Effect of ultrasonic-induced selenium crystallization behavior during selenium reduction

In this work, the crystallization process of selenium was accelerated by ultrasonic wave. The effects of ultrasonic waves and conventional conditions of selenium crystallization were compared to understand the effects of different conditions on crystallization, including ultrasonic time, ultrasonic power, reduction temperature, and H₂SeO₃ concentration. The mechanism of ultrasound affecting selenium crystallization was also investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The experimental results showed that ultrasonic time, ultrasonic power, and reduction temperature significantly influenced the crystallization process and morphology of selenium. Ultrasonic time had a large effect on the completeness (all products have been crystallized) and integrity of the crystallization of the products. Meanwhile, ultrasonic power and reduction temperature had no effect on the completeness of crystallization. However, it had a significant effect on the morphology and integrity of the crystallized products, and different morphologies of the nano-selenium materials could be obtained by changing the ultrasonic parameters. Both primary and secondary nucleation are important in the process of ultrasound-accelerated selenium crystallization. The cavitation effect and mechanical fluctuant effects generated by ultrasound could reduce the crystallization induction time and accelerate the primary nucleation rate. The high-speed micro-jet formed in the rupture of the cavitation bubble generated is the most important reason to influence the secondary nucleation of the system.     

Ultrasound-assisted intensified crystallization of L-glutamic acid: Crystal nucleation and polymorph transformation

In this paper, the crystallization of L-glutamic acid with application of ultrasound was explored in detail, including the process of nucleation, polymorphic control and polymorphic transformation. The induction time and metastable zone widths (MSZWs) were measured with and without ultrasound during the nucleation process. The induction time and MSZWs were decreased by ultrasound and the induction time was further decreased by higher ultrasonic power. The calculated nucleation parameters (such as interfacial energy, critical nucleus size and critical Gibbs energy) showed an obvious decrease in the presence of ultrasound, indicating that the nucleation was enhanced with application of ultrasound. By adjusting the ultrasonic power in the quench cooling process, the difference in nucleation temperatures would determine the distribution of polymorphs. In further, the polymorphic transformation was investigated quantitatively, and to the best of our knowledge, it was the first time to study the transformation kinetics with ultrasound using Avrami-Erofeev model. In the transformation process, the crystallization mechanism of the stable form was modified by ultrasound. The ultrasound eliminated the nucleation element in the rate-limiting step and facilitated the crystal growth of stable form. Thus, the ultrasound has a profound influence on L-glutamic acid crystallization.     

A study on the primary and secondary nucleation of ice by power ultrasound

Several different investigations have been carried out to study the primary and secondary nucleation of ice by sonocrystallisation. Firstly, the primary nucleation of discrete ice crystals in a supercooled sucrose solution has been observed. For increasing concentrations of sucrose solutions from 0 to 45 wt%, the nucleation temperature consistently occurs at a higher nucleation temperature in the presence of ultrasound. The nucleation temperature also increases as the power output and duty cycle of a commercial ultrasonic horn are increased. Snap shot images of the bubble clouds obtained from the ultrasonic horn also show that the number of bubbles appears to increase as the ultrasonic output is increased. This suggests that the nucleation of ice is related to the power output and number of cavitation bubbles. The effect of a single bubble on the sonocrystallisation of ice is discussed. High-speed movies (1120 fps) have shown that the crystallisation appears to occur in the immediate vicinity of the single bubble. In most cases, many crystals are observed and it is not known whether a single ice crystal is being fragmented by the bubble or whether many crystals are being initiated. The bubble appears to undergo a dancing regime, frequently splitting and rejoining and also emitting some small microbubbles. A study on the secondary nucleation of ice in sucrose solutions has been carried out using a unique ultrasonic cold stage device. Images taken using a microscope system show that the pre-existing ice dendrite crystals can be broken up into smaller fragments by an ultrasonic field. Cavitation bubbles appear to be important during the fragmentation process, possibly melting any ice crystals in their path. Flow patterns around cavitation bubbles have also been observed, and these may be responsible for the fragmentation of ice crystals.     

