Microbubble dynamics at high mechanical index: ultrasound stimulated behaviour of SonoVue from optically predefined 'stand-off' positions

Introduction of a temporally periodic pressure field within a fluid can induce forced oscillations to bubbles present therein. The resultant [radial] bubble dynamics are a complex function of several parameters, including the driving pressure amplitude, and proximity to nearby boundaries, such as vessel walls, or indeed, other bubbles. Recently, experimentation gauged towards the development of a quantitative understanding of [acoustically] driven bubbles of micrometer dimensions, especially when close to boundaries, has become a challenge of heightened academic and industrial interest. In pursuit of this, the present authors pioneered a new approach to such measurements that exploits optical trapping to locate microbubbles at prescribed displacements from a boundary [1,2]. Here, we extend our previous method and report the first comprehensive study that has observed the dynamical behavior of isolated single micro-bubbles (the commercially available ultrasound contrast agent: SonoVue) that had been optically trapped over a range of well-defined displacements from a rigid boundary. All of the measurements were conducted at a mechanical index (MI) > 3. We noted a distinct variance in micro-bubble behavior across all quiescent radii and stand-off parameter, and also correlated bubble outcome statistics with measured radial dynamics. Finally, we suggest that the procedure outlined can be exploited to design 'next-generation' micro-bubbles with specific response characteristics.