Quantification of microbubble-induced streaming across ultrasound frequencies using particle image velocimetry

Abstract

Functional microbubbles are widely used in therapeutic ultrasound for drug delivery, blood-brain barrier opening, clot lysis, and other biomedical interventions. Their therapeutic effects arise largely from acoustic microstreaming, which generates localized shear stresses and enhances transport around oscillating bubbles. Despite the central role of microstreaming, its dependence on ultrasound excitation conditions is not well defined. Existing studies use diverse and often incomparable frequencies, pressures, and duty cycles, leaving a gap in systematic understanding of how these parameters govern the streaming velocity and shear fields produced by functional microbubbles under physiologically relevant conditions. In this study, we employ particle image velocimetry (PIV) in a popliteal-vein lab-on-chip model to directly quantify the microstreaming velocity and shear-rate fields generated by microbubbles across a wide range of ultrasound conditions including frequency, duty cycle, and acoustic pressure. Measurements at 150 kHz, 300 kHz, 500 kHz, and 1 MHz reveal a pronounced frequency dependence. Counterintuitively, the strongest microstreaming is observed at 150 kHz, with progressively weaker streaming at 300 and 500 kHz, and the weakest flows at 1 MHz. This trend contrasts with the common assumption that microbubble oscillations peak near their [~]1 MHz resonance frequency. Duty cycle further modulated the response: 10% duty cycle generated the weakest fields, whereas 100% duty cycle produced maximal streaming. The results reveal a strong nonlinear pressure dependence at low frequencies, particularly at 300 kPa where flow amplification was most pronounced. These findings underscore that acoustic frequency plays a substantial role in intensity of microbubble-induced streaming, with low-frequency excitation in the 150-300 kHz range producing markedly more efficient streaming and associated shear. Adopting acoustic parameters that maximize microstreaming within this low-frequency window while avoiding conditions that promote inertial cavitation may improve both the efficacy and safety of microbubble-mediated treatments.