The influence of distance between microbubbles on the fluid flow produced during ultrasound exposure

The collapse dynamics of lipid monolayer-coated microbubbles in the clinically-relevant size range under 6 μm in diameter have not been studied directly due to their small size obscuring the collapse visualization. This study investigates the influence of inter-microbubble distance on the shape of lipid debris clouds created by the collapse of the microbubble destroying the microbubble lipid monolayer. The shape was highly influenced by the fluid motion that occurred as the microbubbles collapsed. It was observed that at inter-microbubble distances smaller than 37 μm the microbubbles began to interact with one another resulting in distorted and ellipsoid-shaped debris clouds. At inter-microbubble distances less than 10 μm, significantly elongated debris clouds were observed that extended out from the original microbubble location in a single direction. These distortions show a significant distance-dependent interaction between microbubbles. It was observed that microbubbles in physical contact with one another behaved in the same manner as separate microbubbles less than 10 μm apart creating significantly elongated debris clouds. It can be hypothesized that small inter-microbubble distances influence the microbubble to collapse asymmetrically resulting in the creation of fluid jets that contribute to the formation of debris fields that are elongated in a single direction.