Microfoam formation in a capillary
Résumé
The ultrasound-induced formation of bubble clusters may be of interest for the manufacture of microfroths. We analyse how lipid-encapsulated microbubble clusters are formed in a cellulose capillary (Fig. 1) and what happens to the clusters if sonication is continued, using continuous driving frequencies in the range 1-10 MHz, using high-speed photography. We demonstrate that as soon as the bubble clusters are formed (Fig. 2) and as long as they are in the sound field, they behave as one entity (Fig. 3). At our acoustic settings, it takes seconds to force the bubble clusters to positions approximately a quarter wavelength apart (Fig. 4). It also just takes seconds to drive the clusters towards the capillary wall (Fig. 5). The following stages of microfroth formation are observed within a dense population of microbubbles before ultrasound arrival (Fig. 6). After the sonication starts, the lipid-encapsulated microbubbles collide, forming small clusters, owing to secondary radiation forces. These clusters coalesce within the space of a quarter of the ultrasonic wavelength, owing to primary radiation forces. The resulting microfroths translate in the direction of the ultrasound field, hitting the capillary wall, also owing to primary radiation forces. Subjecting lipid-encapsulated microbubbles of given concentration to a continuous low-amplitude signal makes it cluster to a microfroth of known position and known size, allowing for sonic manipulation.
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