Experimental Investigation of the directional collapse and microjet dynamics of single acoustic bubbles in confined tubes.

Understanding the behavior of single cavitation bubbles under ultrasonic excitation is crucial for biomedical applications, particularly targeted drug delivery. This study experimentally investigates the physics of acoustically driven bubble shape instabilities within confined tubes (inner diameters 600-800 μm) designed to mimic human micro-vessels. Utilizing an ultrafast photomicrographic system, we captured high-resolution recordings of the spatiotemporal evolution and translational trajectories of micrometric air bubbles. The experimental results demonstrate that when a bubble is positioned on the tube's axial line, an increase in acoustic pressure significantly alters bubble morphology and reduces collapse time. Notably, the formation of high-velocity upward microjets was observed during collapse phases, with peak jet velocities scaling positively with ultrasonic intensity. A comparative analysis of varying initial bubble sizes reveals that the tube boundary induces a fourth-harmonic acoustic response, which substantially enhances the intensity of the bubble's horizontal migration, rotational dynamics, and collapse violence. Furthermore, the initial proximity of the bubble to the tube wall was identified as a critical determinant of cavitation dynamics. Detailed analysis shows that a shorter initial distance to the boundary accelerates the rotational path of the bubble towards the wall, leading to an earlier and more intense asymmetric collapse. These findings provide new quantitative insights into the microscopic mechanisms of ultrasound-induced cavitation in confined geometries, offering a framework for optimizing ultrasonic applications in clinical and microfluidic environments.