Green ultrasound-assisted extraction of chlorogenic acids from sweet potato peels and sonochemical hydrolysis of caffeoylquinic acids derivatives

Sweet potato peels are rich in chlorogenic acids. In this work, we applied ultrasound technology to extract the main compounds from sweet potato peel and used multivariate analysis and principal component analysis (PCA) to evaluate the effects of different extraction conditions on the extraction of chlorogenic acids. The extraction was studied varying ultrasonic power density (20, 35 and 50 W/L) and processing time (5, 10, 20 and 40 min) using an ultrasonic bath operating at 25 kHz. The chemical analysis was carried out by UPLC-qTOF-MS, and the results were evaluated by PCA and PLS-DA chemometric analysis. Results show that both ultrasonic power density and processing time influences in the extraction of different chlorogenic acid, and that different extraction conditions can be used to selectively extract specific caffeoylquinic acids and feruloylquinic acids in higher amounts. Ultrasound promoted the hydrolysis of tricaffeoylquinic acid when subjected to ultrasonic waves (20–50 W/L), and of 3,4-caffeyolquinic acid at high ultrasonic power density (50 W/L).     

Improving effect of oleic acid-mediated sodium caseinate-based encapsulation in an ultrasound field on the thermal stability and bioaccessibility of quercetin

The simultaneous improvement of quercetin (QUE) processing stability and bioavailability has always presented a technical challenge during food processing. This study constructed a water-soluble carrier consisting of oleic acid (OA) and sodium caseinate (NaCas) in an ultrasonic field and investigated the effect of its encapsulation on improving the thermal stability and bioaccessibility of QUE. The results showed that the OA and NaCas generated uniform, stable water-soluble particles with a poly dispersity index (PDI) below 0.3 and an absolute value of Zeta potential above 30 mV in optimized conditions (a protein concentration of 4 mg/mL, ultrasonic power of 300 W, and ultrasonic time of 5 min). OA-NaCas mass ratio of 1:40, 1:15, 1:8, and 1:4 was selected for QUE loading to compare its encapsulation effect at different mass ratios. Compared with the NaCas without OA, the QUE embedding rate reached 95 % at OA-NaCas mass ratios of 1:15 and 1:8. In addition, the transmission electron microscopy (TEM) images confirmed that QUE was embedded in OA-NaCas particles, forming regular, spherical OA-NaCas-QUE particles at mass ratios or 1:15 and 1:8. Next, when heated at 80 °C for 120 min, the OA-NaCas (OA:NaCas, 1:15, 1:8, and W/W) particles significantly improved the QUE retention rate. The simulated in vitro gastrointestinal digestion experiments showed that the QUE bioaccessibility increased from 25 % to more than 60 % when it was encapsulated in OA-NaCas (OA:NaCas, 1:15, 1:8, and W/W) particles. These results indicated that the OA-NaCas complex was suitable as a hydrophilic delivery carrier of fat-soluble polyphenols.     

Enhancing the adhesion strength of micro electroforming layer by ultrasonic agitation method and the application

Micro electroforming is widely used for fabricating micro metal devices in Micro Electro Mechanism System (MEMS). However, there is the problem of poor adhesion strength between micro electroforming layer and substrate. This dramatically influences the dimensional accuracy of the device. To solve this problem, ultrasonic agitation method is applied during the micro electroforming process. To explore the effect of the ultrasonic agitation on the adhesion strength, micro electroforming experiments were carried out under different ultrasonic power (0 W, 100 W, 150 W, 200 W, 250 W) and different ultrasonic frequencies (0 kHz, 40 kHz, 80 kHz, 120 kHz, 200 kHz). The effects of the ultrasonic power and the ultrasonic frequency on the micro electroforming process were investigated by polarization method and alternating current (a.c.) impedance method. The adhesion strength between the electroforming layer and the substrate was measured by scratch test. The compressive stress of the electroforming layer was measured by X-ray Diffraction (XRD) method. The crystallite size of the electroforming layer was measured by Transmission Electron Microscopy (TEM) method. The internal contact surface area of the electroforming layer was measured by cyclic voltammetry (CV) method. The experimental results indicate that the ultrasonic agitation can decrease the polarization overpotential and increase the charge transfer process. Generally, the internal contact surface area is increased and the compressive stress is reduced. And then the adhesion strength is enhanced. Due to the different depolarization effects of the ultrasonic power and the ultrasonic frequency, the effects on strengthening the adhesion strength are different. When the ultrasonic agitation is 200 W and 40 kHz, the effect on strengthening the adhesion strength is the best. In order to prove the effect which the ultrasonic agitation can improve the adhesion strength of the micro devices, micro pillar arrays were fabricated under ultrasonic agitation (200 W, 40 kHz). The experimental results show that the residual rate of the micro pillar arrays is increased about 17% by ultrasonic agitation method. This work contributes to fabricating the electroforming layer with large adhesion strength.     

