HIGH-SPEED OBSERVATIONS OF THE NUCLEATION OF ICE BY POWER ULTRASOUND.

Transcription

1 1 HIGH-SPEED OBSERVATIONS OF THE NUCLEATION OF ICE BY POWER ULTRASOUND. Chow,R.C.1,3*, Atkins,D.1, Singleton,S.1, Mettin,R.2, Lindinger,B.2, Kurz,T.2, Lauterborn,W. 2, Povey,M.3, Chivers,R.4 1 Unilever R&D, Colworth, Sharnbrook, Bedfordshire, UK MK44 1LQ 2 Drittes Physikalisches Institut, Universität Göttingen, Göttingen, Germany 3 Procter Dept of Food Science, University of Leeds, Leeds, West Yorkshire, UK LS2 9JT 4 DAMPT, University of Cambridge, Cambridge, UK CB3 9EW * Rachel.chow@Unilever.com 1.1 Introduction The crystallisation of water to produce ice crystals is important for a wide range of foods ranging from ice cream to frozen foods. The control of the crystallisation process is one of the major factors affecting the stability and sensory characteristics of these products. Crystallisation can be divided into two main stages: nucleation and growth. Nucleation can be difficult to control as under ambient pressure conditions, the nucleation temperature can occur at any temperature between 0 C to 41 C. This temperature range is called the supercooling temperature and in this region water exists in a metastable state. There are several methods which can be used to promote the nucleation of ice (Kennedy, 1998). One promising technique is the application of power ultrasound. In the presence of an acoustic wave, ice can be initiated at a higher nucleation temperature, than under control conditions. It is also thought that smaller crystals which are evenly distributed throughout a material can be achieved. There is a limited body of literature on the nucleation of ice by power ultrasound. The results from theoretical studies have indicated that the cavitation bubbles produced by the ultrasonic wave are responsible for the nucleation process (Hickling, 1965; Hunt and Jackson, 1967). The exact mechanism is still under debate (Hickling, 1994) although it is thought that the high pressure emitted from a transient cavitation bubble is responsible. One of the main problems in the experimental validation has been that the processes involved occur on a microscopic level and take place over very short timescales making them difficult to observe. In these studies, we have utilised high-speed optical photography to investigate the nucleation of ice in relation to the cavitation bubbles produced by power ultrasound. A variety of different ultrasonic systems were used. Firstly, a commercial ultrasonic probe was used to study the nucleation and density population of

2 2 ice crystals produced by a multicavitation system. The nucleation of ice was studied in more detail by examining the dynamics of a single oscillating bubble. The single bubble was levitated in a fixed location by using a standing wave system. The nucleation of ice was also studied using a laser induced bubble system. The laser induced bubble underwent a single cycle of growth and collapse. Lastly, an ultrasonic cold stage device was constructed to observe the fragmentation of pre-existing ice dendrites and the production of more crystal nuclei. Details of all these studies can be found in Chow-McGarva (2004) 1.2 Ultrasonic horn studies The first measurements were performed using a standard commercial 20 khz ultrasonic horn (450L, Branson Ultrasonics). A quantitative study of the nucleation temperature in both pure water and a 15 wt% sucrose solution showed that the ice was always stimulated at a higher temperature, and required less supercooling in the presence of ultrasound (Chow et al., 2003a; Chow et al., 2004a). Table 1 compares the evolution of the multibubble cloud produced at power output level 7 both above and below the equilibrium freezing point in pure water. These images were obtained by using a highspeed camera (Phantom V, Photosonics Ltd) to film a single 0.1 second pulse of ultrasound (3700 f.p.s. and a 61 μs shutter time). The probe tip (12.7 mm diameter) can be seen at the top of each image. The dense clouds observed at 2.8 C and the presence of the dendritic ice crystals after the end of the ultrasonic pulse strongly suggests that the crystals are nucleated only within the body of the cloud, thus confirming the requirement of cavitation bubbles for the nucleation process. Figure 1 shows the density population of crystals filmed under increasing ultrasonic power output levels. A single 0.1 second pulse of ultrasound was applied at 3.8 C in a 15 wt% sucrose solution. The growing crystals are shown 0.40 seconds later. As the power output level was increased, the level of transient cavitation was recorded to increase (Chow-McGarva, 2004). This resulted in an increase in the number of crystals. These results highlight that a commercial ultrasonic horn can be used to control the nucleation and the size distribution of ice crystals produced within the supercooled liquid. 1.3 Single bubble cavitation studies In order to study the mechanism by which a cavitation bubble may stimulate ice crystallisation, a standing wave system was used to levitate a single bubble in a fixed location within pure water. The water

