Axisymmetric pressure field of laser-induced underwater shockwaves: non-contact high-speed measurement and analysis

Yoshiyuki Tagawa

Tokyo University of Agriculture and Technology Abstract


Image sequence of a laser-induced underwater shock wave

The axisymmetric pressure field of laser-induced underwater shockwaves is investigated. The shock waves in a microchannel play essential roles for transporting liquids quickly in microfluidic devices. Typical examples include the generation of supersonic microjets applicable for needle-free injection systems.

The measurement of the underwater shockwave is, however, difficult due to ultra-high velocity of the shockwave in water and small density fluctuation of water. In order to measure the axisymmetric pressure field of laser-induced underwater shockwaves, we adopt background-oriented schlieren (BOS) technique. The BOS technique is able to visualize density gradients and quantify pressure fields with a simple setup compared to other visualization techniques e.g. schlieren. For its validation, the shock wave has been measured by shadowgraph and hydrophones simultaneously. We construct an ultrahigh-speed BOS system, which is able to take images every 0.2 µs with high spatial resolution ∼10 µm/pixel. Furthermore, the physics-based optical flow method is incorporated into the BOS technique. This technique can obtain the displacement vector field at the theoretical resolution of one vector per pixel. The tomographic reconstruction is proposed and the corresponding pressure field is calculated, which gives the pressure field consistent with the result obtained by the hydrophone.

Based on the aforementioned measurements, we analyze the shockwave paying special attention to the pressure impulse, the time integral of the pressure evolution. Remarkably the distribution of the pressure impulse of a shock wave is spherically symmetric for a wide range of experimental parameters even when the distribution of the peak pressure is not spherically symmetric. The isotropic distribution of the pressure impulse may provide high degrees of freedom for the design of needle-free injection devices using laser-induced high-speed microjets.