The control of ice crystallisation is of capital importance for industrial processes where the crystals' size determines entirely the viability of the product, namely:
freeze drying of pharmaceutical products,
freezing of food and biological products,
‘cold' storage and cooling by means of ice slurries,
cryopreservation of living tissues
The noticeable impact of ultrasound waves on the nucleation and growth rate of ice crystals in a super-cooled aqueous solution has been clearly established in the literature. However, the long way between the evidence of the phenomenon and the definition of operating conditions pertaining to an industrial process is quite unexplored.
According to literature, the number of nuclei which determines the final number of crystals in a frozen product, mainly depends on the following parameters: the super-cooling level, the amplitude and frequency of the applied acoustic signal and the initial size distribution of gas bubbles. However, neither empirical nor theoretical correlations between all these parameters and the final crystals number or mean crystal size have yet been published. In the scope of industrial development of ultrasound assisted processes, there is an urgent need for an experimental design or a theoretical model for distinguishing the most influent parameters and for correlating them to mean crystal size. Providing both the empirical correlations and the theoretical model will be the global objective of this project.
Both experimental and modelling work will be carried out at two different levels: by a fundamental approach and by an application oriented approach. The project's first stage will be fundamental research. This stage will be aimed at explaining the physics of isolated nucleation phenomenon by analysing the acoustic cavitation of a single gas bubble in super-cooled water. During the second stage, a real multi-bubbles water sample in a vial (glass container for freeze-drying of pharmaceuticals) will be considered. This applied research step will be aimed at providing ice crystals mean size predictive tools to be used directly for freeze-drying pilot device design. At each project's stage, the theoretical modelling and simulation results will be validated by measurements obtained by means of special designed experimental devices.
This project continues several years' effort aimed on freeze drying process optimization and carried out by the group ‘Coupled heat and mass transfer' of the LAGEP (ESCPE/UCB, Lyon) research unit altogether with industrial partners. This group, which is the first submissioner, has recently demonstrated that ice nucleation can be initiated in a reproducible manner at different temperatures by applying ultrasound pulses. By the way, the crystal's morphology after freezing was observed by cold chamber microscopy and a strong correlation was put in evidence between the nucleation temperature and the mean crystal size. These results have confirmed the great ultrasounds potential as a mean of controlling ice crystallisation and have established the leading position of the LAGEP in the international research in this area. Very recently, preliminary studies concerned with modelling and design of a new experimental refrigeration/sonication device have been started.
The second submissioner of the project, the group ‘Solid elaboration' of the RAPSODEE (EMAC, Albi) research unit, has since many years carried out experimental studies on ultrasound-assisted crystallisation of solutes and on acoustic cavitation. This latter work concerned essentially the interaction between the applied acoustic pressure and the bubbles size distribution within a liquid sample. Such an interaction is a key issue of our project because it concerns two of the factors that impact most the nucleation kinetic. It was proven experimentally that the attenuation and the velocity of an acoustic wave in a liquid depends strongly on the concentration and size of the gas bubbles and that reciprocally the bubble population evolves in number, size, and shape when submitted to an acoustic wave. Besides, the RAPSODEE team has recently elaborated a new theoretical concept possibly explaining the triggering of solute nucleation by cavitation, based on the pressure-gradient forced species diffusion around a cavitation bubble. This concept can be adapted to ice crystallisation. Apart from the contribution to the theoretical ice nucleation model, the partners from the RAPSODEE team will adapt to ice crystallisation the single gas bubble levitation experimental cell they have extensively used for solute crystallisation studies. A special device will be developed allowing for water freezing in a compartment where a single gas bubble levitates and oscillates under the influence of an acoustic field.