Effects of thermal treatment and sonication on quality attributes of Chokanan mango (Mangifera indica L.) juice

Ultrasonic treatment is an emerging food processing technology that has growing interest among health-conscious consumers. Freshly squeezed Chokanan mango juice was thermally treated (at 90 °C for 30 and 60 s) and sonicated (for 15, 30 and 60 min at 25 °C, 40 kHz frequency, 130 W) to compare the effect on microbial inactivation, physicochemical properties, antioxidant activities and other quality parameters. After sonication and thermal treatment, no significant changes occurred in pH, total soluble solids and titratable acidity. Sonication for 15 and 30 min showed significant improvement in selected quality parameters except color and ascorbic acid content, when compared to freshly squeezed juice (control). A significant increase in extractability of carotenoids (4–9%) and polyphenols (30–35%) was observed for juice subjected to ultrasonic treatment for 15 and 30 min, when compared to the control. In addition, enhancement of radical scavenging activity and reducing power was observed in all sonicated juice samples regardless of treatment time. Thermal and ultrasonic treatment exhibited significant reduction in microbial count of the juice. The results obtained support the use of sonication to improve the quality of Chokanan mango juice along with safety standard as an alternative to thermal treatment.     

Dual-frequency multi-angle ultrasonic processing technology and its real-time monitoring on physicochemical properties of raw soymilk and soybean protein

To improve the soybean protein content (SPC), flavor and quality of soymilk, the effects of dual-frequency ultrasound at different angles (40 + 20 kHz 0°, 40 + 20 kHz 30°, 40 + 20 kHz 45°) on physicochemical properties and soybean protein (SP) structure of raw soymilk were mainly studied and compared with the conventional single-frequency (40 kHz, 20 kHz) ultrasound. Furthermore, the intensity of the ultrasonic field in real-time was monitored via the oscilloscope and spectrum analyzer. The results showed that 40 + 20 kHz 45° treatment significantly increased SPC. The ultrasonic field intensity of 40 + 20 kHz 0° treatment was the largest (8.727 × 10⁴ W/m²) and its distribution was the most uniform. The emulsifying stability of SP reached the peak value (233.80 min), and SP also had the largest particle size and excellent thermal stability. The protein solubility of 40 + 20 kHz 30° treatment attained peak value of 87.09%. 20 kHz treatment significantly affected the flavor of okara. The whiteness and brightness of raw soymilk treated with 40 kHz were the highest and the system was stable. Hence, the action mode of ultrasonic technology can be deeply explored and the feasibility for improving the quality of soymilk can be achieved.     

The effect of ultrasonic intensity on physicochemical properties of Chinese fir

Cavitation intensity is affected by ultrasonic intensity (UI) and is a key parameter to describe experimental results during ultrasonic treatment. The relationship between the UI and physicochemical properties of Chinese fir was investigated. In this study, four frequencies (25, 28, 40, and 59 kHz) were used at the same intensity of 240 W and the same duration of 35 min. The UI during the ultrasonic treatment was determined, and the chemical components were determined. The chemical structure, crystallinity, morphology, and extractives of wood were respectively analyzed by Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and gas chromatography-mass spectrometry (GC–MS). The results showed that higher crystallinity was associated with a larger integrated area under the curve of the ultrasonic intensity (UIA). The largest UIA was observed at 25 kHz, followed by those at 40, 59, and 28 kHz. The relative content of hemicellulose was strongly affected by the ultrasonic treatment. No chemical reactions were observed in the wood, whereas the ultrasonic treatment affected the torus of the bordered pits and facilitated the migration of extractives. In general, the higher the UIA, the stronger the effect of the cavitation was. The most significant changes in the physicochemical properties were observed at 25 kHz. The instantaneous ultrasonic intensity (IUI) changed over time, and the UIA was closely associated with changes in the physicochemical properties of the wood. The results of this study demonstrate that UI has a significant influence on the physicochemical properties of wood.     

