Ultrasonic Standing Wave Research Articles

Acoustophoresis of monodisperse oil droplets in water: Effect of symmetry breaking and non-resonance operation on oil trapping behavior.

Acoustic manipulation of particles in microchannels has recently gained much attention. Ultrasonic standing wave (USW) separation of oil droplets or particles is an established technology for microscale applications. Acoustofluidic devices are normally operated at optimized conditions, namely, resonant frequency, to minimize power consumption. It has been recently shown that symmetry breaking is needed to obtain efficient conditions for acoustic particle trapping. In this work, we study the acoustophoretic behavior of monodisperse oil droplets (silicone oil and hexadecane) in water in the microfluidic chip operating at a non-resonant frequency and an off-center placement of the transducer. Finite element-based computer simulations are further performed to investigate the influence of these conditions on the acoustic pressure distribution and oil trapping behavior. Via investigating the Gor'kov potential, we obtained an overlap between the trapping patterns obtained in experiments and simulations. We demonstrate that an off-center placement of the transducer and driving the transducer at a non-resonant frequency can still lead to predictable behavior of particles in acoustofluidics. This is relevant to applications in which the theoretical resonant frequency cannot be achieved, e.g., manipulation of biological matter within living tissues.

Possibility of Controlled Variation of the Atomic Crystal Structure of Paratellurite by Means of the Excitation of an Ultrasonic Standing Wave

Possibility of Controlled Variation of the Atomic Crystal Structure of Paratellurite by Means of the Excitation of an Ultrasonic Standing Wave

Practical scale modification of oleogels by ultrasonic standing waves

Lipid-based materials, such as substitutes for saturated fats (oleogels) structurally modified with ultrasonic standing waves (USW), have been developed by our group. To enable their potential application in food products, pharmaceuticals, and cosmetics, practical and economical production methods are needed. Here, we report scale-up of our procedure of structurally modifying oleogels via the use of USW by a factor of 200 compared to our previous microfluidic chamber. To this end, we compared three different USW chamber prototypes through finite element simulations (FEM) and experimental work. Imaging of the internal structure of USW-treated oleogels was used as feedback for successful development of chambers, i.e., the formation of band-like structures was the guiding factor in chamber development. We then studied the bulk mechanical properties by a uniaxial compression test of the sonicated oleogels obtained with the most promising USW chamber, and sampled local mechanical properties using scanning acoustic microscopy. The results were interpreted using a hyperelastic foam model. The stability of the sonicated oleogels was compared to control samples using automated image analysis oil-release tests. This work enabled the effective mechanical-structural manipulation of oleogels in volumes of 10–100 mL, thus paving the way for USW treatments of large-scale lipid-based materials.

An Effective Microfluidic Chip with Serpentine Microchannels for Particle Separation Powered by Ultrasonic Standing Waves

An Effective Microfluidic Chip with Serpentine Microchannels for Particle Separation Powered by Ultrasonic Standing Waves

Effect of the ultrasonic standing wave frequency on the attractive mineralization for fine coal particle flotation

Froth flotation for mineral beneficiation is one of the most important separation techniques; however, it has several challenges for processing fine and ultrafine particles. Attractive mineralization between particles and bubbles by ultrasonic standing wave (USW) is a novel and high-efficiency method that could assist fine particle flotation. Frequency is an important ultrasound parameter, whose effectiveness mechanisms on the attractive mineralization did not compressively address. This study explored the effect of the USW field with various frequencies on the fine coal flotation for filling this gap. Herein, a high-speed camera and a focused beam reflectance measurement (FBRM) were used to analyze three sub-processes of the attractive mineralization, including the microbubbles’ formation, the conventional flotation bubbles (CFBs)’ dispersion, and the particles’ movement. It was found that the maximum flotation metallurgical responses were obtained under the highest examined USW frequency (600 kHz). However, the flotation outcomes by a low USW frequency (50 kHz) were even lower than the conventional flotation tests. Observation and theoretical calculation results revealed these results were originated from the influence of frequency on the carrier bubbles’ formation and the action of the secondary acoustic force during USW-assisted flotation. These outcomes demonstrated that frequency is a key factor determining the success of attractive mineralization for fine particles’ flotation.

