Whispering Acoustics – Understanding and controlling nucleation by acoustics/ultrasound in soft matter

Summary

Many high-value industrial processes involve crystallisation as a separation and/or purification stage or to achieve final product morphology. Examples of industries dependent on crystallisation processes include active pharmaceutical ingredients (APIs), agro-chemicals, food and polymers. Nucleation control is essential to optimise crystal size distribution, morphology and polymorphic form. It is well known that high-power acoustic fields (>10 kWm-2) can have a dramatic effect on materials. For instance, sonotrodes are used to initiate crystal nucleation and to control crystal growth. A big drawback of high-power acoustics is the associated cavitation which creates large amounts of free radicals, especially but not exclusively in aqueous systems, leading to undesirable product oxidation – a particular problem for food applications where it can be associated with unpleasant off flavours. Bubbles generated by cavitation can also interfere with the instrumentation used to monitor
suspended crystals (e.g., laser backscattering, turbidity) making efficient process control difficult. In addition, the cavitation field itself can be difficult to control and cavitational damage to the processing (sonotrode) and control apparatus may lead to shortened device lifetimes and product contamination (a particular issue for food and pharma). By cavitation, we refer to both stable/non-inertial cavitation and transient/inertial cavitation. These challenges have prevented the wide spread use of high power cavitation-producing ultrasound in food and some other sectors.
We propose to carry out fundamental investigations into a low-power ultrasonic technique (using continuous low-intensity non-cavitating fields), recently patented, that has the potential to reduce energy use, optimise processes, reduce equipment damage by cavitation, produce novel materials, control product properties and enhance product purity. An earlier patent, whilst referring to lower power ultrasound, actually deploys stable cavitation as the nucleating principle.

However, many questions remain regarding the mechanisms at work in low power continuous ultrasound: is it a linear/non-linear effect; what is the relationship of thermal and mass transfer mechanisms to the oscillating pressure field, what is the effect of power level and particle size? How does the acoustic field modify phase change and how do the particle dynamics (including particleparticle interactions) in the acoustic field influence crystal nucleation and growth? We expect that elucidation of the fundamental mechanisms of the phenomenon and building on the understanding developed in our current project, will lead to robust methodologies for implementation in industrial processes, involving mathematical modelling, computational modelling and rigorous experimental investigation and validating developed models. We believe that the developed technology would have transformational potential in all industrially important processes involving crystallisation and enable hitherto unimagined materials to be conceived. Systematic research into the phenomenon of cavitation began in 20th century with the discovery of the material-altering properties of high intensity sound waves generated in water and oil. Later on, cavitational damage to engineered structures such as ship propellers became an economic issue. The term ‘sonocrystallisation’ emerged in 1993 and the first reference to the use of sound waves to influence crystal growth in 1962 41. Works of reference in the area include 42 and 43. When it became apparent that the drawbacks of transient cavitation obviated the technology’s potential for our industrial partners, research into high power ultrasound ceased in our group and attention turned to stable cavitation. The aforementioned patent claim by MP collaborators stated that nucleation could be achieved in the absence of transient cavitation. However, it became clear that nucleation at the power levels referred to in the patent was actually a result of stable cavitation; this as a result of systematic studies of the impact of ultrasound on foods which began in our laboratory in 2000 with the appointment of Rachel Chow, a Unilever employee as ‘Royal Commission for the Exhibition of 1851 Student’. In that collaboration we demonstrated unequivocally that stable (non-inertial) cavitation could be an effective nucleator of ice. Our interest in the area has revived due to the realisation that power levels much lower even than those needed to induce stable cavitation nevertheless profoundly influence nucleation. This effect can be exploited in developing systems that provide superior control on critical quality attributes of crystals at low input energy levels. Vision: Through the use of continuous, low-intensity, sub-cavitational acoustic fields, our proposed programme aims to transform processes across a wide range of industries where crystallisation is a key production, purification or separation stage including the substantial economic sectors of pharma, food and polymer processing. This potentially disruptive technology will be investigated through the fundamental experimental study of acoustically controlled nucleation modelling. Proposed research and its context ACOUSTIC PROCESSING OF CRYSTALS thermal and mass-transfer processes within the nucleation stage, the design of acoustic nucleators for model validation and their implementation at laboratory-scale for feasibility demonstration towards the next stage of pilot-plant process construction (leading on from this project). Our recent project referred to above has led to enhanced understanding and modelling capability for the thermal and hydrodynamic effects around particles in acoustic fields and the effects of multiple particles on those field patterns. This has led to preliminary models for acoustic effects on nucleation and will be used as the basis for the proposed model development of crystal nucleation and growth, and its interaction with acoustic fields. Our development over recent years of direct nucleation control for continuous crystallisation processes rests on a detailed understanding of crystallisation through modelling and experimental investigation, coupled with the integration of advanced process technologies into an automated process control system. These techniques will form the starting point for implementation of low-power acoustic nucleation control strategies.

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