Ultrasound-induced dynamics of microbubbles near a boundary: vibrations, shape modes and jets.

Despite the demonstrated clinical utility of microbubbles in targeted drug delivery using ultrasound, the fundamental mechanism driving this process is still not fully understood. The occurrence of microdamage on soft biomaterials at the low acoustic intensities used in clinical practice cannot be attributed to inertial cavitation. Our objective is to address this knowledge gap by exploring the rich dynamics of acoustically driven single bubbles near a boundary and the role of their shape modes in inducing microjets. We leverage simultaneous high-speed phase-contrast X-ray and visible light shadowgraphy imaging to capture both the side and top views of single gas microbubbles that rest against a flat substrate and are driven by an ultrasound pulse. Through this, we are able to reconstruct the time-dependent morphology of the bubble revealing a specific sequence of events, starting with volumetric oscillation, followed by the inception of harmonic axisymmetric shape-modes, transitioning to half-harmonic ones, and finally, experiencing symmetry breakage and developing non-zonal shape modes. Notably, we discover that when the interfacial acceleration exceeds a certain threshold, the valleys of the shape deformation transition into jets directed toward the substrate at every cycle.