Effects of multi-frequency ultrasound on physicochemical properties,structural characteristics of gluten protein and the quality of noodle

In this study, the influence of multi-frequency ultrasound irradiation on the functional properties and structural characteristics of gluten, as well as the textural and cooking characteristics of the noodles were investigated. Results showed that the textural and cooking characteristics of noodles that contain less gluten pretreated by multi-frequency ultrasonic were ultrasonic frequency dependent. Moreover, the noodles that contain a smaller amount of sonicated gluten could achieve the textural and cooking quality of commercial noodles. There was no significant difference in the cooking and texture characteristics between commercial noodles and noodles with 12%, 11%, and 10% gluten pretreated by single-frequency (40 kHz), dual-frequency (28/40 kHz), and triple-frequency sonication (28/40/80 kHz), respectively. Furthermore, the cavitation efficiency of triple-frequency ultrasound was greater than that of dual-frequency and single-frequency. As the number of ultrasonic frequencies increased, the solubility, water holding capacity and oil holding capacity of gluten increased significantly (p < 0.05), and the particle size was reduced from 197.93 ± 5.28 nm to 110.15 ± 2.61 nm. Furthermore, compared to the control group (untreated), the UV absorption and fluorescence intensity of the gluten treated by multi-frequency ultrasonication increased. The surface hydrophobicity of gluten increased from 8159.1 ± 195.87 (untreated) to 11621.5 ± 379.72 (28/40/80 kHz). Raman spectroscopy showed that the α-helix content of all sonicated gluten protein samples decreased after sonication, while the β-sheet and β-turn content increased, and tryptophan and tyrosine residues were exposed. Through scanning electron microscope (SEM) analysis, the gluten protein network structure after ultrasonic treatment was loose, and the pore size of the gluten protein network increased from about 10 μm (untreated) to about 26 μm (28/40/80 kHz). This work elucidated the effect of ultrasonic frequency on the performance of gluten, indicating that with increasing frequency combination increases, the ultrasound effect became more pronounced and protein unfolding increased, thereby impacting the functional properties and the quality of the final product. This study provided a theoretical basis for the application of multi-frequency ultrasound technology in the modification of gluten protein and noodle processing.     

Effect of slit dual-frequency ultrasonic emulsification technology on the stability of walnut emulsions

A highly hygienic walnut emulsion beverage was prepared by using a slit dual-frequency emulsification technique. The optimal ultrasonic parameters were studied as a model system: the ultrasonic time of 50 min, the ultrasonic power density of 260 W/L, and a dual-frequency ultrasonic combination of 28/68 kHz. Walnut emulsion with an average mean volume diameter of 2.05 µm, a Zeta potential absolute value of 40 mV was obtained after ultrasonic treatment, and the emulsion stability could be maintained for more than 14 days without phase separation. At the lowest ultrasonic energy input, the vibrating emulsion could promote droplet aggregation. However, excessive energy input could result in sample overtreatment and reduced emulsion activity. The laser scanning confocal microscope (LSCM) and transmission electron microscope (TEM) confirmed that walnut emulsion processed by slit dual-frequency ultrasonic had pretty high stability. Therefore, the slit dual-frequency ultrasonic emulsification technique was found to be well suited for the preparation of complex and fine oil-in-water food emulsions.     

Lactobacillus plantarum-derived biosurfactant: Ultrasound-induced production and characterization

The aim of the present study was to investigate the effect of ultrasonic treatment (25 kHz) on biosurfactant production by Lactobacillus plantarum ATCC 8014. The impacts of the ultrasonication (with a frequency of 25 kHz and power of 7.4 W for 30 min time duration) were examined at different stages of the fermentation process to obtain the optimum stimulation instant(s). The optimum scenario was found to be one-time sonication at the 12^th hour of fermentation which can be beneficial from an economic point of view (compared with multiple applications of sonication). Ultrasonic treatment at this time resulted in enhancement of the productivities of biomass (4.5 g/L) and biosurfactant (2.01 g/L) which was almost 1.3 times higher than those of the non-sonicated control samples. According to our results, it was clearly observed that glucose consumption increased after ultrasonic treatment representing the improved substrate uptake and progression of the cellular metabolism. Furthermore, the transmission electron microscopic images immediately after sonication clarified the pore formation on the cell surfaces. The results also indicated the enhancement of plasma membrane permeability of the sonicated cells. Fourier transform infrared spectroscopy and scanning electron microscopy coupled with energy dispersive x-ray spectroscopy analyses also disclosed respectively no structural differences before and after ultrasonic exposure in the produced biosurfactant and bacterial cell membrane. The biosurfactant was characterized to be a mixture of carbohydrate (28%), protein (23%) and lipid (specified by gas chromatography-mass spectrometry) known as glycolipoprotein. The sustainable critical micelle concentration and the stability of the synthesized biosurfactant can feature its potential applicability in various processes in the food and pharmaceutical industries.     

