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A study on the primary and secondary nucleation of ice by power ultrasound
Ultrasonics 43 (2005) 227–230 www.elsevier.com/locate/ultras
A study on the primary and secondary nucleation of ice by power ultrasound q R. Chow
a,b,*
, R. Blindt a, R. Chivers c, M. Povey
b
a b
Unilever R&D, Colworth Laboratory, Sharnbrook, Bedford MK44 1LQ, UK Procter Department of Food Science, University of Leeds, Leeds LS2 9JT, UK c DAMTP, University of Cambridge, Cambridge CB3 9EW, UK Available online 8 July 2004
Abstract 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. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Sonocrystallisation; Single bubble; Ice; Nucleation; Sucrose solution
1. Introduction Although the potential of an ultrasonic wave to stimulate the nucleation of solid crystals in a variety of liquids has been known for 70 years, the first evidence q This article is based on a presentation given at the Ultrasonics International 2003. * Corresponding author. Address: Unilever R&D, Colworth Laboratory, Sharnbrook, Bedford MK44 1LQ, UK. Tel.: +44 1234 222382; fax: +44 1234 222259. E-mail address: [email protected] (R. Chow).
0041-624X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ultras.2004.06.006
for the effect in water was that of Chalmers [1]. The ability to exert some degree of control over the freezing process for a wide range of potential applications: the production of artificial snow, cryogenics, cold energy and transport (air conditioning) systems and food processing and beverage production. There is a significant body of literature (see Ref. [2]) on the sonocrystallisation of ice (including the theoretical work), but the detailed mechanisms involved remain elusive. As far as experimental studies are concerned, cavitation in water has a ubiquitous presence whenever the sonocrystallisation of ice has been detected.
228
R. Chow et al. / Ultrasonics 43 (2005) 227–230
Although Chalmers raised the question in his original work it is still not known whether or not, cavitation, either stable or transient, is a pre-requisite for the sonocrystallisation of ice in water. (There is evidence that transient cavitation is not a pre-requisite for sonocrystallisation of fat blends [3].) Certainly the theoretical studies (e.g. Hickling [4], Hunt and Jackson [5]) have focussed on the behavior of a single collapsing spherical bubble as a potential source of the necessary thermodynamic conditions. In particular the high pressure created as a bubble collapses has been hypothesized as the stimulus for ice nucleation [4,6], but this is not universally accepted [5,7]. Practical applications (including most of the experimental reports on sonocrystallisation effects) usually involve the production of a bubble population with its own specific dynamics, rather than the individual bubbles of the theoreticians. This raises the difficult, but essential question of the means of characterizing cavitation activity in a liquid. Solutions to this problem are still being sought [8]. The present work involves new studies undertaken to try and elucidate the specific mechanisms involved in the sonocrystallisation of ice. The detailed results are contained in recently published papers [2,9–11]. Here we summarize the approaches used and the key results that can be obtained. It is perhaps important to mention that several of the investigations were conducted using video cameras or high-speed photography and the images reproduced represent a gross selection of the amount of data analyzed.
the effect of an ultrasonic field on ice crystal growth, or final crystal morphology. Only recently [9] have the present authors demonstrated the production of secondary nucleation of ice ultrasonically.
