Enhancement of Focused Liquid Jets by Surface Bubbles

We investigate the enhancement of the velocity of focused liquid jets by surface bubbles preformed on the inner surface of the container. The focused jets are created from the impact on a liquid-filled cylindrical tube at cavitation numbers of 0.37 (strong impact where cavitation is likely to occur on unprocessed surfaces) and 2.1 (weak impact where cavitation does not occur from the impact). The bubbles with a base diameter up to hundreds of micrometers were formed via the process of solvent exchange using air-equilibrated ethanol and water. Our measurements by high-speed imaging show that at both cavitation numbers, the jet velocities with preformed bubbles are significantly higher than those without preformed bubbles. Furthermore, our results show that after the process of solvent exchange, a large number of expanding bubbles are observed at cavitation number of 0.37, indicating that possibly both sub-millimeter and sub-micrometer bubbles on the surface contribute to the jet velocity enhancement. At the cavitation number of 2.1, the surface bubbles are observed to oscillate immediately after the impact. The measurements of the liquid pressure after the impact reveal that at both cavitation numbers, the negative pressure is damped by the preformed surface bubbles, contributing to the increase of the jet velocity. This work sheds light on the crucial role of surface bubbles on the impulsive motion of liquids. Our findings have significant implications for the focusing jet technology, opening the opportunities for jetting fragile samples such as biological samples