Institute of Fundamental Technological Research

We investigated gas release from two hard- shelled ultrasound contrast agents by subjecting them to high-mechanical index (MI) ultrasound and simultaneously capturing high-speed photographs. At an insonifying frequency of 1.7 MHz, a larger percentage of contrast bubbles is seen to crack than at 0.5 MHz. Most of the released gas bubbles have equilibrium diameters between 1.25 and 1.75 m. Their disappearance was observed optically. Free gas bubbles have equilibrium diameters smaller than the bubbles from which they have been released. Coalescence may account for the long dissolution times acoustically observed and published in previous studies. After sonic cracking, the cracked bubbles stay acoustically active.

Sonic cracking

When gas bubbles collide, the following stages of bubble coalescence have been reported: flattening of the opposing bubble surfaces prior to contact, drainage of the interposed liquid film toward a critical minimal thickness, rupture of the liquid film, and formation of a single bubble. During insonification, expanding contrast agent microbubbles may collide with each other, resulting in coalescence or bounce.
In this study, we investigate the validity of the film drainage formalism for expanding free bubbles, by subjecting rigid-shelled contrast agent microbubbles to ultrasound, in order to release gas, and photograph the coalescence of these free gas bubbles. As with colliding bubbles, bubble surface flattening is related to the Weber number. Only inertial film drainage between free interfaces explains the observed coalescence times. In accordance with theory, smaller bubble fragments coalesce on very small time scales, while larger bubbles bounce off each other.