Session: P1N

The nucleation of ice by power ultrasound at elevated temperatures (compared to control experiments) has been well documented for the past four decades, however there is a limited understanding of the influence, if any of cavitation bubbles. This work reports high-speed observations of the crystallisation of ice in the presence of acoustic cavitation. The experimental system allowed a small sample of pure water to be cooled rapidly to sub-zero temperatures. Ultrasound could then be applied at the desired frequency and amplitude and a single acoustic bubble could, if required be levitated in the liquid. The observations were made using a high-speed camera capable of 1100 f.p.s. and a microsecond flash unit. Three main experiments were performed. The first results show that the presence of bubbles in the absence of an ultrasonic field have no effect on the nucleation temperature. The second set of results shows that ice cannot be nucleated at a higher temperature in the presence of an ultrasound field, and absence of a cavitation bubble. In both these cases, nucleation occurs at similar temperatures to ice crystallisation under control conditions, and the ice is initiated at the liquid container surface. These results confirm that acoustic cavitation is essential for the sonocrystallisation of ice. The third set of experiments shows that ice is nucleated at elevated temperatures in the presence of a single levitated bubble. There appears to be a driving pressure amplitude threshold, above which the ice can be nucleated. In this case, the ice is initiated in the immediate vicinity of the acoustic bubble. The bubble apparently does not undergo the rapid collapse phase that is often associated with the mechanism of ice crystallisation and which is typically found for sonoluminescing or chemically active bubbles. Instead, the levitated bubble