3. Primary nucleation In the initial experiments, a commercial sonochemical (20 kHz) insonicator was used. Ice crystals produced in its field could be clearly seen and recorded on video (Fig. 1). The irradiated sample was set to cool to approximately 1 °C/min down to 20 °C in a system described in [2]. The initiation of (primary) nucleation was detected as a sharp temperature rise (caused by the latent heat of crystallisation). Fig. 2 shows the nucleation temperature of ice in dilute sucrose solutions as a function of concentration. For all concentrations up to 45 wt% sucrose the effect of sonication is to raise the nucleation temperature by between 1 and 3 °C. The insonication equipment includes an output control with a dial marked from 1 to 10. For the above experiment (Fig. 2) it was set at output level 5 (30% pulsed). Varying the output level shows clearly, at least, the cloud of cavitation bubbles produced by the device increases steadily (Fig. 3) as the power output is increased. In order to investigate the effect of output level on the nucleation temperature it was necessary to pulse the excitation of the probe (using continuous waves caused sufficient heating of the transducer and the irradiated liquid as to preclude the formation of ice). Using a 10% duty cycle, the nucleation temperature rose monotonically as the output level increased (Fig. 4). Heating effects pre-
2. The nucleation of ice Ice has a very unusual phase diagram [12] exhibiting eight different phases. The freezing point of water decreases as the ambient pressure increases up to about 2 kbar. While the melting temperature of ice is constant at a given pressure, the nucleation of pure water to form ice can occur within a range of temperatures (the ‘‘supercooling range’’) which, for water extends from 0 to 40 °C at ambient pressure. Two distinct processes are identified in the nucleation of crystals. Primary nucleation involves the formation of a crystal in a solution containing no existing crystals. Secondary nucleation involves the production of new crystals in a solution containing pre-existing crystals, e.g. by fragmentation. In general sonocrystallisation exhibits a number of features specific to the ultrasonic wave. For most materials these include: (i) faster primary nucleation, (ii) the initiation of secondary nucleation and (iii) the production of smaller crystals with greater size uniformity. Most of the reports to date on the sonocrystallisation of ice have been concerned with primary nucleation. There have been few reports on
Fig. 1. Photographs of ice crystals nucleated in a 15 wt% sucrose solution at 3.4 °C by a commercial ultrasonic device (output 4, 10% duty cycle): (a) ice crystals following an ultrasonic pulse, (b) crystals 5 s later.
R. Chow et al. / Ultrasonics 43 (2005) 227–230
4. Studies on a single bubble
0 Control Ultrasound
Nucleation temperature (C)
229
-2
-4
-6
-8
-10 -5
5
15 25 35 Sucrose concentration (wt %)
45
55
Fig. 2. The effect of ultrasound (output 5, 30% duty cycle) on the primary nucleation temperature of ice in sucrose solutions (error bars indicate standard deviations).
In order to investigate the role of cavitation, highspeed photographic studies (1120 fps) of a bubble levitated in an ultrasonic standing wave (at approximately 27 kHz) were carried out in conjunction with the group of Prof. W. Lauterborn and Dr. Robert Mettin at Gottingen University, Germany. These are reported elsewhere [11], but the main preliminary observations are the following: The bubble tends to ‘‘dance’’ around, sometimes splitting and rejoining or emitting some small microbubbles in the ultrasonic field. At a certain pressure amplitude in the standing wave (about 0.8 bar) at least one crystal of ice can be seen to nucleate in the immediate vicinity of the bubble. They grow at a rate (3 cm/s) similar to that reported by other authors in the literature. It is not clear whether it is primary or secondary nucleation that is observed. However the bubble oscillations in the ultrasonic field exhibit a ratio of maximum to minimum radius of approximately 3–1, indicating that the sonocrystallisation is not necessarily initiated by the strong collapse of HicklingÕs theory [4,6].
5. Microscopic studies (secondary nucleation)
Fig. 3. Photographs of bubble clouds produced at different output settings in a 15 wt% sucrose solution: (a) output level 2 and (b) output level 4.
-2 10% pulsed 50% pulsed
Nucleation temperature (C)
-2.5 -3 -3.5 -4 -4.5 -5 -5.5 -6 -6.5
Direct microscopic visualisation (and recording) of the sonocrystallisation process was performed with a purpose built stage which not only allowed controlled cooling, but also the simultaneous subjection of the sample to a variable level of pressure oscillating at ultrasonic frequencies (67 kHz). Construction of the cell has been described elsewhere [9,10]. Fig. 5(a) shows typical dendritic ice crystals grown by cooling the specimen. Switching on the ultrasound (Fig. 5(b)) shows their fragmentation. Thus an ultrasonic wave can clearly stimulate the formation of secondary nuclei. As may have been expected the onset of the ultrasonic stimulus is accompanied by the presence of cavitation bubbles. These often appear to track through the ice crystals melting them as they go. Furthermore the cavitation bubbles can act as foci for circulatory flow in the liquid adjacent to the crystals (Fig. 6).
-7 0
1
2
3 4 Power output (a.u.)