Effect of the Ultrasonic Standing Wave Frequency on the Attractive Mineralization for Fine Particle Flotation

Froth flotation for mineral beneficiation is the most important separation technique; however, it has several challenges for processing fine and ultrafine particles. Attractive mineralization between particles and bubbles by ultrasonic standing wave (USW) is a novel and high-efficiency method that could assist fine particle flotation. Frequency is an important ultrasound parameter, which its effectiveness mechanisms on the attractive mineralization did not compressively address. This study explored the effect of the USW field with various frequencies on the fine coal flotation for filling this gap. Herein, a high-speed camera and a focused beam reflectance measurement (FBRM) were used to analyze three sub-processes of the attractive mineralization, including the microbubbles' formation, the conventional flotation bubbles (CFBs)' dispersion, and the particles' movement. It was found that the maximum flotation metallurgical responses were obtained under the highest examined USW frequency (600 kHz). However, the flotation outcomes by a low USW frequency (50 kHz) were even lower than the conventional flotation tests. Observation and theoretical calculation results revealed these results were originated from the influence of frequency on the carrier bubbles' formation and the action of the secondary acoustic force during USW-assisted flotation. These outcomes demonstrated that frequency is a key factor determining the success of attractive mineralization for fine particles' flotation.

Suppression of Air Disturbance by Ultrasonic Standing Wave for Improvement of Vision Sensing

In the previous paper, we proposed a method to stabilize the air over a heater by ultrasonic standing wave for better vision sensing. The phenomenon was not fully understood. This paper reports the observation results by particle image velocimetry.

Slit-jet 노즐을 통과한 초음파 무화 에어로졸 화염에 정상초음파가 미치는 영향

Slit-jet 노즐을 통과한 초음파 무화 에어로졸 화염에 정상초음파가 미치는 영향

Controlling oleogel crystallization using ultrasonic standing waves

Oleogels are lipid-based soft materials composed of large fractions of oil (> 85%) developed as saturated and hydrogenated fat substitutes to reduce cardiovascular diseases caused by obesity. Promising oleogels are unstable during storage, and to improve their stability careful control of the crystalline network is necessary. However, this is unattainable with state-of-the-art technologies. We employ ultrasonic standing wave (USSW) fields to modify oleogel structure. During crystallization, the growing crystals move towards the US-SW nodal planes. Homogeneous, dense bands of microcrystals form independently of oleogelator type, concentration, and cooling rate. The thickness of these bands is proportional to the USSW wavelength. These new structures act as physical barriers in reducing the migration kinetics of a liposoluble colorant compared to statically crystallized oleogels. These results may extend beyond oleogels to potentially be used wherever careful control of the crystallization process and final structure of a system is needed, such as in the cosmetics, pharmaceutical, chemical, and food industries.

Optimization of acoustic parameters for ultrasonic separation of emulsions with different physical properties.

Ultrasound is an emerging and promising method for demulsification, which is highly affected by acoustic parameters and emulsion properties. Herein, a series of microscopic and dehydration experiments are carried out to investigate the parameter optimization of ultrasonic separation. The results show that the optimal acoustic parameters highly depend on the emulsion properties. For low frequency ultrasonic standing waves (USWs), mechanical vibrations not only facilitate droplet collision and coalescence, but also disperse the surfactant absorbed on the interface to decrease the interfacial strength. Therefore, low frequency ultrasound is suitable for separating emulsions with high viscosity and high interfacial strength. Increasing the energy density to produce moderate cavitation can increase demulsification efficiency. However, excessive cavitation results in secondary emulsification. In high frequency USWs, the droplets migrate directionally and form bandings, thereby promoting droplet coalescence. Therefore, high frequency ultrasound is favorable for separating emulsions with low dispersed phase content and small droplet size. Increasing the energy density can accelerate the aggregation of droplets, however, excessive energy density causes acoustic streaming that disturbs the aggregated droplets, resulting in reduced demulsification efficiency. This work presents rules for acoustic parameter optimization, further advancing industrial applications of ultrasonic separation.