3 3 was then supercooled. It was found that the nucleation of ice could only be stimulated at a higher nucleation temperature in the presence of the oscillating cavitation bubble (Chow et al., 2004b). Figure 2 shows the nucleation of ice which was filmed (HiSIS 2000) using 1120 f.p.s. (444 μs shutter time). Prior to the nucleation event, the bubble appeared deformed, or dancing, and sometimes microbubble (<5 μm) ejections were observed within one frame. As the temperature was decreased, the acoustic pressure had to be adjusted to maintain a stable bubble. In this example, the pressure was increased from 0.69 to 0.98 bar. When the pressure was increased, several ice crystals were observed in the immediate vicinity of the bubble. Further repetitions of the experiment showed that ice could only be stimulated with a minimum acoustic pressure of 0.60 bar. The dynamics of the single bubble were investigated. Figure 3 shows the bubble radius time curves measured for a range of different acoustic pressures using a synchronous flash illumination (~ 5 μs). There were several interesting observations. Firstly, the bubble responded in phase with the ultrasonic field. Secondly, it exhibited a radius maximum to minimum compression value of ~3:1. These were both characteristics of a stable cavitation bubble. In contrast to transient cavitation, this type of cavitation bubble does not undergo the rapid collapse, or emit the pressure shock wave that is thought to be responsible for the sonocrystallisation of ice. Other characteristics of a non-transient cavitation bubble which were noted included no bubble luminescence and the unstable dancing motion. The current mechanisms relating to the nucleation process are based upon the theoretical calculations of a symmetrically collapsing transient cavitation bubble (Hickling, 1965; Hunt and Jackson, 1966). These findings suggest that the nucleation of ice by power ultrasound may occur by another mechanism. This could be related to the flow field established by the cavitation bubble (Zhang et al., 2003), or the presence of the asymmetrical bubble. 1.4 Laser induced cavitation studies Optical laser induced cavitation is a technique which utilises a laser to produce a single cycle of the nucleation, growth and collapse of a single cavitation bubble (Lauterborn et al., 1999). A Q-switched NdYAG laser (Lumonics HY750) was focussed into supercooled samples of water and dilute sucrose solutions, and an ultra high-speed camera (Imacon 468, Hadland Phototonics) capable of frame rates of upto 100

4 4 million frames per second was used to film the nucleation event after a single laser shot (8 ns pulse width, up to 20 mj energy) was shot into the sample. Figure 4 shows the nucleation of an ice crystal from a single laser induced bubble in a 15 wt% sucrose solution. Figure 4(a) shows the single plasma produced by the laser. Figures 4(b-c) shows the growth and then collapse of a symmetrical bubble. At 250 μs after the plasma (figure 4(d)), the bubble seemed to split into some smaller bubbles. The fragments remained in figures 4(e-f). After μs (figures 4 (g-h)), an ice crystal could be observed. The ice crystal was produced in the immediate vicinity of one bubble fragment. Further experiments showed that the laser bubble did not always induce ice. The probability of ice nucleation in a 15 wt% sucrose solution supercooled to -4.74±1.75 C was 91.7%. The probability of nucleation was increased if the bubble became asymmetric or fragmented. These results were in contrast to the previous experiments described above, where nucleation was always observed. It is thought that as nucleation is a stochastic process, a single oscillation of a laser induced bubble may not always lead to the nucleation. The probability of nucleation is increased by the continuously oscillating bubble produced by the standing wave system, and it is increased further by the multicavitation events produced by the ultrasonic horn. 1.5 Ultrasonic cold stage studies Previous studies using an ultrasonic cold stage device have shown that an alternating pressure can lead to the fragmentation of pre-existing ice crystals (Chow et al., 2003a; Chow et al., 2004a). This was shown to occur by either the production of a crack around the base of the dendritic structure, or a cavitation melting process. A high-speed camera (Phantom V, Photosonics Ltd) was used to study the process of the effect of cavitation in a 15 wt% sucrose solution in more detail. Figure 5 shows the effect of a cavitation bubble on the fragmentation of pre-existing ice dendrites. These were filmed at 100 f.p.s. (shutter time 22 μs) at 66 khz and 30 Vrms. A cavitation bubble can be seen to move along the primary dendrite, breaking off the dendritic structures to produce fragmented crystals. This process occurred very quickly and the solution contained many small fragments after a couple of seconds of sonication. These results suggest that the density population of crystals observed in figure 1 was produced by a secondary nucleation (fragmentation) process.

5 5 1.6 Conclusions A detailed study of the nucleation of ice by power ultrasound has been performed using a variety of high-speed photography systems with a particular focus on the influence of cavitation. The nucleation of ice has been shown to occur predominantly within the bubble cloud produced by a commercial ultrasonic horn. An investigation of a single oscillating bubble has confirmed that ice crystals are nucleated in the immediate vicinity of the bubble. It is widely thought that the high pressure emitted from a transient cavitation bubble is responsible for the nucleation process (Hickling, 1994), however experiments utilising a single oscillating bubble have shown that ice can be initiated by a stable cavitation bubble. The mechanism of nucleation may be related to the asymmetric bubble shape, the flow field associated with the cavitation bubble, or the production of microbubbles. The nucleation of ice was also shown to not always occur after a single cycle of a cavitation bubble. It is thought that the probability of ice nucleation is increased by a continuously oscillating single bubble, and further increased by the multibubble cavitation system. The number of ice crystals produced at increasing ultrasonic output levels of the ultrasonic horn was observed to increase as the power output level was increased. Microscopic studies using a unique ultrasonic cold stage device have shown that this may occur by the fragmentation of pre-existing ice dendrites by cavitation bubbles.

