Probing the mechanisms of enhanced crystallisation of APS in the presence of ultrasound

Understanding the origins of the enhancement of crystallisation of a lipid (all-purpose shortening, APS) through the application of ultrasound is a fundamental pre-requisite for the exploitation of this technique in a wider context. To this end, we show here a number of measurements designed to probe the mechanisms responsible for this effect. For example, we show how the type of bubble cluster, produced at the sound source, alters the bubble population and residency time. In addition, to probe the various contributions to the enhanced crystallisation rate, isolation of the cluster environment below the piston like emitter (PLE) used as the ultrasonic source was shown to reduce the enhancement observed, but did not remove it entirely. This implied that the exposure of the liquid to pressure shocks and the environment around the cluster has a positive effect on the crystallisation kinetics. In turn the addition of extra seed crystals and mechanical agitation also enhances the rate of crystallisation. Finally, the time at which ultrasonic irradiation of the fluid is applied is shown to alter the kinetics observed. These observations suggest that two components are important: large bubble populations and mechanical effects on pre-existing crystals. These findings suggest that maximising these effects could be an eloquent way to enhance and control the material characteristics of materials produced in this manner.

Ultrasonic Formation of Fe3O4-Reduced Graphene Oxide-Salicylic Acid Nanoparticles with Switchable Antioxidant Function

We demonstrate a single-step ultrasonic in situ complexation of salicylic acid during the growth of Fe₃O₄-reduced graphene oxide nanoparticles (∼10 nm) to improve the antioxidant and antiproliferative effects of pristine drug molecules. These nanoparticles have a precisely defined electronic molecular structure with salicylic acid ligands specifically complexed to Fe(III)/Fe(II) sites, four orders of magnitude larger electric surface potential, and enzymatic activity modulated by ascorbic acid molecules. The diminishing efficiency of hydroxyl radicals by Fe₃O₄-rGO-SA nanoparticles is tenfold higher than that by pristine salicylic acid in the electro-Fenton process. The H⁺ production of these nanoparticles can be switched by the interaction with ascorbic acid ligands and cause the redox deactivation of iron or enhanced antioxidation, where rGO plays an important role in enhanced charge transfer catalysis. Fe₃O₄-rGO-SA nanoparticles are nontoxic to erythrocytes, i.e., human peripheral blood mononuclear cells, but surpassingly inhibit the growth of three cancer cell lines, HeLa, HepG2, and HT29, with respect to pristine salicylic acid molecules.

In-situ observations and acoustic measurements upon fragmentation of free-floating intermetallics under ultrasonic cavitation in water

Grain refinement in alloys is a well-known effect of ultrasonic melt processing. Fragmentation of primary crystals by cavitation-induced action in liquid metals is considered as one of the main driving mechanisms for producing finer and equiaxed grain structures. However, in-situ observations of the fragmentation process are generally complex and difficult to follow in opaque liquid metals, especially for the free-floating crystals. In the present study, we develop a transparent test rig to observe in real time the fragmentation potential of free-floating primary Al3Zr particles under ultrasonic excitation in water (an established analogue medium to liquid aluminium for cavitation studies). An effective treatment domain was identified and fragmentation time determined using acoustic pressure field mapping. For the first time, real-time high-speed imaging captured the dynamic interaction of shock waves from the collapsing bubbles with floating intermetallic particles that led to their fragmentation. The breakage sequence as well as the cavitation erosion pattern were studied by means of post-treatment microscopic characterisation of the fragments. Fragment size distribution and crack patterns on the fractured surface were then analysed and quantified. Application of ultrasound is shown to rapidly (<10 s) reduce intermetallic size (from 5 mm down to 10 μm), thereby increasing the number of potential nucleation sites for the grain refinement of aluminium alloys during melt treatment.