Effect of Bulk Viscosity and Emulsion Droplet Size on the Separation Efficiency of Model Mineral Oil-in-Water (O/W) Emulsions under Ultrasonic Standing Wave Fields: A Theoretical and Experimental Investigation

Ultrasound standing waves can be used to separate emulsions. So far, they have been applied to oil-in-water emulsions with low continuous phase viscosity. This technique has the potential to be used for novel applications such as separating lipids from algal biomass; however, this requires the methodology to be optimized to process viscous emulsions. We have addressed this issue by studying the effects of bulk phase viscosity (1–23 mPa·s), emulsion droplet size (4.5–20 μm), power (10–54 W/L), and frequency (1 and 2 MHz) of ultrasound on the separation efficiency of model mineral oil-in-water–glycerol-mixture emulsions. For the small droplet size (4.5 μm) emulsion in water, the maximum separation achieved increased from 36 to 79% when ultrasound power increased from 10 to 54 W/L. However, for the large droplet size (11 μm) emulsion, the maximum separation was greater than 95% and was independent of ultrasound power. The maximum separation efficiency for small droplet size (4.5–6 μm) emulsions decreased from 80 to 14% when the viscosity increased from 1 to 23 mPa·s. However, for the large droplet size (11–20 μm) emulsion, the maximum separation efficiency decreased from 98 to 62% when the viscosity of the bulk phase was increased from 1 to 23 mPa·s. The experimental results were then interpreted using analytical and numerical simulations by calculating the time required for the emulsion droplets to migrate to the nearest pressure antinodal plane under the influence of ultrasound standing waves. Further experiments showed that increasing the ultrasound frequency from 1 to 2 MHz increased the maximum separation from 36 to 86% for fine emulsions and water as the continuous phase.

Optimization of Ultrasonic Acoustic Standing Wave Systems

Ultrasonic acoustic standing wave systems find use in many industrial applications, such as sonochemical reactions, atomization of liquids, ultrasonic cleaning, and spray dry. In most applications, highest possible sound pressure levels are needed to achieve optimum results. Until now, the atomization of liquids is limited to fluids with low viscosity, as systems generating sufficient sound pressure for atomizing fluids with higher viscosities are often not marketable due to their low throughput or high costs. For the production of polymer or metal powders or the dispensing of adhesives, highest sound pressures should be achieved with systems in suitable size, with good efficiency and at low cost but without contamination of sonotrodes and reflectors by the dispersed media. An alternative to the use of more powerful transducers is increasing the intensity of the acoustic standing wave field by optimizing the boundary conditions of the acoustic field. In most existing standing wave systems a part of the radiating sound waves does not contribute to the process, as the waves spread into the wrong direction or wipe themselves out due to interference. In order to obtain maximum sound pressure amplitudes in the standing wave field, all waves should be trapped between the sonotrode and the reflector. In addition, the resonance condition should be met for all radiated waves. These conditions can be fulfilled by optimizing the shapes of sonotrode and resonator as well as the distance between them. This contribution reports on a model, which is able to simulate the sound field between a transducer surface and a reflector. Using a linear finite-element model, the boundary conditions of the standing wave system are optimized. Sound pressure levels of the standing wave field are calculated for different shapes of reflectors and boundary conditions like the distance between the transducer and the reflector. The simulation results are validated by sound-field measurements via refracto-vibrometry and a microphone. Finally, optimization guidelines for the generation of high-intensity acoustic standing wave fields are shown and verified by measurements.

Toward efficient interactions of bubbles and coal particles induced by stable cavitation bubbles under 600kHz ultrasonic standing waves.