High yield synthesis of Ni-BTC metal–organic framework with ultrasonic irradiation: Role of polar aprotic DMF solvent

Nickel based porous solid was synthesized with 20 kHz ultrasonic irradiation. The reaction of Ni(II) nitrate hexahydrate with 1,3,5-benzene tricarboxylic acid in N,N-Dimethylformamide (DMF) as the sole solvent under ultrasonic radiation produced porous Ni-BTC MOF. Choice of correct solvent for the ultrasonic treatment was proven important. The effect of varying ultrasonic powers (40%, 60% and 80% of 750 W) along with different temperature conditions (50 °C, 60 °C, 70 °C and 80 °C) influenced the respective yield. A very high yield of 88% Ni-BTC MOF was obtained from 80% ultrasonic power at 60 °C. BET surface areas of the MOF crystals measured by N₂ gas adsorption isotherms were in the range of 960–1000 m²/g.     

Intensification of alkaline delignification of sugarcane bagasse using ultrasound assisted approach

Ultrasound-assisted approach has been investigated for delignification so as to develop green and sustainable technology. Combination of NaOH with ultrasound has been applied with detailed study into effect of various parameters such as time (operating range of 15–90 min), alkali concentration (0.25 M−2.5 M), solvent loading (1:15–1:30 w/v), temperature (50–90 ˚C), power (40–140 W) and duty cycle (40–70 %) at fixed frequency of 20 kHz. The optimized operating conditions established for the ultrasonic horn were 1 M as the NaOH concentration, 1 h as treatment time, 70˚C as the operating temperature, 1:20 as the biomass loading ratio, 100 W as the ultrasonic power and 70% duty cycle yielding 67.30% as the delignification extent. Comparative study performed using conventional and ultrasonic bath assisted alkaline treatment revealed lower delignification as 48.09% and 61.55% respectively. The biomass samples were characterized by SEM, XRD, FTIR and BET techniques to establish the role of ultrasound during the treatment. The morphological changes based on the ultrasound treatment demonstrated by SEM were favorable for enhanced delignification and also the crystallinity index was more in the case of ultrasound treated material than that obtained by conventional method. Specific surface area and pore size determinations based on BET analysis also confirmed beneficial role of ultrasound. The overall results clearly demonstrated the intensification obtained due to the use of ultrasonic reactors.     

Ultrasound assisted intensified recovery of lactose from whey based on antisolvent crystallization

The current work deals with understanding the fundamental aspects of intensified recovery of lactose from paneer (cottage cheese) whey using the anti-solvent induced sonocrystallization. Ultrasonic horn (22 kHz) with varying power levels over the range of 40–120 W has been used for initial experiments at 100% duty cycle and two different levels of ultrasonic exposure time as 10 min and 20 min. Similar experiments were also performed using ultrasonic bath for the same time of exposure but with at two ultrasonic frequencies (22 kHz and 33 kHz). It was observed that the lactose recovery as well as purity increased with an increase in ultrasonic power at 100% duty cycle for the case of treatment time as 10 min whereas the lactose recovery and purity increased only till an optimum power for the 20 min treatment. In the case of ultrasonic bath, lactose purity increased with an increase in the ultrasonic frequency from 22 kHz to 33 kHz though the lactose recovery marginally decreased. Overall, it was observed that the maximum lactose recovery was ∼98% obtained using ultrasonic horn while the maximum lactose purity was ∼97%. It was also observed that maximum lactose recovery was ∼94% for the case of ultrasonic bath while the maximum lactose purity was ∼92%. The work has enabled to understand the optimized application of ultrasound so as to maximize both the lactose yield and purity during the recovery from whey.     