Polymorph control by designed ultrasound application strategy: The role of molecular self-assembly

Molecular self-assembly plays a vital role in the nucleation process and sometimes determines the nucleation outcomes. In this study, ultrasound technology was applied to control polymorph nucleation. For the first time, different ultrasonic application methods based on the nucleation mechanisms have been proposed. For PZA-water and DHB-toluene systems that the molecular self-assembly in solution resembles the synthon in crystal structure, ultrasound pretreatment strategy was conducted to break the original molecular interactions to alter the nucleated form. When the solute molecular self-associates can’t give sufficient information to predict the nucleated polymorph like INA-ethanol system, the method of introducing continuous ultrasonic irradiation in the nucleation stage was applied. The induction of ultrasound during nucleation process can break the original interactions firstly by shear forces and accelerate the occurrence of nucleation to avoid the reorientation and rearrangement of solute molecules. These strategies were proved to be effective in polymorph control and have a degree of applicability.     

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

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-assisted theophylline polymorphic transformation: Selective polymorph nucleation,molecular mechanism and kinetics analysis

In this paper, the ultrasound-assisted solvent-mediated polymorphic transformation of theophylline was explored in detail. The induction time and reconstruction time were significantly decreased by ultrasound, thereby decreasing the total transformation time and promoting the transformation process. The ultrasound-promoted efficiency of nucleation was different in three alcoholic solvents, which was difficult to explain by traditional kinetic effects. To resolve the above confusion, binding energies calculated by Density Functional Theory were applied to explore the relationship between the ultrasound-promoted efficiency of nucleation and solute–solvent interactions. Then, a possible molecular self-assembly nucleation pathway affected by ultrasound was proposed: the ultrasound could change and magnify the crucial effect of the specific sites of solute–solvent interactions in the nucleation process. Finally, the transformation kinetics with different effective ultrasonic energies was quantitatively analyzed by Avrami-Erofeev model, indicating that the dissolution element in the rate-limiting step was gradually eliminated by higher ultrasonic energy. Fortunately, the elusive crystal form V could be easily obtained by the ultrasound-assisted polymorph transformation. This proved to be a robust method to produce high purity form V of theophylline. The outcome of this study demonstrated that the proper ultrasonic irradiation had the potential to produce specific polymorphs selectively.     

Lysozyme crystallization in hydrogel media under ultrasound irradiation

Sonocrystallization implies the application of ultrasound radiation to control the nucleation and crystal growth depending on the actuation time and intensity. Its application allows to induce nucleation at lower supersaturations than required under standard conditions. Although extended in inorganic and organic crystallization, it has been scarcely explored in protein crystallization. Now, that industrial protein crystallization is gaining momentum, the interest on new ways to control protein nucleation and crystal growth is advancing. In this work we present the development of a novel ultrasound bioreactor to study its influence on protein crystallization in agarose gel. Gel media minimize convention currents and sedimentation, favoring a more homogeneous and stable conditions to study the effect of an externally generated low energy ultrasonic irradiation on protein crystallization avoiding other undesired effects such as temperature increase, introduction of surfaces which induce nucleation, destructive cavitation phenomena, etc. In-depth statistical analysis of the results has shown that the impact of ultrasound in gel media on crystal size populations are statistically significant and reproducible.     

Theoretical model of ice nucleation induced by inertial acoustic cavitation. Part 2: Number of ice nuclei generated by a single bubble

In the preceding paper (part 1), the pressure and temperature fields close to a bubble undergoing inertial acoustic cavitation were presented. It was shown that extremely high liquid water pressures but quite moderate temperatures were attained near the bubble wall just after the collapse providing the necessary conditions for ice nucleation. In this paper (part 2), the nucleation rate and the nuclei number generated by a single collapsing bubble were determined. The calculations were performed for different driving acoustic pressures, liquid ambient temperatures and bubble initial radius. An optimal acoustic pressure range and a nucleation temperature threshold as function of bubble radius were determined. The capability of moderate power ultrasound to trigger ice nucleation at low undercooling level and for a wide distribution of bubble sizes has thus been assessed on the theoretical ground.     

