Sub-wavelength scale randomly frozen microbubble during short-pulsed-ultrasound-driven microbubble cluster dynamics in microfluidic channel

Microbubble cloud dynamics under short-pulse ultrasound in flowing environments remain insufficiently understood. Here, we investigate microbubble cloud behaviour in a vessel-mimicking microfluidic channel under short-pulse ultrasound (1.125 MHz) and controlled laminar flow (37.5-150 µL/min). High-speed visualization reveals two distinct regimes: an actively interacting regime characterized by clustering and coalescence, and a 'frozen' regime in which microbubbles exhibit minimal displacement despite continued ultrasound excitation. Both regimes lead to the formation of spatially frozen, yet oscillating, microbubbles at the subwavelength scale. We observe clustered frozen microbubbles in the frozen region and isolated relative larger microbubble in the actively interacting region. A theoretical model including hydrodynamic drag with wall correction captures the transition between actively moving and frozen states. The results indicate that microbubble cloud dynamics under short-pulse excitation is determined by a dynamic competition between acoustic radiation forces and near-wall hydrodynamic drag, with flow rate and pulse duration acting as coupled control parameters.