The ultrasound-induced formation of bubble clusters may be of interest as a therapeutic means. If the clusters behave as one entity, i.e., one mega-bubble, its ultrasonic manipulation towards a boundary is straightforward and quick. If the clusters can be forced to accumulate to a microfoam, entire vessels might be blocked on purpose using an ultrasound contrast agent and a sound source. Alternatively, the microfoam could be removed from the blood pool. The latter tech- nique might be applicable in highly toxic ultrasound-guided drug delivery. We analysed how ultrasound contrast agents with different shell compositions form clusters in a capillary and what happens to the clusters if sonication is contin- ued, using continuous driving frequencies in the range 1–10 MHz. We observed the following stages of microfoam formation within a dense population of microbubbles before ultrasound arrival. After the sonication started, contrast microbubbles collided, forming small clusters, owing to secondary radiation forces. These clusters coalesced within the space of a quarter of the ultrasonic wavelength, owing to primary radiation forces. The resulting microfoams translated in the direction of the ultrasound field, hitting the capillary wall, also owing to primary radiation forces. We have demonstrated that as soon as the bubble clusters are formed and as long as they are in the sound field, they behave as one entity. At our acoustic settings, it takes seconds to force the bubble clusters to positions approximately a quarter wavelength apart. The clusters contain approximately 2,000 ultrasound contrast agent microbubbles. Clusters streaming trough a capillary interact, forming morphing microfoam. Subjecting an ultrasound contrast agent of given concentration to a continuous low-amplitude signal makes it cluster to a microfoam of known position and known size, allowing for sonic manipulation, including the release of its contents.