Several different investigations have been carried out to study the primary and secondary nucleation of ice by sonocrystallisation. Firstly, the primary nucleation of discrete ice crystals in a supercooled sucrose solution has been observed. For increasing concentrations of sucrose solutions from 0 to 45 wt%, the nucleation temperature consistently occurs at a higher nucleation temperature in the presence of ultrasound. The nucleation temperature also increases as the power output and duty cycle of a commercial ultrasonic horn are increased. Snap shot images of the bubble clouds obtained from the ultrasonic horn also show that the number of bubbles appears to increase as the ultrasonic output is increased. This suggests that the nucleation of ice is related to the power output and number of cavitation bubbles. The effect of a single bubble on the sonocrystallisation of ice is discussed. High-speed movies (1120 fps) have shown that the crystallisation appears to occur in the immediate vicinity of the single bubble. In most cases, many crystals are observed and it is not known whether a single ice crystal is being fragmented by the bubble or whether many crystals are being initiated. The bubble appears to undergo a dancing regime, frequently splitting and rejoining and also emitting some small microbubbles. A study on the secondary nucleation of ice in sucrose solutions has been carried out using a unique ultrasonic cold stage device. Images taken using a microscope system show that the pre-existing ice dendrite crystals can be broken up into smaller fragments by an ultrasonic field. Cavitation bubbles appear to be important during the fragmentation process, possibly melting any ice crystals in their path. Flow patterns around cavitation bubbles have also been observed, and these may be responsible for the fragmentation of ice crystals.
[1]
Malcolm Povey,et al.
The sonocrystallisation of ice in sucrose solutions: primary and secondary nucleation.
,
2003,
Ultrasonics.
[2]
Hickling.
Transient, high-pressure solidification associated with cavitation in water.
,
1994,
Physical review letters.
[3]
R. Blindt,et al.
The effect of ultrasonic intensity on the crystal structure of palm oil.
,
2004,
Ultrasonics sonochemistry.
[4]
B. Chalmers.
Principles of Solidification
,
1964
.
[5]
J. Hunt,et al.
Nucleation of Solid in an Undercooled Liquid by Cavitation
,
1966
.
[6]
J. Hunt,et al.
Nucleation of the Solid Phase by Cavitation in an Undercooled Liquid which expands on Freezing
,
1966,
Nature.
[7]
R. Chivers,et al.
The microscopic visualisation of the sonocrystallisation of ice using a novel ultrasonic cold stage.
,
2004,
Ultrasonics sonochemistry.
[8]
Bajram Zeqiri,et al.
High-frequency acoustic emissions generated by a 20 kHz sonochemical horn processor detected using a novel broadband acoustic sensor: a preliminary study.
,
2003,
Ultrasonics sonochemistry.
[9]
R. Hickling,et al.
Nucleation of Freezing by Cavity Collapse and its Relation to Cavitation Damage
,
1965,
Nature.