Sonofragmentation of Organic Molecular Crystals vs Strength of Materials

Mechanochemistry, the interface between the chemical and the mechanical worlds, includes the relationship between the chemical and mechanical properties of solids. In this work, fragmentation of organic molecular crystals during ultrasonic irradiation of slurries has been quantitatively investigated. This has particular relevance to nucleation processes during sonocrystallization, which is increasingly used in the processing and formulation of numerous pharmaceutical agents (PAs). We have discovered that the rates of sonofragmentation are very strongly correlated with the strength of the materials (as measured by Vickers hardness and Young's modulus). This is a mechanochemical extension of the Bell-Evans-Polanyi Principle or Hammond's Postulate: the kinetics (i.e., rates) of solid fracture correlate with thermodynamic properties of solids (e.g., Young's modulus). The mechanism of the particle breakage is consistent with a direct interaction between the shockwaves or localized microjets created by the ultrasound (through acoustic cavitation) and the solid particles in the slurry. Comparisons of the sonofragmentation patterns of ionic and molecular crystals showed that ionic crystals are more sensitive to sonofragmentation than molecular crystals for a given Young's modulus. The rates of sonofragmentation are proposed to correlate with the types and densities of imperfections in the crystals.

On the governing fragmentation mechanism of primary intermetallics by induced cavitation

One of the main applications of ultrasonic melt treatment is the grain refinement of aluminium alloys. Among several suggested mechanisms, the fragmentation of primary intermetallics by acoustic cavitation is regarded as very efficient. However, the physical process causing this fragmentation has received little attention and is not yet well understood. In this study, we evaluate the mechanical properties of primary Al3Zr intermetallics by nano-indentation experiments and correlate those with in-situ high-speed imaging (of up to 1 Mfps) of their fragmentation process by laser-induced cavitation (single bubble) and by acoustic cavitation (cloud of bubbles) in water. Intermetallic crystals were chemically extracted from an Al-3 wt% Zr alloy matrix. Mechanical properties such as hardness, elastic modulus and fracture toughness of the extracted intermetallics were determined using a geometrically fixed Berkovich nano-diamond and cube corner indenter, under ambient temperature conditions. The studied crystals were then exposed to the two cavitation conditions mentioned. Results demonstrated for the first time that the governing fragmentation mechanism of the studied intermetallics was due to the emitted shock waves from the collapsing bubbles. The fragmentation caused by a single bubble collapse was found to be almost instantaneous. On the other hand, sono-fragmentation studies revealed that the intermetallic crystal initially underwent low cycle fatigue loading, followed by catastrophic brittle failure due to propagating shock waves. The observed fragmentation mechanism was supported by fracture mechanics and pressure measurements using a calibrated fibre optic hydrophone. Results showed that the acoustic pressures produced from shock wave emissions in the case of a single bubble collapse, and responsible for instantaneous fragmentation of the intermetallics, were in the range of 20-40 MPa. Whereas, the shock pressure generated from the acoustic cavitation cloud collapses surged up to 1.6 MPa inducing fatigue stresses within the crystal leading to eventual fragmentation.

Sonocrystallization as an Efficient Way to Control the Size, Morphology, and Purity of Coordination Compound Microcrystallites: Application to a Single-Chain Magnet

The size, morphology, and purity control of coordination compound powders is a key stage for their conversion into materials and devices. In particular, surface science techniques require highly pure bulk materials with a narrow crystallite-size distribution together with straightforward, scalable, and reproducible crystallization procedures. In this work we demonstrate how sonocrystallization, i.e. the application of ultrasound during the crystallization process, can afford very quickly powders made of crystallites with controlled size, morphology, and purity. We show that this process drastically diminishes the crystallite-size distribution (low polydispersity indexes, PDI) and crystallite aspect ratio. By comparing sonicated samples with samples obtained by various silent crystallization conditions, we unambiguously show that the improvement in the crystallite morphology and size distribution is not due to any thermal effect but to the sonication of the crystallizing media. The application of sonocrystallization on crystallization batches of single-chain magnets (SCMs) maintains the chemical integrity of the SCMs together with their original magnetic behavior. Moreover, luminescent measurements show that sonocrystallization induces an efficient micromixing that drastically enhances the purity of the SCM powders. We thus propose that sonocrystallization, which is already used on organic or MOF compounds, can be applied to (magnetic) coordination compounds to readily afford bulk powders for characterization or shaping techniques that require pure, morphology- and crystallite-size-controlled powder samples.

