The importance of acoustic cavitation in the sonocrystallisation of ice-high speed observations of a single acoustic bubble

The importance of cavitation bubbles in the nucleation of ice crystals in pure water is examined using a single bubble acoustic levitation and high speed imaging system. The dynamic images have shown that neither air bubbles (manually added to the solution), nor an acoustic pressure wave alone (in the absence of cavitation) is sufficient to cause the nucleation of ice crystals in supercooled water up to the undisturbed nucleation temperature. The high-speed analysis of the crystallisation event in the presence of ultrasound and an acoustic bubble has shown that a single as well as many ice crystals can be produced in the immediate vicinity of the bubble. The bubble is typically not destroyed by the nucleation processes. In the investigated regime, it undergoes a weak "dancing" motion, frequently splitting and rejoining, and occasionally emitting small microbubbles. The bubble radius time curve has been roughly measured, and collapse ratios (Rmax/Rmin) of 3:1 have been observed. However, this does not exclude that stronger collapse events of the bubble, or split microbubbles could trigger the ice nucleation.