Experimental investigation of flow boiling heat transfer enhancement under ultrasound fields in a minichannel heat sink

Ultrasound is considered to be an effective active heat transfer enhancement method, which is widely used in various fields. But there is no clear understanding of flow boiling heat transfer characteristics in micro/mini-channels under ultrasonic field since the studies related are limited up to now. In this paper, a novel minichannel heat exchanger with two ultrasonic transducers inside the inlet and outlet plenum respectively is designed to experimentally investigate the impacts of ultrasound on flow boiling heat transfer enhancement in a minichannel heat sink. Flow visualization analyses reveal that ultrasound can promote rapid bubble motion, bubble detachment from heating wall surface and thereby new bubble generation, and decrease the length of confined bubble. Furthermore, the flow boiling experiments are initiated employing working fluid R141b at different ultrasonic parameters (e.g., frequency, power, angle of radiation) and heat flux under three types of ultrasound excitations: no ultrasound (NU), single inlet ultrasound (IU), inlet and outlet ultrasound (IOU). The results indicate that ultrasound has obvious augmentation effects on flow boiling heat transfer even though the intensification effects will be limited with the heat flux increases. The higher ultrasonic power, the lower ultrasonic frequency and the higher ultrasonic radiation angle, the better intensification efficiency. The maximum enhancement ratio of h_ave in the saturated boiling section reaches 1.88 at 50 W, 23 kHz and 45° under the experimental conditions. This study will be beneficial for future applications of ultrasound on flow boiling heat transfer in micro/mini-channels.     

Flow boiling heat transfer enhancement under ultrasound field in minichannel heat sinks

The enhancement of the heat transfer assisted by ultrasound is considered to be an interesting and highly efficient cooling technology, but the investigation and application of ultrasound in minichannel heat sinks to strengthen the flow boiling heat transfer are very limited. Herein, a novel installation of ultrasound transducers in the flow direction of a minichannel heat sink is designed to experimentally study the characteristics of heat transfer in flow boiling and the influence of operation parameters (e.g., heat flux, mass flux rate) and ultrasound parameters (e.g., frequency, power) on the flow boiling heat transfer in a minichannel heat sink with and without ultrasound field. Bubble motion and flow pattern in the minichannel are analyzed by high-speed flow visualization, revealing that the ultrasound field induces more bubbles at the same observation position and a forward shift of the onset of nucleation boiling along the flow direction, as ultrasonic cavitation produces a large number of bubbles. Moreover, bubbles hitting the channel wall on the left and right sides are found, and the motion speed of the bubbles is increased by 31.9% under the ultrasound field. Our results demonstrate that the heat transfer coefficient obtained under the ultrasound field is 53.9% higher than in the absence of the ultrasound field under the same conditions, and the enhancement ratio is decreased in the high heat flux region due to the change of the flow regime with increasing heat flux. This study provides a theoretical basis for the application of an ultrasound field in minichannel heat sinks for the enhancement of flow boiling heat transfer.     

Heat transfer enhancement using 2 MHz ultrasound

The present work focuses on possible heat transfer enhancement from a heating plate towards tap water in forced convection by means of 2 MHz ultrasound. The thermal approach allows to observe the increase of local convective heat transfer coefficients in the presence of ultrasound and to deduce a correlation between ultrasound power and Nusselt number. Heat transfer coefficient under ultrasound remains constant while heat transfer coefficient under silent conditions increases with Reynolds number from 900 up to 5000. Therefore, heat transfer enhancement factor ranges from 25% up to 90% for the same energy conditions (supplied ultrasonic power = 110 W and supplied thermal power = 450 W). In the same time cavitational activity due to 2 MHz ultrasound emission was characterized from mechanical and chemical viewpoints without significant results. At least, Particle Image Velocimetry (PIV) measurements have been performed in order to investigate hydrodynamic modifications due to the presence of 2 MHz ultrasound. It was therefore possible to propose a better understanding of heat transfer enhancement mechanism with high frequency ultrasound.     

