Cavitation bubble interaction with compliant structures on a microscale: a contribution to the understanding of bacterial cell lysis by cavitation treatment

date: 14.06.2022

category: Sporočila za javnost

 

Researchers at FME UL have published an article in which Microbubble collapse in vicinity of a bacterial cell is investigated numerically. Asist. raz. Jure Zevnik and prof. dr. Matevž Dular published the article Cavitation bubble interaction with compliant structures on a microscale: A contribution to the understanding of bacterial cell lysis by cavitation treatment in Ultrasonic Sonochemistry (IF: 7.491).

Numerous studies have already shown that the process of cavitation can be successfully used for water treatment and eradication of bacteria. However, most of the relevant studies are being conducted on a macro scale, so the understanding of the processes at a fundamental level remains poor. In attempt to further elucidate the process of cavitation-assisted water treatment on a scale of a single bubble, the present paper numerically addresses interaction between a collapsing microbubble and a nearby compliant structure, that mechanically and structurally resembles a bacterial cell.

zevnik1

A schematic representation of the considered setup – an initially stable microbubble (left) in vicinity of a freely submerged spherical bacterial cell (right).

A fluid–structure interaction methodology is employed, where compressible multiphase flow is considered and the bacterial cell wall is modeled as a multi-layered shell structure. Simulations are performed for two selected model structures, each resembling the main structural features of Gram-negative and Gram-positive bacterial cell envelopes. The contribution of two independent dimensionless geometric parameters is investigated, namely the bubble-cell distance δ and their size ratio ς. Three characteristic modes of bubble collapse dynamics and four modes of spatiotemporal occurrence of peak local stresses in the bacterial cell membrane are identified throughout the parameter space considered. The former range from the development of a weak and thin jet away from the cell to spherical bubble collapses.

zevnik2

Pressure (top) and velocity (bottom) contours along with bubble (left) and bacterial cell (right) shape progression for a sample case that resembles bubble collapse mode J - weak jet away from the bacterium: Gram-positive model organism, ς = 1, and δ = 1.01.

zevnik3

Peak values of stresses in the inner cell membrane across the considered parameter space δ-ς, as predicted by the numerical model for a) Gram-negative and b) Gram-positive model organism. Solid black lines represent the interpolated peak stress contours with spacing of 1 MPa and both bold lines the estimated contours at the membrane poration threshold of 20 MPa.

The results show that local stresses arising from bubble-induced loads can exceed poration thresholds of cell membranes and that bacterial cell damage could be explained solely by mechanical effects in absence of thermal and chemical ones. Based on this, the damage potential of a single microbubble for bacteria eradication is estimated, showing a higher resistance of the Gram-positive model organism to the nearby bubble collapse. Microstreaming is identified as the primary mechanical mechanism of bacterial cell damage, which in certain cases may be enhanced by the occurrence of shock waves during bubble collapse. The results are also discussed in the scope of bacteria eradication by cavitation treatment on a macro scale, where processes of hydrodynamic and ultrasonic cavitation are being employed.

Link to the article: https://doi.org/10.1016/j.ultsonch.2022.106053

zevnik4

Estimated critical non-dimensional bubble-bacterium distances δ* (solid black line) for poration of the inner cell membrane in relation to the bubble-bacterium size ratio ϛ. Regions in δ-ϛ parameter space where membrane poration threshold is exceeded are given for both Gram-negative (blue fill) and Gram-positive (orange fill) model cell.

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