generated between two 0.8-mm spacing parallel plates coated with hydrophobic materials. A pulsed laser is focused at the center of the droplet through a convex lens to generate pulsed bubbles so that the movement of the interface is accelerated. The evolution of the liquid jets is observed by high-speed photography. A high-precision numerical method of the interface is established based on the volume of fluid method (VOF) and large eddy simulation (LES), which is enabled to capture the gas-liquid interface and small-scale flow structures exactly. An intriguing jet close to the plate is observed while the concave interface only forms that near the centerline. A charming entrainment phenomenon captured in the development of the jets, is mainly related to the big difference (Δθ) in contact angles between two plates, with a phase diagram given in the present work. Finally, a discussion responding to the influence of different contact angles on the tip velocity of a jet is done, concluding that the jet velocity increases gradually with the enlargement of Δθ to some extent. In addition, the hydrophobic plate, which is regarded as a mirrored plane and reflects the momentum of the fluid, plays a significant role in the formation of hydrophobic jets as well. Our findings offer a new understanding both of the formation of jets and surface cleaning.