A review on possible mechanisms of sonocrystallisation in solution

Sonocrystallisation is the application of ultrasound to the crystallisation process. The benefits obtained by sonication have been widely studied since the beginning of the 20th century and so far it is clear that ultrasound can be a very useful tool for enhancing crystallisation and controlling the properties of the final product. Crystal size, polymorphs, purity, process repeatability and lower induction time are only some of the advantages of sonocrystallisation. Even though the effects of sonication on crystallisation are quite clear, the physical explanation of the phenomena involved is still lacking. Is the presence of cavitation necessary for the process? Or is only the bubbles surface responsible for enhancing crystallisation? Are the strong local increases in pressure and temperature induced by cavitation the main cause of all the observed effects? Or is it the strong turbulence induced in the system instead? Many questions still remain and can only be appreciated with an understanding of the complexity behind the individual processes of crystallisation and acoustic cavitation. Therefore, this review will first summarise the theories behind crystallisation and acoustic cavitation, followed by a description of all the current proposed sonocrystallisation mechanisms, and conclude with an overview on future prospects of sonocrystallisation applications.

Investigations on sonofragmentation of hydroxyapatite crystals as a function of strontium incorporation

Hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, HA) is chemically similar to the mineral component of bones and hard tissues in mammals. Various cations can substitute calcium in the crystal structure of hydroxyapatite. Among them strontium triggers interest, because strontium incorporated hydroxyapatite increases the number of bone forming sites in addition to having good biocompatibility. In the present investigation strontium substituted hydroxyapatite (SHA) in the compositions range 0, 10, 20, 50 and 100 mol% have been synthesized by precipitation method and subjected to ultrasonic treatment for different time intervals to gain insight on the role of ultrasound in modifying the morphology of SHA. This study reveals that the aspect ratio of SHA varied with the duration of ultrasonication. SHA samples subjected to 5 min ultrasound experienced an increase in aspect ratio. Further increase of ultrasonication time tends to decrease the aspect ratio invariably for all SHA samples indicating particle fragmentation. The extent of sonofragmentation as a function of percentage incorporation of strontium in HA lattice has been studied as a part of the present investigation. This study indicated that strain in the HA lattice has correlation with strontium incorporation, leading to varied extent of sonofragmentation. Also, the investigation suggested that 100% substitution of calcium sites by strontium leads to a relatively lesser strain and hence poor fragmentation. To the best of our knowledge the report on sonofragmentation of HA crystallites as a function of strontium incorporation is first of its kind in the literature.

Applications of ultrasound to chiral crystallization, resolution and deracemization

Industrial synthesis of enantiopure compounds is nowadays heavily based on the separation of racemates through crystallization processes. Although the application of ultrasound in solution crystallization processes (sonocrystallization) has become a promising emerging technology, offering several benefits (e.g. reduction of the induction time and narrowing of the metastable zone width, control over the product size, shape and polymorphic modification), little attention has been paid so far to the effects of ultrasound on chiral crystallization processes. Several recent studies have reported on the application of acoustic energy to crystallization processes that separate enantiomers, ranging from classical (diastereomeric) resolution and preferential crystallization to new and emerging processes such as attrition-enhanced deracemization (Viedma ripening). A variety of interesting effects have been observed, which include among others, enhanced crystallization yield with higher enantiomeric purity crystals, spontaneous mirror symmetry breaking crystallization, formation of metastable conglomerate crystals and enhanced deracemization rates. The objective of this review is to provide an overview of the effects of ultrasound on chiral crystallization and outline several aspects of interest in this emerging field.

Role of ultrasonic treatment, inoculation and solute in the grain refinement of commercial purity aluminium

The present study investigates the influence of ultrasonic treatment on the grain refinement of commercial purity aluminium with a range of Al3Ti1B master alloy additions. When the aluminium contains the smallest amount of added master alloy, ultrasonics caused significant additional grain refinement compared to that provided by the master alloy alone. However, the influence of ultrasonics on grain size reduces with increasing addition of the master alloy which adds additional TiB₂ particles and Ti solute with each incremental addition. Applying the Interdependence model to analyse the experimentally measured grain sizes revealed that the results of this study and those from similar experiments on an Al-2Cu alloy were consistent when the alloy compositions are converted to their growth restriction factors (Q) and that increasing Q had a major effect on reducing grain size and increasing grain number density. Compared with the application of ultrasonic treatment where an order of magnitude increase in the grain number density is achieved, an increase in the Ti content over the range of master alloy additions, causes the grain number density to increase by approximately three times.