Cavitation bubble interaction with a rigid spherical particle on a microscale.

Cavitation bubble collapse close to a submerged sphere on a microscale is investigated numerically using a finite volume method in order to determine the likelihood of previously suspected mechanical effects to cause bacterial cell damage, such as impact of a high speed water jet, propagation of bubble emitted shock waves, shear loads, and thermal loads. A grid convergence study and validation of the employed axisymmetric numerical model against the Gilmore's equation is performed for a case of a single microbubble collapse due to a sudden ambient pressure increase. Numerical simulations of bubble-sphere interaction corresponding to different values of nondimensional bubble-sphere standoff distance δ and their size ratio ε are carried out. The obtained results show vastly different bubble collapse dynamics across the considered parameter space, from the development of a fast thin annular jet towards the sphere to an almost spherical bubble collapse. Although some similarities in bubble shape progression to previous studies on larger bubbles exist, it can be noticed that bubble jetting is much less likely to occur on the considered scale due to the cushioning effects of surface tension on the intensity of the collapse. Overall, the results show that the mechanical loads on a spherical particle tend to increase with a sphere-bubble size ratio ε, and decrease with their distance δ. Additionally, the results are discussed with respect to bacteria eradication by hydrodynamic cavitation. Potentially harmful mechanical effects of bubble-sphere interaction on a micro scale are identified, namely the collapse-induced shear loads with peaks of a few megapascals and propagation of bubble emitted shock waves, which could cause spatially highly variable compressive loads with peaks of a few hundred megapascals and gradients of 100 MPa/μm.