5
6
7
Fig. 4. The primary nucleation temperature of ice in a 30 wt% sucrose solution for varying ultrasonic output levels and duty cycles (error bars indicate standard deviations).
vented the use of the 50% duty cycle above output level 5, but at the higher outputs there is some evidence that the larger pulses, which produce more cavitation, produce higher nucleation temperatures.
6. Conclusions Specific evidence has been presented to confirm the impression from the literature that the primary nucleation of ice can be induced by power ultrasound. The first clear evidence is presented that the nucleation temperatures of ice increase with the increasing power of the ultrasonic power input to the sample. Cavitation bubbles have been seen to nucleate ice, but both stable and transient cavitation appear to be important.
230
R. Chow et al. / Ultrasonics 43 (2005) 227–230
nucleation may be caused by cavitation bubbles or by high shear flow in the vicinity of the cavitation. It is clear that considerable further investigations are needed to clarify and quantify the mechanisms involved in the ultrasonic nucleation of ice, but the present work represents a significant step in elucidating the processes involved.
Acknowledgments The authors are grateful to colleagues at Unilever R&D and the University of Leeds for their expert support, to Prof. Lauterborn, Dr. R. Mettin, Dr. T. Kurz and B. Lindinger and the group at Gottingen for collaboration in the use of their excellent facilities, and to Dr. M. Lowe and his colleagues and Linkam Scientific, UK for their contributions to the development of the ultrasonic cell. The first named author gratefully acknowledges an Industrial Fellowship from the Royal Commission for the Exhibition of 1851, London which made the presentation of this work possible.
References
Fig. 5. Secondary nucleation of ice in a 15 wt% sucrose solution with ultrasound (a) ice dendrite (no ultrasound) and (b) fragments of crystals remaining after sonication (4 s of ultrasound). Image width=0.92 mm (from Ref. [9]).
Fig. 6. Circular flow patterns observed around two cavitation bubbles in a 15 wt% sucrose solution (from Ref. [2]).
The direct investigation of secondary nucleation is also new and is based upon a novel ultrasonic cold stage. Pre-existing ice crystals can be fragmented by ultrasound leaving a population of crystal nuclei. The secondary
[1] B. Chalmers, in: Principles of Solidification, John Wiley & Sons Inc. 1964, pp. 62–90. [2] R. Chow, R. Blindt, R. Chivers, M. Povey, The sonocrystallisation of ice in sucrose solutions: primary and secondary nucleation, Ultrasonics 41 (2003) 595–604. [3] M. Patrick, R. Blindt, J. Janssen, The effect of ultrasonic intensity on the crystal structure of palm oil, Applications of Power Ultrasound in Physical and Chemical Processing Proceedings, 2003, pp. 53–60. [4] R. Hickling, Nucleation of freezing by cavity collapse and its relation to cavitation damage, Nature 206 (1965) 915–917. [5] J.D. Hunt, K.A. Jackson, Nucleation of solid in an undercooled liquid by cavitation, J. Appl. Phys. 37 (1966) 254–257. [6] R. Hickling, Transient, high pressure solidification associated with cavitation in water, Phys. Rev. Lett. 73 (1994) 2853–2856. [7] J.D. Hunt, K.A. Jackson, Nucleation of the solid phase by cavitation in an undercooled liquid which expands on freezing, Nature 211 (1966) 1080–1081. [8] M. Hodnett, R. Chow, B. Zeqiri, High frequency acoustic emissions generated by a 20 kHz sonochemical horn processor detected using a novel broadband acoustic sensor: a preliminary study, Ultrason. Sonochem., in press. [9] R.C.Y. Chow, R.A. Blindt, A. Kamp, P. Grocutt, R.C. Chivers, Stimulation of ice crystallisation with ultrasonic cavitation–– microscopic studies, Indian J. Phys. 77A (4) (2003) 315–318. [10] R. Chow, R. Blindt, A. Kamp, P. Grocutt, R. Chivers, The microscopic visualisation of the sonocrystallisation of ice using a novel ultrasonic cold stage, Ultrason. Sonochem. 11 (2004) 245– 250. [11] R. Chow, R. Mettin, B. Lindinger, T. Kurz, W. Lauterborn, Highspeed observations of the sonocrystallisation of ice using a single levitated bubble, IEEE International Ultrasonics Symposium Proceedings, 2003, pp. 152–153. [12] P.V. Hobbs, Ice Physics, Clarendon Press, Oxford, 1974.
