Ultra-High-Speed Photography and Hydrophone Measurement of Laser-Induced Shock Waves in Liquids: Viscous Effects on Spherical Shock Dynamics

抄録

We experimentally study viscous effects on spherical shock waves in glycerol-water solution. A spherical shock wave is generated through rapid expansion of plasma by focusing a nanosecond laser pulse (532 nm) of energy fixed at 1.66 ± 0.22 mJ into the solution with varying glycerol concentration. In the near field, the shock propagation is recorded by an ultra-high-speed camera with temporal resolution of 10 ns. The recorded images are used to obtain the shock evolution, allowing for constructing the shock pressure evolution with the Rankine-Hugoniot relation. It turns out that the plasma pressure is reduced as the solution gets more viscous with increasing the glycerol concentration; wave steepening effect is deemphasized in the more viscous solution, leading to slower decay of the near-field shock pressure. In the far field, the shock pressure is recorded by a hydrophone at 2 mm to 10 mm from the laser focus and compared to linear acoustic theories with or without viscous absorption. The comparison shows that for the case of higher glycerol concentration (say, beyond 80 wt%), the inviscid assumption will fail, meaning that the viscous theory needs to be used for more accurate prediction of the far-field shock pressure.