When a cavitation bubble collapses in vicinity to a solid surface, high flow velocities are induced. They involve remarkably high but unsteady wall shear rates. Even though they are crucial in ultrasonic surface cleaning or cavitation erosion, and their knowledge is needed for validation of numerical methods, they have not been measured so far due to experimental difficulties. Here, a wall shear rate raster microscope was developed. It bases on an electrochemical principle and involves a model to solve the appropriate convection-diffusion equation. As wall shear rate sensor, a microelectrode was flush-mount into a solid surface. With this method, wall shear rates on micrometer and microsecond scales can be resolved. The wall shear rates produced during the collapse of a single, laser generated bubble (maximum radius about 400 μm) were measured in planes. Via the synchronously performed high speed imaging, the wall shear rates can be clearly related to details and the different stages of the bubble dynamics. This way the respective impacts of the flow phenomena involved on the generation of wall shear stress were evaluated. For example, the jet that accompanies the bubble collapse was resolved in terms of wall shear rates during its impact on the wall and its subsequent spreading in radial direction. The above experimental data are compared with results of numerical simulations of the collapse of a single bubble obtained with a compressible two-phase flow solver that uses barotropic equations of state.
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