Dynamics of cylindrical crevice microbubbles

Microbubbles are theorized to form in crevices on mineralizations causing twinkling, a rapid color shift observed when imaging hard mineralizations with Doppler ultrasound. While the classic Rayleigh–Plesset equation and subsequent modifications describe bubble dynamics in a free field, they do not account for crevice boundaries. In this study, cylindrical boundary conditions were incorporated into the derivation of the Rayleigh–Plesset equation and validated experimentally. Cylindrical crevices with diameters ranging from 0.08–1.2 μm and depths of 1 μm were etched into a silicon wafer. Bubbles that formed in these crevices were photographed at high-speed through an inverted microscope while being driven with ultrasound at 0.75, 2.5, and 5 MHz. Experimentally, for all tested crevice sizes, the bubbles did not visibly grow. In contrast, computational results using standard water surface tension (72.5 mN/m) predicted the bubbles should grow ∼6 mm. However, when applying a higher surface tension that was measured on the silicon-water interface (∼3000 mN/m), the bubbles were only predicted to grow ∼1 nm, supporting the experimental findings. These results offer insight into the mechanism causing twinkling and provides avenues for future investigation into the effect of crevice size and shape on twinkling. Work supported by NSF CAREER 1943937 and GRFP DGE1255832.