6 6 References: Chow, R.C.Y., Blindt, R.A., Kamp, A., Grocutt, P. and Chivers, R.C. (2003a) Stimulation of ice crystallisation with ultrasonic cavitation microscopic studies. Indian Journal of Physics 77A, Chow, R.C.Y., Blindt, R.A., Chivers, R.C. and Povey, M.J. (2003b) The sonocrystallisation of ice in sucrose solutions: primary and secondary nucleation. Ultrasonics 41, Chow, R.C.Y., Blindt, R.A., Kamp, A., Grocutt, P. and Chivers, R.C. (2004a) The microscopic visualisation of the sonocrystallisation of ice using a novel ultrasonic cold stage. Ultrasonics Sonochemistry 11, Chow, R.C.Y., Blindt, R.A., Chivers, R.C. and Povey, M.J.W. (2004b) A study on the primary and secondary nucleation of ice by power ultrasound. Ultrasonics, (In Press). Chow-McGarva, R.C.Y. (2004) A study on the sonocrytsallisation of ice. PhD submitted to the University of Leeds. Hickling, R. (1965) Nucleation of freezing by cavity collapse and its relation to cavitation damage. Nature 206 (29 May), Hunt, J.D. and Jackson, K.A. (1966) Nucleation of solid in an undercooled liquid by cavitation. Journal of Applied Physics 37 (1), Kennedy, C.J. (1998) Formation of ice in frozen foods and its control by physical stimulus. In: The Properties of Water in Foods ISOPOW 6, 1 edn. pp Reid, D.S., (Ed.) Blackie Academic & Professional. Lauterborn, W., Kurz, T., Mettin, R., and Ohl, C. D. (1999) Experimental and theoretical bubble dynamics. Advances in Chemical Physics 110, Zhang, X., Inada, T., and Tezuka, A. (2003) Ultrasonic-induced nucleation of ice in water containing air bubbles. Ultrasonics Sonochemistry 10, Acknowledgements: The primary author would like to thank the Royal Commission for the Exhibition of 1851 for their Industrial Fellowship award, and funding provided Unilever R&D.

7 7 Table 1 High-speed observations of the cloud produced by an ultrasonic horn (power output 7, 0.1 second pulse) above and below the equilibrium freezing point of ice in pure water Time Temperature above equilibrium freezing Temperature below equilibrium freezing (seconds) t=0 point (1.0 C) First bubbles observed point (-2.8 C) First bubbles observed 3.15mm 3.15mm t=0.025 Bubble cloud and density increase Bubble cloud and density increase Denser cloud suggests presence of ice crystals t=0.050 Bubble cloud and density increase Bubble cloud and density increase Denser cloud suggests presence of ice crystals t=0.938 No bubbles observed Ice nuclei only observed

8 8 Figure 1 High-speed photography of ice crystals (-3.8 C) nucleated in a 15 wt% sucrose solution by an ultrasonic horn at increasing ultrasonic output levels (0.1 second pulse) t= 0.41 seconds Ultrasonic output level -3.8 C Output 2 3mm Output 4 Output 7

9 9 Figure 2 High-speed photography of the nucleation of ice from a single acoustic bubble at 3.26 C. Prior to nucleation the pressure was increased from 0.69 bar to 0.98 bar (27.34 khz): (a) t= 0 s, (b) t= s, (c) t= s, (d) t= s, (e) t= s, (f) t= s a b single bubble deformed bubble 0.2 mm c d ice crystal(s) e f dendritic ice crystal nuclei ice crystals

10 10 Figure 3 Observed radius time curve for a single oscillating bubble in pure water using the single bubble levitation system at khz (temperatures between -0.5 to 1.1 C) 45 Radius (micrometers) bar 0.71 bar 0.82 bar 0.85 bar 0.98 bar Time (microseconds) 80 Figure 4 Ultra high-speed photography of the nucleation of an ice crystal (-4.2 C) from a single laser induced bubble in a 15 wt% sucrose solution (each image width = 3.5 mm): (a) μs, (b) μs, (c) μs, (d) μs, (e) μs, (f) μs, (g) μs a c e g b d f h

11 11 Figure 5 The microscopic effect of a cavitation bubble on the fragmentation of ice in a 15 wt% sucrose solution at -3.0 C: (a) t= 0 s (b) t= 2 s, (c) t= 2.2 s, (d) t= 2.8 s, (e) t= 3.0 s, (f) t= 3.2 s, (g) t= 3.4 s, (h) t= 3.6 s a cavitation b solution dendritic ice 30μm c d e f fragmented crystals g h fragmented crystals