A study on the primary and secondary nucleation of ice by power ultrasound q R. Chow
a,b,*
, R. Blindt a, R. Chivers c, M. Povey
b
a b
Unilever R&D, Colworth Laboratory, Sharnbrook, Bedford MK44 1LQ, UK Procter Department of Food Science, University of Leeds, Leeds LS2 9JT, UK c DAMTP, University of Cambridge, Cambridge CB3 9EW, UK Available online 8 July 2004
Abstract 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. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Sonocrystallisation; Single bubble; Ice; Nucleation; Sucrose solution
1. Introduction Although the potential of an ultrasonic wave to stimulate the nucleation of solid crystals in a variety of liquids has been known for 70 years, the first evidence q This article is based on a presentation given at the Ultrasonics International 2003. * Corresponding author. Address: Unilever R&D, Colworth Laboratory, Sharnbrook, Bedford MK44 1LQ, UK. Tel.: +44 1234 222382; fax: +44 1234 222259. E-mail address: [email protected] (R. Chow).
0041-624X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ultras.2004.06.006
for the effect in water was that of Chalmers [1]. The ability to exert some degree of control over the freezing process for a wide range of potential applications: the production of artificial snow, cryogenics, cold energy and transport (air conditioning) systems and food processing and beverage production. There is a significant body of literature (see Ref. [2]) on the sonocrystallisation of ice (including the theoretical work), but the detailed mechanisms involved remain elusive. As far as experimental studies are concerned, cavitation in water has a ubiquitous presence whenever the sonocrystallisation of ice has been detected.
228
R. Chow et al. / Ultrasonics 43 (2005) 227–230
Although Chalmers raised the question in his original work it is still not known whether or not, cavitation, either stable or transient, is a pre-requisite for the sonocrystallisation of ice in water. (There is evidence that transient cavitation is not a pre-requisite for sonocrystallisation of fat blends [3].) Certainly the theoretical studies (e.g. Hickling [4], Hunt and Jackson [5]) have focussed on the behavior of a single collapsing spherical bubble as a potential source of the necessary thermodynamic conditions. In particular the high pressure created as a bubble collapses has been hypothesized as the stimulus for ice nucleation [4,6], but this is not universally accepted [5,7]. Practical applications (including most of the experimental reports on sonocrystallisation effects) usually involve the production of a bubble population with its own specific dynamics, rather than the individual bubbles of the theoreticians. This raises the difficult, but essential question of the means of characterizing cavitation activity in a liquid. Solutions to this problem are still being sought [8]. The present work involves new studies undertaken to try and elucidate the specific mechanisms involved in the sonocrystallisation of ice. The detailed results are contained in recently published papers [2,9–11]. Here we summarize the approaches used and the key results that can be obtained. It is perhaps important to mention that several of the investigations were conducted using video cameras or high-speed photography and the images reproduced represent a gross selection of the amount of data analyzed.
the effect of an ultrasonic field on ice crystal growth, or final crystal morphology. Only recently [9] have the present authors demonstrated the production of secondary nucleation of ice ultrasonically.
