The microscopic visualisation of the sonocrystallisation of ice using a novel ultrasonic cold stage

Abstract

This work reports dynamic video images of the influence of ultrasonic cavitation on the sonocrystallisation of ice at a microscopic level. This has been achieved through the construction of a unique ultrasonic system for an optical microscope. The system consists of (1). an ultrasonic cold stage, (2). a temperature control system, and (3). a microscope and imaging setup. This allows the temperature of a sample to be systematically controlled while it is subjected to simultaneous excitation with alternating pressures in the ultrasonic frequency range. Both the amplitude of excitation and the frequency can be varied. Experiments on ice crystals in pure water and sucrose solutions were conducted. Three distinct phenomena were observed. Firstly, there is a tendency for cavitation bubbles to form at the grain boundaries between ice crystals. Secondly, there is a progressive melting of ice by cavitation bubbles which appear to eat their way into the ice phase. Thirdly, the dendritic ice structures may fragment under the influence of ultrasound, thus increasing the number of nuclei which may subsequently grow (secondary nucleation). These observations form the basis of a significantly enhanced understanding and exploitation of the sonocrystallisation of ice.

Ultrasound for Improved Crystallisation in Food Processing

“…They also found that nucleation rate shows an order of magnitude rise with sonication, while growth rate (and hence the dominant crystal size) reduces with sonication as compared to a mechanically agitated system used for crystallisation. Observations have pointed towards bubbles as a cause for melting due to their random motion [15]. Such random motion of the cavitation bubble is also associated with breakdown of ice dendrites as they form (shown in Fig.…”

The Effect of Ultrasound on the Crystallisation of Paracetamol in the Presence of Structurally Similar Impurities

“…It is logical to anticipate that these phenomenon are amplified by application ultrasound. Cavitation occurring on, or near, crystal surfaces causes intense shear stresses which can lead to secondary nucleation [18,20,72,73]. It is suggested that ultrasound energy overcomes the kinetic barriers to nucleation by reducing the activation energy, and hence reducing the induction time.…”

The ultrasonic processing of dairy products – An overview

“…It was shown with 15% sucrose solution [27] that microbubbles generated by cavitation acted as nucleation sites for ice formation (Fig. 8).…”

“…Probably the most significant application in this respect is the use of ultrasonic cutting devices. These machines utilize the high-frequency acoustic vibrations (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35) to provide a smooth and clean cut for both hard and soft cheeses, minimizing wastage [4,5,56]. Ultrasound has also been commercialized as a method of sealing milk containers and packages of grated cheese -in a process also referred to as ultrasonic welding, the temperature induced by the cavitation processes described above is sufficient to seal the plastic [38,83].…”

“…When ultrasound was applied at~70 kHz, cavitation bubbles were formed between the growing boundaries of ice crystals. Further observations were made of these bubbles causing melting due to their random motion [27]. Such random motion of cavitation bubbles on ice crystals broke down any ice dendrites that were formed (Fig.…”