The effect of ultrasound irradiation on the convective heat transfer rate during immersion cooling of a stationary sphere

It has been proven that ultrasound irradiation can enhance the rate of heat transfer processes. The objective of this work was to study the heat transfer phenomenon, mainly the heat exchange at the surface, as affected by ultrasound irradiation around a stationary copper sphere (k=386W m(-1)K(-1), C(p)=384J kg(-1)K(-1), ρ=8660kg m(-3)) during cooling. The sphere (0.01m in diameter) was immersed in an ethylene glycol-water mixture (-10°C) in an ultrasonic cooling system that included a refrigerated circulator, a flow meter, an ultrasound generator and an ultrasonic bath. The temperature of the sphere was recorded using a data logger equipped with a T-type thermocouple in the center of the sphere. The temperature of the cooling medium was also monitored by four thermocouples situated at different places in the bath. The sphere was located at different positions (0.02, 0.04 and 0.06m) above the transducer surface of the bath calculated considering the center of the sphere as the center of the reference system and was exposed to different intensities of ultrasound (0, 120, 190, 450, 890, 1800, 2800, 3400 and 4100W m(-2)) during cooling. The frequency of the ultrasound was 25kHz. It was demonstrated that ultrasound irradiation can increase the rate of heat transfer significantly, resulting in considerably shorter cooling times. Higher intensities caused higher cooling rates, and Nu values were increased from about 23-27 to 25-108 depending on the intensity of ultrasound and the position of the sphere. However, high intensities of ultrasound led to the generation of heat at the surface of the sphere, thus limiting the lowest final temperature achieved. An analytical solution was developed considering the heat generation and was fitted to the experimental data with R(2) values in the range of 0.910-0.998. Visual observations revealed that both cavitation and acoustic streaming were important for heat transfer phenomenon. Cavitation clouds at the surface of the sphere were the main cause of heating effect. The results showed that closer distances to the transducer surface showed higher cooling rates. On the other hand, despite having a bigger distance from the transducer, when the sphere was located close to the gas-liquid interface the enhancement factor of heat transfer was higher. Ultrasound irradiation showed promising effect for the enhancement of convective heat transfer rate during immersion cooling. More investigations are required to demonstrate the behavior of ultrasound assisted heat transfer and resolve the proper way of the application of ultrasound to assist the cooling and/or freezing processes.     

Polymer-mediated and ultrasound-assisted crystallization of ropivacaine: Crystal growth and morphology modulation

The objective of this research was to modify the crystal shape and size of poorly water-soluble drug ropivacaine, and to reveal the effects of polymeric additive and ultrasound on crystal nucleation and growth. Ropivacaine often grow as needle-like crystals extended along the a-axis and the shape was hardly controllable by altering solvent types and operating conditions for the crystallization process. We found that ropivacaine crystallized as block-like crystals when polyvinylpyrrolidone (PVP) was used. The control over crystal morphology by the additive was related to crystallization temperature, solute concentration, additive concentration, and molecular weight. SEM and AFM analyses were performed providing insights into crystal growth pattern and cavities on the surface induced by the polymeric additive. In ultrasound-assisted crystallization, the impacts of ultrasonic time, ultrasonic power, and additive concentration were investigated. The particles precipitated at extended ultrasonic time exhibited plate-like crystals with shorter aspect ratio. Combined use of polymeric additive and ultrasound led to rice-shaped crystals, which the average particle size was further decreased. The induction time measurement and single crystal growth experiments were carried out. The results suggested that PVP worked as strong nucleation and growth inhibitor. Molecular dynamics simulation was performed to explore the action mechanism of the polymer. The interaction energies between PVP and crystal faces were calculated, and mobility of the additive with different chain length in crystal-solution system was evaluated by mean square displacement. Based on the study, a possible mechanism for the morphological evolution of ropivacaine crystals assisted by PVP and ultrasound was proposed.     