3. Primary nucleation In the initial experiments, a commercial sonochemical (20 kHz) insonicator was used. Ice crystals produced in its field could be clearly seen and recorded on video (Fig. 1). The irradiated sample was set to cool to approximately 1 °C/min down to 20 °C in a system described in [2]. The initiation of (primary) nucleation was detected as a sharp temperature rise (caused by the latent heat of crystallisation). Fig. 2 shows the nucleation temperature of ice in dilute sucrose solutions as a function of concentration. For all concentrations up to 45 wt% sucrose the effect of sonication is to raise the nucleation temperature by between 1 and 3 °C. The insonication equipment includes an output control with a dial marked from 1 to 10. For the above experiment (Fig. 2) it was set at output level 5 (30% pulsed). Varying the output level shows clearly, at least, the cloud of cavitation bubbles produced by the device increases steadily (Fig. 3) as the power output is increased. In order to investigate the effect of output level on the nucleation temperature it was necessary to pulse the excitation of the probe (using continuous waves caused sufficient heating of the transducer and the irradiated liquid as to preclude the formation of ice). Using a 10% duty cycle, the nucleation temperature rose monotonically as the output level increased (Fig. 4). Heating effects pre-
2. The nucleation of ice Ice has a very unusual phase diagram [12] exhibiting eight different phases. The freezing point of water decreases as the ambient pressure increases up to about 2 kbar. While the melting temperature of ice is constant at a given pressure, the nucleation of pure water to form ice can occur within a range of temperatures (the ‘‘supercooling range’’) which, for water extends from 0 to 40 °C at ambient pressure. Two distinct processes are identified in the nucleation of crystals. Primary nucleation involves the formation of a crystal in a solution containing no existing crystals. Secondary nucleation involves the production of new crystals in a solution containing pre-existing crystals, e.g. by fragmentation. In general sonocrystallisation exhibits a number of features specific to the ultrasonic wave. For most materials these include: (i) faster primary nucleation, (ii) the initiation of secondary nucleation and (iii) the production of smaller crystals with greater size uniformity. Most of the reports to date on the sonocrystallisation of ice have been concerned with primary nucleation. There have been few reports on
Fig. 1. Photographs of ice crystals nucleated in a 15 wt% sucrose solution at 3.4 °C by a commercial ultrasonic device (output 4, 10% duty cycle): (a) ice crystals following an ultrasonic pulse, (b) crystals 5 s later.
R. Chow et al. / Ultrasonics 43 (2005) 227–230
4. Studies on a single bubble
0 Control Ultrasound
Nucleation temperature (C)
229
-2
-4
-6
-8
-10 -5
5
15 25 35 Sucrose concentration (wt %)
45
55
Fig. 2. The effect of ultrasound (output 5, 30% duty cycle) on the primary nucleation temperature of ice in sucrose solutions (error bars indicate standard deviations).
In order to investigate the role of cavitation, highspeed photographic studies (1120 fps) of a bubble levitated in an ultrasonic standing wave (at approximately 27 kHz) were carried out in conjunction with the group of Prof. W. Lauterborn and Dr. Robert Mettin at Gottingen University, Germany. These are reported elsewhere [11], but the main preliminary observations are the following: The bubble tends to ‘‘dance’’ around, sometimes splitting and rejoining or emitting some small microbubbles in the ultrasonic field. At a certain pressure amplitude in the standing wave (about 0.8 bar) at least one crystal of ice can be seen to nucleate in the immediate vicinity of the bubble. They grow at a rate (3 cm/s) similar to that reported by other authors in the literature. It is not clear whether it is primary or secondary nucleation that is observed. However the bubble oscillations in the ultrasonic field exhibit a ratio of maximum to minimum radius of approximately 3–1, indicating that the sonocrystallisation is not necessarily initiated by the strong collapse of HicklingÕs theory [4,6].
5. Microscopic studies (secondary nucleation)
Fig. 3. Photographs of bubble clouds produced at different output settings in a 15 wt% sucrose solution: (a) output level 2 and (b) output level 4.
-2 10% pulsed 50% pulsed
Nucleation temperature (C)
-2.5 -3 -3.5 -4 -4.5 -5 -5.5 -6 -6.5
Direct microscopic visualisation (and recording) of the sonocrystallisation process was performed with a purpose built stage which not only allowed controlled cooling, but also the simultaneous subjection of the sample to a variable level of pressure oscillating at ultrasonic frequencies (67 kHz). Construction of the cell has been described elsewhere [9,10]. Fig. 5(a) shows typical dendritic ice crystals grown by cooling the specimen. Switching on the ultrasound (Fig. 5(b)) shows their fragmentation. Thus an ultrasonic wave can clearly stimulate the formation of secondary nuclei. As may have been expected the onset of the ultrasonic stimulus is accompanied by the presence of cavitation bubbles. These often appear to track through the ice crystals melting them as they go. Furthermore the cavitation bubbles can act as foci for circulatory flow in the liquid adjacent to the crystals (Fig. 6).
-7 0
1
2
3 4 Power output (a.u.)
5
6
7
Fig. 4. The primary nucleation temperature of ice in a 30 wt% sucrose solution for varying ultrasonic output levels and duty cycles (error bars indicate standard deviations).
vented the use of the 50% duty cycle above output level 5, but at the higher outputs there is some evidence that the larger pulses, which produce more cavitation, produce higher nucleation temperatures.