The interactions of bubbles and coal particles in 600kHz ultrasonic standing waves (USW) field has been investigated. A high-speed camera was employed to record the phenomena occurred under the USW treatment. The formation and behaviors of cavitation bubbles were analyzed. Under the driving of these cavitation bubbles, whose size is from several microns to dozens of microns, coal particles were aggregated and then attracted by large bubbles due to the acoustic radiation forces. The results of USW-assisted flotation show a significant improvement in recoveries at 600kHz, which indicates that the interactions of bubbles and particles in the USW field are more efficient than that in the conventional gravitational field. Furthermore, the sound pressure distribution of the USW was measured and predicted by a hydrophone. The analysis of gravity and buoyancy, primary and secondary Bjerknes forces shows that bubble-laden particles can be attracted by the rising bubbles under large acoustic forces. This study highlights the potential for USW technology to achieve efficient bubble-particle interactions in flotation.

超声波驻波创培土壤团粒结构的构想

超声波驻波创培土壤团粒结构的构想

Selective aggregation by ultrasonic standing waves through gas nuclei on the particle surface.

Gas nuclei in water are usually too small to be directly observed. They will grow into bubbles under the negative pressure, which is called cavitation (heterogeneous cavitation). In this study, the gas nuclei in the hydrophilic and hydrophobic silica particle suspension were investigated using the transient cavitation threshold measured by a high-intensity focused ultrasound (HIFU). The transient cavitation bubbles were also observed by a high-speed camera. The results showed that the nuclei only exist on the surface of hydrophobic particles. Furthermore, the aggregation experiments revealed that the aggregates were only formed in the hydrophobic silica suspension by ultrasonic standing waves (USW) at 200kHz. This distinct difference was mainly due to the formation of gas nuclei on hydrophobic silica particles, which grew and coalesced into stable bubbles under the 200kHz USW. The aggregation process in suspension was observed by a CCD camera. Moreover, the cavitation thresholds and acoustic radiation forces were analyzed to explain the mechanism of the acoustic aggregation. This study showed a very promising acoustic method for the selective aggregation of hydrophobic particles, which might be efficiently used in the mineral separation industry.

Particle Patterning by Ultrasonic Standing Waves in a Rectangular Cavity

By means of the acoustic radiation force, standing waves of ultrasound can form a two-dimensional microarray to hold dispersed particles or cells in solution. This phenomenon can be used in lab-on-a-chip technology for single-cell analysis and analytical chemistry, though the physics behind ultrasound patterning is not fully understood. This study provides an analytical expression for the acoustic potential energy that traps cells, and reveals that the trapping points at pressure nodes, antinodes, and internode midpoints depend on a cell's size and mechanical properties. This theoretical approach should help to engineer acoustic-patterning devices, especially for ``one cell per well'' analysis.

Particle aggregation and flow patterns induced by ultrasonic standing wave and acoustic streaming: An experimental study by PIV and PTV

In this study, two flow phenomena induced by the acoustic force, ultrasonic standing wave (USW) and acoustic streaming, were examined by means of flow visualization (FV), particle image velocimetry (PIV) and micro particle tracking velocimetry (μPTV). The control of the aggregation of particles in a suspension solution by ultrasonic standing wave (USW) and acoustic streaming was studied for a PMMA (Poly(methyl methacrylate)) cuvette of length 12.6 mm, width 12.6 mm and height 46 mm. The fluid inside was filled with water and μm-size particles as the suspension solution, and the flow is excited with a piezoelectric plate operating at MHz frequency oscillation of voltage signal. This oscillation generates USW inside the liquid space, while at the same time induce acoustic streaming flows in the confined space. Three particle sizes (5–18 μm) were tested with frequencies vary from 0.847 to 2.032 MHz in the present study. Experimental results show that the ultrasonic standing wave is successfully generated, and the error between the standing wavelength in the theory and in the experiment does not exceed 6%. USW is accompanied by the acoustic streaming flow, and within the performed frequency range, the degree of aggregation for particles in different diameters is 18 µm > 10 µm > 5 µm. With all driving frequencies it is found that the acoustic streaming flow starts as the oscillation is turned on and affecting the flow patterns immediately. At driving frequencies of 0.847, 1.863, 2.032 MHz for 10 µm and 18 µm particles, a stable streaming flow coexists with the USW pattern that cause the flow to move with minimum reduction of the particle aggregation can be observed.The micro-scale flow patterns of acoustic streaming flows induced by a single triangular obstruction were investigated by flow visualization (FV) and μPTV. Different base to altitude ratios from 0.25 to 4 of the isosceles triangular obstructions were tested, corresponding to the top angles varied from 127° to 18°, the heights from 0.3 mm to 0.6 mm, and the base widths from 1.2 mm to 0.1 mm. By using the piezoelectric plates, the steady acoustic streaming flow patterns were successfully created and observed at a range of driving voltages from 5 to 35 V and frequencies from 0.5 to 3 kHz. The vector field calculated using μPTV results show that the streaming flow is more concentrated at the vertex of the triangular obstructions, and flow acceleration at the vertex is more significant in the case of small base to altitude ratio. These experimental results suggest the probabilities to utilize both acoustic streaming and USW for applications such as particle sorting and manipulation.

