Abstract
When a cavitation bubble oscillates and collapses in the vicinity of a solid boundary (a substrate), it induces intense microconvection in the surrounding liquid and—of high practical importance—directly at the substrate. As the involved flows are fast, highly unsteady, and possess an impressive shear, experiments are difficult and data are scarce. Here, insight into the generation and dynamics of the liquid flows from individual cavitation bubbles collapsing in the vicinity of a solid boundary is provided. Single laser-induced cavitation bubbles (maximum radius around 375 μm) are seeded at precisely defined standoff distances to a substrate by a focused laser pulse. The bubble shape dynamics are imaged by synchronized high-speed cameras from two perpendicular viewing angles. Recording of the shape dynamics is combined with the simultaneous time-resolved measurement of the full flow field on a micrometer-resolution. Measurements employ a high-speed hybrid particle imaging velocimetry and particle tracking velocimetry technique, with a temporal sampling of up to 135 kHz, using fluorescent microparticles as tracers. The time evolution of the unsteady flow field induced by one and the same bubble over a time period much longer than the bubble lifetime is determined. The shear flow at the substrate is analyzed and a liquid transport toward and away from the substrate surface is demonstrated. Depending on the bubble standoff distance, very different flow patterns are observed. The dominant liquid displacement is caused by the long-lived vortex ring being produced during bubble collapse. Most peculiar, the bubble standoff distance determines the sense of direction of the circulation associated with the vortex ring and, consequently, whether the vortex is ejected from the substrate or radially stretches over it. The results are relevant for the understanding of cavitation effects, such as surface cleaning, erosion, and mixing or in biomedical context and may serve as basis for numerical simulations.
10 More- Received 6 December 2016
DOI:https://doi.org/10.1103/PhysRevFluids.2.064202
©2017 American Physical Society