Surface Nucleation Kinetics on Active Centers from Supersaturated Vapor

The formation of nuclei from a supersaturated vapor on a surface with active centers is treated within standard self-consistent classical model with exhaustion of active centers. Basic characteristics of nucleation process (total number of nuclei, nucleation rate, time lag and size distribution of nuclei) are determined by numerical solution of kinetic equations. It is shown that standard approach to nucleation on active centers based on Avrami model coincides with our approach in the values of time lag of nucleation process, but it differs in the total number of nuclei.     

Fermentation of Saccharomyces cerevisiae in a 7.5 L ultrasound-enhanced fermenter: Effect of sonication conditions on ethanol production,intracellular Ca2+ concentration and key regulating enzyme activity in glycolysis

In this study, the effect of sonication on the fermentation process of a single-celled fungus was examined. During the experiment, Saccharomyces cerevisiae (S. cerevisiae) was used as the starting strain for ethanol fermentation (batch fermentation) in a 7.5 L automated fermentation tank. The fermentation tank connected with a six-frequency ultrasonic equipment. Non-sonication treatment was set up as the control. Sonication treatment with power density of 280 W/L and 48 h of treatment time were set up as trial groups for investigating the influence of different ultrasound frequency including 20, 23, 25, 28, 33 and 40 kHz on the changes in dry cell-weight, glucose consumption rate, and ethanol yield. The results showed that the dry cell-weight, glucose consumption rate, and ethanol content reached the best results under the ultrasonic condition of 28 kHz ultrasound frequency in comparison with other ultrasound frequency. The dry cell-weight and ethanol content of the 28 kHz ultrasonic treatment group increased by 17.30% and 30.79%, respectively in comparison with the control group The residual sugar content dropped to a lower level within 24 h, which was consistent with the change in ethanol production. Besides, the results found that the glucose consumption rate increased compared to the control. It indicated that ultrasound accelerated glucose consumption contributed to increase the rate of ethanol output. In order to explore the mechanism of sonication enhanced the content of ethanol output by S. cerevisiae, the morphology, permeability of S. cerevisiae and key enzyme activities of ethanol synthesis were investigated before and after sonication treatment. The results showed that after sonication treatment, the extracellular nucleic acid protein content and intracellular Ca²⁺ concentration increased significantly. The morphology of S. cerevisiae was observed by SEM and found that the surface of the strain had wrinkles and depressions after ultrasonic treatment. furthermore after sonication treatment, the activities of three key enzymes which catalyze three irreversible reactions in glycolysis metabolism, namely, hexokinase, phosphofructokinase and pyruvate kinase increased by 59.02%, 109.05% and 87.27%, respectively. In a word, low-intensity ultrasound enhance the rate of ethanol output by S. cerevisiae might due to enhancing the growth and cell permeability of strains, and increasing the activities of three key enzymes of ethanol biosynthesis.     

Free surface entrainment of oxide particles and their role in ultrasonic treatment performance of aluminum alloys

Although the ultrasonic treatment of molten aluminum has been studied for long period, there is still much to be revealed for this process. Many studies have focused on the investigation of acoustic cavitation and streaming under the horn tip and their effects on the treatment efficiency. However, to the best of our knowledge, no attempt has been done to explain phenomena occurring near or on the melt free surface. Thus, the goal of this study is to investigate phenomena occurring at the free surface during ultrasound irradiation and clarify their possible influence on the ultrasound treatment performance. The results of high temperature and water model experiments reveal that ultrasound irradiation significantly promotes the formation of alumina particles on the melt free surface around sonotrode, and part of these particles can be entrained into aluminum melts. Furthermore, TEM observation results suggested that the entrained alumina inclusions can serve as nucleation sites for the primary Al₃Zr compounds. Most importantly, the oxidation and entrainment of particles from free surface are likely to be controllable by the immersion depth of sonotrode into molten aluminum.     

