Cavitation erosion by shockwave self-focusing of a single bubble

In situ measurement of cavitation damage from single bubble collapse using high-speed chronoamperometry

“…Clearly, the bubble dynamics are significantly damaging only for . This has been previously reported and is a result of shockwave self-focusing during the collapse [6] . The highest damage is found here for .…”

“…Here we assume that the material removal mainly occurs in the passive film through a skimming mechanism, i.e., layer-like progression induced by the shear stresses. Some weight-lossless plastic deformation in the form of indentation is also expected [6] though this limited plastic deformation could not result in formation of cracks or material removal in the form of detached particles on the surface of ductile aluminium. As the surface will be quickly repassivated after the bubble collapse, the weight loss of the sample due to the removal of passive layer is roughly equal to the weight of elementary aluminium which reacts with oxygen, i.e., Δw in the Faraday equation.…”

“…We suspect that the peaks get wider for subsequent collapses as the bubble tends to fragment during the first collapse and loses some of its cylindrical symmetry. This can result in an “asynchronous collapse” where parts of the bubble are already collapsing while others are still expanding [6] .

Fig.

…”

“…Instead, when parts of the ring bubble are already in the rebound stage, others are still collapsing. Whereas, this process is particularly fast in the heavily erosive small stand-off case where it takes only about 75 ns, it can take several microseconds during the second collapse because the sphericity of the rebounded bubble is significantly lower and the bubble tends to fragment [6] . Further, it should be considered that after the first collapse the sensor, i.e.…”

“…For smallest bubble to wall stand-offs, the bubble collapse is amplified by a shockwave self-focusing mechanism that drives an erosive collapse of part of the gas phase and produces erosion away from the axis of symmetry. This has been shown to be the only significant mechanism behind cavitation erosion which is able to generate visible damage already after a single bubble impact on hard metal surfaces, without fatigue at play [6] . In the later work the damage detection could be done in-situ but required specialized imaging equipment and the damage quantification relied on ex-situ measurements.…”