Shape Oscillation and 3-D Microstreaming Profile of a Phospholipid-Coated Microbubble Attached to a Wall.

Ultrasound-activated microbubbles display diverse dynamics, including shape oscillations and microstreaming, both of which are important for applications in particle transport, drug delivery, and bacterial interventions. Despite advancements, a knowledge gap persists in our understanding of the correlation between shape oscillations, corresponding microstreaming, and resulting mechanical impact to neighboring boundaries for microbubbles of clinically relevant sizes. This study investigates this correlation for biotinylated phospholipid-coated microbubbles with radii of 3.0- $7.3~\mu $ m (N = 79) when attached to a streptavidin-coated wall, imaged at 5 million and 10 000 frames per second to visualize both shape oscillations and microstreaming. Driven at 1.25-MHz frequency, 25 000 cycles, and varying peak negative acoustic pressures (170-420 kPa), the larger sized microbubbles showed higher shape modes (up to mode 6) than the smaller ones. A shape stability model predicted the dominant shape mode with 89% agreement with experimental findings. Four distinct 3-D microstreaming patterns were observed by employing astigmatic particle tracking velocimetry (APTV): patternless, radial, dipole, and quadrupole. Modal decomposition of shape oscillations revealed that the quadrupole pattern occurred from self-interaction of the dominant shape mode, while the dipole pattern resulted from two strongly interacting shape modes. The quantified microstreaming showed maximum velocities ranging from 2 to 10 mm/s, which decayed with increasing distance from the microbubble center. Notably, lower shape modes and the quadrupole pattern were associated with higher wall shear stress (up to 13 Pa). These unique 3-D microstreaming profiles have the potential to enhance the efficiency of therapeutic microbubble-mediated ultrasound applications through controlled shape oscillation and mechanical impact.