A study of coal aggregation by standing-wave ultrasound

Recovery of fine coal particles by flotation has always been difficult due to the low collision efficiency between air bubbles and fine coal particles. One method to improve the flotation recovery of fine coal particles is the aggregation of these particles prior to the flotation process. This paper introduces a new method for the aggregation of fine coal particles by utilizing an ultrasonic standing-wave treatment. Under a 200 kHz ultrasonic standing-wave, cavitation bubbles formed on the hydrophobic coal particle surfaces. The bubble-laden coal particles moved to the nodes of the ultrasonic standing wave by the acoustic radiation force if the size of the cavitation bubbles were larger than the resonance radius, resulting in rapid aggregation of the fine coal particles. Micro-flotation results indicated that the flotation rate significantly increased after the ultrasonic standing-wave treatment. This study demonstrated an efficient ultrasonic standing-wave method for improving the flotation rate of fine coal particles without addition of chemicals or collectors.

Ultrasonic Based Tissue Modelling and Engineering.

Systems and devices for in vitro tissue modelling and engineering are valuable tools, which combine the strength between the controlled laboratory environment and the complex tissue organization and environment in vivo. Device-based tissue engineering is also a possible avenue for future explant culture in regenerative medicine. The most fundamental requirements on platforms intended for tissue modelling and engineering are their ability to shape and maintain cell aggregates over long-term culture. An emerging technology for tissue shaping and culture is ultrasonic standing wave (USW) particle manipulation, which offers label-free and gentle positioning and aggregation of cells. The pressure nodes defined by the USW, where cells are trapped in most cases, are stable over time and can be both static and dynamic depending on actuation schemes. In this review article, we highlight the potential of USW cell manipulation as a tool for tissue modelling and engineering.

Effective cell trapping using PDMS microspheres in an acoustofluidic chip

We present a facile particle-based cell manipulation method using acoustic radiation forces. In this work, we selected several representative particles including poly(lactic-co-glycolic acid) (PLGA) microspheres, silica-coated magnetic microbeads, polydimethylsiloxane (PDMS) microspheres and investigated the responses of these particle systems to ultrasonic standing waves (USWs) in a microfluidic chip. We show that depending on the nature (positive or negative acoustic contrast factors) of the particles, these particle systems display different alignment behaviors along the microfluidic channel under USWs. Specifically, PLGA microspheres and silica-coated magnetic microbeads are able to be aligned in the middle of the microfluidic channel, while PDMS microspheres are translocated to the side walls of the channel, which is beneficial for cell trapping and manipulation. Further results demonstrate that the functional PDMS microspheres with a negative acoustic contrast factor can be used to trap cells to the pressure antinodes in the acoustofluidic chip. Cell viability tests reveal that the ultrasonic manipulation does not exert any harmful effect to the cells. This acoustic-based particle and cell manipulation technique may hold a great promise for the development of rapid, noninvasive, continuous assays for detecting of cells and separation of biological samples.