High-speed video microscopy and numerical modeling of bubble dynamics near a surface of urinary stone

Abstract

Ultra-high-speed video microscopy and numerical modeling were used to assess the dynamics of microbubbles at the surface of urinary stones. Lipid-shell microbubbles designed to accumulate on stone surfaces were driven by bursts of ultrasound in the sub-MHz range with pressure amplitudes on the order of 1 MPa. Microbubbles were observed to undergo repeated cycles of expansion and violent collapse. At maximum expansion, the microbubbles' cross-section resembled an ellipse truncated by the stone. Approximating the bubble shape as an oblate spheroid, this study modeled the collapse by solving the multicomponent Euler equations with a two-dimensional-axisymmetric code with adaptive mesh refinement for fine resolution of the gas-liquid interface. Modeled bubble collapse and high-speed video microscopy showed a distinctive circumferential pinching during the collapse. In the numerical model, this pinching was associated with bidirectional microjetting normal to the rigid surface and toroidal collapse of the bubble. Modeled pressure spikes had amplitudes two-to-three orders of magnitude greater than that of the driving wave. Micro-computed tomography was used to study surface erosion and formation of microcracks from the action of microbubbles. This study suggests that engineered microbubbles enable stone-treatment modalities with driving pressures significantly lower than those required without the microbubbles.

On the governing fragmentation mechanism of primary intermetallics by induced cavitation

“…Moreover and according to Ref. [23] , pressure surges in the range of 1.6 MPa as measured with the fibre-optic hydrophone in our study can be achieved at a distance of 100–150 μm from the point of the bubble collapse. So, if an individual bubble of a few tens of microns size collapses beyond this distance, the crystal is not likely to be damaged in a short period of time.…”

“…This study also highlights that shock waves emitted from the active cavitation zone (just below the sonotrode surface) travel up to considerable distances carrying sufficient potential energy to damage intermetallic crystals upon impact; this is unlike other studies [21] , [23] , [29] , [70] which demonstrate that cavitation bubbles should be in very close proximity to the solid boundaries in order to produce severe damage. Thus, the limits of the active cavitation zone can be potentially quantified and optimised promoting effective cavitation treatment.…”

“…According to Wang et al [21] , the probable reason for this prolonged breaking time of crystal corresponds to the emitted pressure waves from the individual pulsating bubbles with radii in the range of 50–80 μm which decay significantly with increasing distance. Specifically, a recent study by Pishchalnikov et al [23] reported that a bubble of a similar size as observed in [21] generates pressure as high as 50 MPa at a distance of about 30 μm from the collapsing spot. This essentially indicates that for instantaneous breakage to happen (as in the case of the laser-induced bubble experiments), the bubble collapse should occur immediately next to the crystal.…”

“…From other studies, it is also anticipated that fragmentation of dendrites in real melts happens by the combined effect of acoustic streaming jet impact [12] and shock waves propagation [22] from imploding bubbles. On the other hand, a recent similar study on the fragmentation of kidney stones suggested that the potential mechanism of stones’ fragmentation is primarily associated with impinged liquid jet followed by powerful shock wave emissions [23] .…”

Towards an understanding of the chemo-mechanical influences on kidney stone failure via the material point method

“…Finally, the work in this paper focused on the direct interaction between the traveling pressure wave and the effect on the submerged stone. However, as has been shown in several works [49][50][51][52] cavitation of bubbles at the surface of the stone can lead to a significant amount of damage in the stone, which was not modeled in this work. While this work shows novel approaches in the field of kidney stone modeling, there are still several limitations present that should be addressed.…”