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