Emerging applications of nano-optical sensors combined with near-infrared spectroscopy for detecting tea extract fermentation aroma under ultrasound-assisted sonication

The current innovative work combines nano-optical sensors with near-infrared spectroscopy for rapid detection and quantification of polyphenols and investigates the potential of the nano-optical sensor based on chemo-selective colorants to detect the dynamic changes in aroma components during the fermentation of tea extract. The procedure examined the influence of different ultrasound-assisted sonication factors on the changes in the consumption rate of polyphenols during the fermentation of tea extract versus non-sonication as a control group. The results showed that the polyphenol consumption rate improved under the ultrasound conditions of 28 kHz ultrasound frequency, 24 min treatment time, and 40 W/L ultrasonic power density. The metal–organic framework based nano-optical sensors reported here have more adsorption sites for enhanced adsorption of the volatile organic compounds. The polystyrene-acrylic microstructure offered specific surface area for the reactants. Besides, the employed porous silica nanospheres with higher porosity administered improved gas enrichment effect. The nano-optical sensor exhibits good performance with a “chromatogram” for the identification of aroma components in the fermentation process of tea extract. The proposed method respectively enhanced the consumption rate of polyphenol by 35.57%, 11.34% and 16.09% under the optimized conditions. Based on the established polyphenol quantitative prediction models, this work demonstrated the feasibility of using a nano-optical sensor to perform in-situ imaging of the fermentation degree of tea extracts subjected to ultrasonic treatment.     

The effects of ultrasonic treated whey on the structure formation in food systems based on whey in combination with pectin and agar-agar

We studied the effects of ultrasonicated whey in food systems with the structure-forming additives such as pectin and agar-agar. The high-intensity (45KHz, 40 W with cavitation) ultrasonic treated whey was used. The conditions and optimal modes of cavitation based ultrasonic processing of curd milk whey have been determined. The mechanism of structure formation has been studied in detail.From the studies carried out, the scientific basis for the choice of structure-forming agents in food systems was established along with the range of rational concentrations of pectin and agar-agar. It was shown that in the case of processing milk curd whey by the cavitation method, the concentration of the structure former can be reduced by 2 times compared to using non-sonicated whey in the food system thus saving costs on the raw materials.It was established that high-purity cavity treatment minimizes gel-like food systems set time up to 20% compared to the control within 15 min. The duration of high-purity treatment within 15 min contributes to an increase in penetration pressure, which characterizes the texture of the gel-like food two times.     

Effects of ultrasound pretreatment on the quality,nutrients and volatile compounds of dry-cured yak meat

The objective of the present study was to assess the effects of ultrasound pretreatment on the quality of dry-cured yak meat. The ultrasonic power with 0, 200, 300 and 400 W (ultrasonic frequency of 20 kHz) were used to assist processing of dry-cured yak meat. The meat quality, nutrient substances, sensory quality, electronic nose, electronic tongue and volatile compounds of dry-cured yak meat were determined. The results indicated that the moisture content and hardness value of ultrasonic treatment group was significantly lower compared to the control group (P < 0.05). Ultrasonic treatment increased the value of b*, and decreased the value of L*, a*, pH, chewiness, melting temperature and enthalpy. Springiness value significantly increased from control group to 300 W of ultrasonic power group. Shear force significantly decreased with the increase of ultrasonic power (P < 0.05). Ultrasonic treatment had no effect on the TVB-N content, but it could increase the TBARS content. Ultrasonic treatment could significantly increase the essential FAA (EFAA) and total FAA (P < 0.05). In addition, the saturated fatty acid (SFA) content significantly increased with the increase of ultrasonic power (P < 0.05). Ultrasound treatment negatively affected the meat’s color, smell, and taste but increased its tenderness and the overall acceptability. It also significantly increased alcohols and aldehydes contents (P < 0.05), which were consistent with the measurement of electronic nose and electronic tongue. The results demonstrated that the the appropriate ultrasonic power assisted in the processing improves quality of dry-cured yak meat, particularly for the power of 300 W.     

Effect of novel ultrasound based processing on the nutrition quality of different fruit and vegetable juices