6. Conclusions Specific evidence has been presented to confirm the impression from the literature that the primary nucleation of ice can be induced by power ultrasound. The first clear evidence is presented that the nucleation temperatures of ice increase with the increasing power of the ultrasonic power input to the sample. Cavitation bubbles have been seen to nucleate ice, but both stable and transient cavitation appear to be important.
230
R. Chow et al. / Ultrasonics 43 (2005) 227–230
nucleation may be caused by cavitation bubbles or by high shear flow in the vicinity of the cavitation. It is clear that considerable further investigations are needed to clarify and quantify the mechanisms involved in the ultrasonic nucleation of ice, but the present work represents a significant step in elucidating the processes involved.
Acknowledgments The authors are grateful to colleagues at Unilever R&D and the University of Leeds for their expert support, to Prof. Lauterborn, Dr. R. Mettin, Dr. T. Kurz and B. Lindinger and the group at Gottingen for collaboration in the use of their excellent facilities, and to Dr. M. Lowe and his colleagues and Linkam Scientific, UK for their contributions to the development of the ultrasonic cell. The first named author gratefully acknowledges an Industrial Fellowship from the Royal Commission for the Exhibition of 1851, London which made the presentation of this work possible.
References
Fig. 5. Secondary nucleation of ice in a 15 wt% sucrose solution with ultrasound (a) ice dendrite (no ultrasound) and (b) fragments of crystals remaining after sonication (4 s of ultrasound). Image width=0.92 mm (from Ref. [9]).
Fig. 6. Circular flow patterns observed around two cavitation bubbles in a 15 wt% sucrose solution (from Ref. [2]).
The direct investigation of secondary nucleation is also new and is based upon a novel ultrasonic cold stage. Pre-existing ice crystals can be fragmented by ultrasound leaving a population of crystal nuclei. The secondary
[1] B. Chalmers, in: Principles of Solidification, John Wiley & Sons Inc. 1964, pp. 62–90. [2] R. Chow, R. Blindt, R. Chivers, M. Povey, The sonocrystallisation of ice in sucrose solutions: primary and secondary nucleation, Ultrasonics 41 (2003) 595–604. [3] M. Patrick, R. Blindt, J. Janssen, The effect of ultrasonic intensity on the crystal structure of palm oil, Applications of Power Ultrasound in Physical and Chemical Processing Proceedings, 2003, pp. 53–60. [4] R. Hickling, Nucleation of freezing by cavity collapse and its relation to cavitation damage, Nature 206 (1965) 915–917. [5] J.D. Hunt, K.A. Jackson, Nucleation of solid in an undercooled liquid by cavitation, J. Appl. Phys. 37 (1966) 254–257. [6] R. Hickling, Transient, high pressure solidification associated with cavitation in water, Phys. Rev. Lett. 73 (1994) 2853–2856. [7] J.D. Hunt, K.A. Jackson, Nucleation of the solid phase by cavitation in an undercooled liquid which expands on freezing, Nature 211 (1966) 1080–1081. [8] M. Hodnett, R. Chow, B. Zeqiri, High frequency acoustic emissions generated by a 20 kHz sonochemical horn processor detected using a novel broadband acoustic sensor: a preliminary study, Ultrason. Sonochem., in press. [9] R.C.Y. Chow, R.A. Blindt, A. Kamp, P. Grocutt, R.C. Chivers, Stimulation of ice crystallisation with ultrasonic cavitation–– microscopic studies, Indian J. Phys. 77A (4) (2003) 315–318. [10] R. Chow, R. Blindt, A. Kamp, P. Grocutt, R. Chivers, The microscopic visualisation of the sonocrystallisation of ice using a novel ultrasonic cold stage, Ultrason. Sonochem. 11 (2004) 245– 250. [11] R. Chow, R. Mettin, B. Lindinger, T. Kurz, W. Lauterborn, Highspeed observations of the sonocrystallisation of ice using a single levitated bubble, IEEE International Ultrasonics Symposium Proceedings, 2003, pp. 152–153. [12] P.V. Hobbs, Ice Physics, Clarendon Press, Oxford, 1974.