Investigation of the effect of power ultrasound on the nucleation of water during freezing of agar gel samples in tubing vials

Nucleation, as an important stage of freezing process, can be induced by the irradiation of power ultrasound. In this study, the effect of irradiation temperature (−2 °C, −3 °C, −4 °C and −5 °C), irradiation duration (0 s, 1 s, 3 s, 5 s, 10 s or 15 s) and ultrasound intensity (0.07 W cm⁻², 0.14 W cm⁻², 0.25 W cm⁻², 0.35 W cm⁻² and 0.42 W cm⁻²) on the dynamic nucleation of ice in agar gel samples was studied. The samples were frozen in an ethylene glycol-water mixture (−20 °C) in an ultrasonic bath system after putting them into tubing vials. Results indicated that ultrasound irradiation is able to initiate nucleation at different supercooled temperatures (from −5 °C to −2 °C) in agar gel if optimum intensity and duration of ultrasound were chosen. Evaluation of the effect of 0.25 W cm⁻² ultrasound intensity and different durations of ultrasound application on agar gels showed that 1 s was not long enough to induce nucleation, 3 s induced the nucleation repeatedly but longer irradiation durations resulted in the generation of heat and therefore nucleation was postponed. Investigation of the effect of ultrasound intensity revealed that higher intensities of ultrasound were effective when a shorter period of irradiation was used, while lower intensities only resulted in nucleation when a longer irradiation time was applied. In addition to this, higher intensities were not effective at longer irradiation times due to the heat generated in the samples by the heating effect of ultrasound. In conclusion, the use of ultrasound as a means to control the crystallization process offers promising application in freezing of solid foods, however, optimum conditions should be selected.     

Modeling of the surface nucleation and growth on active centers from the vapor phase

The formation of nuclei of a new phase and their growth on the inhomogeneous substrate from a vapor phase are studied. Basic kinetic equations describing such a process are solved numerically. The effect of the depletion of active sites during the growth of nuclei is taken into account. Basic characteristics of the nucleation process, such as size distribution function, nucleation rate and number of nuclei formed on the unit surface are determined. It is shown that the size distribution of nuclei evolves by a nontrivial way as a function of time. This process is fully nonstationary from the viewpoint of nucleation rate. The total number of nuclei reaches the number of active centers for a sufficiently long time. Presented at the VIII-th Symposium on Surface Physics, Třešt’ Castle, Czech Republic, June 28 – July 2, 1999. This work was supported by Grant No. 202/99/0403 of the Grant Agency of the Czech Republic.     

Ultrasonic drying of foodstuff in a fluidized bed: Parametric study

The application of high power ultrasound for dehydration of porous materials may be very effective in processes in which heat-sensitive materials such as foodstuffs have to be treated. In fact, high-intensity ultrasonic vibrations are capable of increasing heat and mass transfer processes in materials. The application of ultrasonic energy can be made alone or in combination with other kind of energy such as hot-air. In this case, ultrasound helps in reducing temperature or treatment time. The aim of this work is to study the effect of air flow rate, ultrasonic power and mass loading on hot-air drying assisted by a new power ultrasonic system. The drying chamber is an aluminium vibrating cylinder, which is able to create a high intensity ultrasonic field in the gas medium. To that purpose the chamber is driven at its centre by a power ultrasonic vibrator at 21.8 kHz. Drying kinetics of carrot cubes and lemon peel cylinders were carried out at 40 degrees C for different air velocities, with and without ultrasound. The results show that the effect of ultrasound on drying rate is affected by air flow rate, ultrasonic power and mass loading. In fact, at high air velocities the acoustic field inside the chamber is disturbed and the effect of ultrasound on drying kinetics diminishes.     

Ultrasonic-induced nucleation of ice in water containing air bubbles

Cavitation induced by ultrasonic vibrations can cause nucleation of ice in supercooled water. In this study, the time required for ultrasonic-induced nucleation of ice was measured for water containing two different size distributions of air bubbles. When the water was supersaturated with air bubbles, there was a time lag of about 0.5 s between the onset of ultrasonic irradiation and the onset of ice nucleation, and the probability of ice nucleation was unusually high within 0.5-1.1 s after the onset of ultrasonic irradiation. These results cannot be explained by conventional models alone, in which the collapse of a cavitation bubble triggers the nucleation of ice. Secondary effects appear to also influence ice nucleation.