Increasing consumer awareness regarding the health benefits of different nutrients in food have led to the requirement of assessing the effect of food processing approaches on the quality attributes. The present work focuses on understanding the effects of novel approaches based on the use of ultrasound and ultraviolet irradiations on the nutritional quality of different fruit and vegetable juices (orange, sweet lime, carrot and spinach juices) and its comparison with the conventional thermal pasteurization operated at 80 °C for 10 min. The ultrasound sterilization parameters were maintained at ultrasound frequency of 20 kHz and power of 100 W with treatment time as 15 min. For the case of ultraviolet irradiations, 2 UVC lamps (254 nm) of 8 W were placed in parallel on either sides of the reactor. The treated juices were analyzed for total phenol content, antioxidant activity, vitamin C, carbohydrates etc. It has been established that ultrasound processed juice retained most of the nutrient components to higher extent in comparison to all the other techniques used in the work. Combination of ultrasound and ultraviolet irradiations used to achieve an effective decontamination of juices (recommended 5 log reduction of microorganisms) also retained nutrients to a higher level in comparison to the thermal method; however some losses were observed as compared to the use of only ultrasound which could be attributed to inefficient heat exchange in the combined approach. A scale up attempt was also made for treatment of spinach juice using ultrasonic reactors and analysis for quality attributes confirmed that the juice satisfied the criteria of required nutrient contents for 18 days shelf life trial in refrigerated storage conditions. The present work has clearly established the usefulness of ultrasound based treatment in maintaining the nutritional quality of beverages while giving enhanced shelf life as compared to the conventional approaches.     

Effects of ultrasound treatment on the starch properties and oil absorption of potato chips

As a non-thermal processing method, the ultrasound treatment prior to the frying process has been demonstrated with great potential in reducing the oil absorption of fried food. This research aimed to evaluate the effect of ultrasound pretreatment on starch properties, water status, pore characteristics, and the oil absorption of potato slices. Ultrasound probe set with two power (360 W and 600 W) at the frequency of 20 kHz for 60 min was applied to perform the pretreatments. The results showed that ultrasound pretreatment led to the surface erosion of starch granules and higher power made the structure of starch disorganized. Moreover, the fraction of bound water and immobilized water were changed after ultrasonic pretreatment. Pores with the minor diameters (0.4–3 μm and 7–12 μm) were formed after ultrasound pretreatment. The penetrated surface oil (PSO) content, and structure oil (STO) content were reduced by 27.31% and 22.25% respectively with lower power ultrasound pretreatment. As the ultrasound power increased, the surface oil (SO) content and PSO content increased by 25.34% and 12.89% respectively, while STO content decreased by 38.05%. By using ultrasonic prior to frying, the quality of potato chips has been greatly improved.     

Medium-high frequency sonication dominates spherical-SiO2 nanoparticle size

Spherical SiO₂ nanoparticles (SSNs) have been inventively synthesized using the Stöber method with sonication at medium–high frequencies (80, 120, and 500 kHz), aiming to control SSN size and shorten reaction time. Compared to the conventional method, such sonication allowed the Stöber reaction complete in 20–60 min with a low molar ratio of NH₄OH/tetraethyl orthosilicate (0.84). The hydrodynamic diameters of 63–117 nm of SSNs were obtained under sonication with 80, 120, and 500 kHz of ultrasonic frequencies. Moreover, the SSNs obtained were smaller at 120 kHz than at 80 kHz in a multi-frequencies ultrasonic reactor, and the SSN size decreased with increasing ultrasonic power at 20 °C, designating the sonochemical unique character, namely, the SSN-size control is associated with the number of microbubbles originated by sonication. With another 500 kHz ultrasonic bath, the optimal system temperature for producing smaller SSNs was proven to be 20 °C. Also, the SSN size decreased with increasing ultrasonic power. The smallest SSNs (63 nm, hydrodynamic diameter by QELS, or 21 nm by FESEM) were obtained by sonication at 207 W for 20 min at 20 °C. Furthermore, the SSN size increased slightly with increasing sonication time and volume, favoring the scale-up of SSNs preparation. The mechanisms of controlling the SSN size were further discussed by the radical’s role and effects of ammonia and ethanol concentration.     

Effect of ultrasonic standing waves on flotation bubbles

Ultrasonic flotation was an effective method to float fine coal. In this study, the effects of the standing waves with different frequencies on ultrasonic flotation were investigated. The dynamic processes of bubble and coal-bubble were revealed by a high-speed camera. The results showed that under the action of Bjerknes force, bubble aggregates were formed within 450 ms and coal bubble aggregates were formed within 20 ms. The bubble aggregates were statistically analyzed by image processing method. The number of aggregates and small bubbles in the ultrasonic field at 100 kHz was greater than those at 80 and 120 kHz. Besides, 100 kHz ultrasonic flotation achieved the highest yields of clean coal (35.89%) and combustible recovery (45.77%). The cavitation bubbles acted as either a “medium” or an “inclusion”, entrapping and entraining the coal particles in the flotation pulp. It promoted the aggregation of bubbles with coal particles, so the flotation efficiency was effectively improved in the presence of ultrasonic standing waves.