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The development of needle-free injection systems utilizing high-speedmicrojets is of great importance to world healthcare. It is thus crucial tocontrol the microjets, which are often induced by underwater shock waves. Inthis contribution from fluid-mechanics point of view, we experimentallyinvestigate the effect of a shock wave on the velocity of a free surface(microjet) and underwater cavitation onset in a microchannel, focusing on thepressure impulse and peak pressure of the shock wave. The shock wave used had anon-spherically-symmetric peak pressure distribution and a sphericallysymmetric pressure impulse distribution [Tagawa et al., J. Fluid Mech., 2016,808, 5-18]. First, we investigate the effect of the shock wave on the jetvelocity by installing a narrow tube and a hydrophone in differentconfigurations in a large water tank, and measuring the shock wave pressure andthe jet velocity simultaneously. The results suggest that the jet velocitydepends only on the pressure impulse of the shock wave. We then investigate theeffect of the shock wave on the cavitation onset by taking measurements in anL-shaped microchannel. The results suggest that the probability of cavitationonset depends only on the peak pressure of the shock wave. In addition, the jetvelocity varies according to the presence or absence of cavitation. The abovefindings provide new insights for advancing a control method for high-speedmicrojets.

Effects of pressure impulse and peak pressure of a shock wave on microjet velocity and the onset of cavitation in a microchannel