Microstreaming profile of a phospholipid-coated wall-attached microbubble undergoing shape oscillation

Ultrasound-activated microbubbles induce shape oscillation and microstreaming. However, a thorough understanding remains elusive. This study investigated the 3D microstreaming profile of clinically relevant lipid-coated microbubbles undergoing shape oscillation when bound to a wall. Size-controlled biotinylated microbubbles (radius 3–8 μm) were produced by a flow-focusing device and bound to a streptavidin-coated glass. Insonification spanned 85–425 kPa at 25,000 cycles and 1.25 MHz. Two cameras, operating at 5 Mfps and 10 kfps, coupled to a microscope, captured microbubble shape oscillation and cavitation microstreaming, respectively. Astigmatic particle tracking velocimetry measured 3D particle trajectories of 500-nm beads. In the dataset (n = 79), four microstreaming profiles were observed: quadrupole, dipole, radial, and patternless. Modal decomposition revealed that quadrupole patterns resulted from self-interaction of the predominant shape mode, while dipole patterns arose from strong interactions of two nearby modes at the same oscillation frequency. Microstreaming reached 0.002–0.01 m/s at varying acoustic pressures. Quadrupole microstreaming generally induced higher shear stresses than dipole and radial patterns at identical acoustic pressure. A lower shape mode induced higher shear stresses than a higher mode at the same acoustic pressure. The unique microstreaming characteristics yield diverse outcomes in mechanical impact, which could aid efficient therapeutic applications of ultrasound-activated microbubbles.