Acoustic Levitation Research Articles

Oscillation characteristics of levitated sample in resonant acoustic field

This study aims to improve and investigate the levitation stability and mechanism of restoring force on an acoustically levitated droplet. To better understand the stable manipulation conditions of a droplet in an acoustic levitator, we experimentally investigated the unsteady translational motion of the acoustically levitated droplet. Furthermore, we visualized its translational motion in the horizontal and vertical (x and y) directions using a high-speed video-camera and estimated the dynamics of a droplet using four distinct levitated samples. The ratio of the levitated samples’ translational frequency, which is obtained by dividing the frequency in the y direction, fy, with that in the x direction, fx, was 3.4, which is equal to the restoring force coefficients. We found that our experimental data agree with the calculation results of a previous study. Our insights could pave the way to manipulate the dynamic behavior of a levitated sample for achieving more stable levitation.

Maximization of Acoustic Levitating Force for a Single-Axis Acoustic Levitation System Using the Finite Element Method**Supported by the Science and Engineering Research Board under Grant No YSS/2015/001245.

We investigate single-axis acoustic levitation using standing waves to levitate particles freely in a medium bounded by a driver and a reflector. The acoustic pressure at the pressure antinode of the standing wave counteracts the downward gravitational force of the levitating object. The optimal relationship between the air gap and the driving frequency leads to resonance and hence maximization of the levitating force. Slight deviation from the exact resonance condition causes a reduction in acoustic pressure at the pressure antinodes. This results in a significant reduction of the levitating force. The driving frequency is kept constant while the air gap is varied for different conditions. The optimal air gap for maximizing the levitation force is studied for first three resonance modes. Furthermore, a levitating particle is introduced between the driver and the reflector. The dependence of the resonance condition on the size of the levitating particle as well as the position of the particle between the driver and the reflector has also been studied. As the size of the levitating particle increases, the resonance condition also gets modified. Finite element results show a good agreement with the validated results available in the literature. Furthermore, the finite element approach is also used to study the variation of acoustic pressure at the pressure antinode with respect to the size of the reflector. The optimum diameter of the reflector is calculated for maximizing the levitating force for three resonance modes.

Experimental investigation of the particle oscillation instability in a single-axis acoustic levitator

Single-axis acoustic levitators are employed in biomedicine, chemistry and physics experiments due to their ability to trap in mid-air objects of a wide range of materials and sizes. Although this type of levitator has been studied for decades, there are effects that are not well understood. One of these effects is the particle oscillation instability, in which the levitating particle starts to oscillate with increasing amplitude until it is ejected out of the levitator. Most of the operations performed with acoustic levitation require high accuracy regarding the positioning of the particle, thus a lack of stability severely hinders the experiments. In this paper, we present an experimental setup that consists of a single-axis levitator, a mechanized stage to control the separation between the emitter and the reflector, a scale to measure the radiation force and a high-speed camera. We experimentally investigate the effect of the distance between the emitter and the reflector on the apparatus resonant frequency and on levitation stability. In accordance with previous theoretical studies, three types of levitation behavior were experimentally identified: stable levitation, oscillation of constant amplitude and unstable oscillation. We also show that the type of levitation behavior can be controlled by changing the distance between the emitter and the reflector.

Overview of acoustical levitation for volumetric visual displays

Acoustical levitation is an area with many applications ranging from medical drug delivery to micro-particle sorting. An application which has gained attention recently is the creation of volumetric displays by using small levitating objects. Advances in inexpensive ultrasonic phased arrays have increased the availability of dynamically controllable beamformers which enables the manipulation of the levitating objects in time and space. This allows for interpreting the levitating objects similarly to pixels in a normal display yet with an additional spatial dimension. Most implementations so far are based on the so-called Gor'kov formulation of radiation force. This formulation coupled with numerical optimization allows for calculation of the phases of the individual elements in the array. By exploiting symmetries in the solution, it is possible to impose an acoustic trap signature phase pattern onto simple focusing methods. Using off-the-shelf mid-air ultrasonic haptics systems to provide multiple focus points to which the phase patterns are applied allows for real-time control of multiple levitating objects. Current systems are limited to a handful of individually controllable objects so visualization is limited to abstract information. We present an overview of the state-of-the-art and discuss limitations and possibilities. [Work supported by Horizon 2020, No. 737087.]

Dynamic Superposition and Levitation Capability of Two Confronting Ultrasonic Waves**Supported by the National Natural Science Foundation of China under Grant Nos 51871186 and 51771156.

The superposition dynamics of two confronting ultrasonic waves and their levitation capability for centimeter-sized thin disks are investigated by numerical analyses and validated by experiments. The sound pressure simulation reveals that two opposite ultrasonic waves provide a more effective standing-wave field than a single ultrasonic wave when the diameter of disk-shaped object approaches the wavelength scale. The dynamic superposition of two confronting beams facilitates the acoustic levitation of the clay disk and aluminum disk with diameters of 0.97λ and 0.90λ. The acoustic radiation forces exerting on these thin disks are measured experimentally, which exhibit a better levitation stability for the centimeter-sized thin disks. The equilibrium levitation positions of the two disks are located near the sound pressure node, and the maximum acoustic radiation pressure on their surfaces is less than one percent of the maximum sound pressure.

Evaporation of droplet in mid-air: Pure and binary droplets in single-axis acoustic levitator.

Acoustic levitation method (ALM) is a container-less processing method with applications in various fields, including material processing, biology, and analytical chemistry. Because it is a container-less processing technique, ALM could prevent nucleation and contamination of materials being processed via contact with a container wall. It is well-known that evaporation of a sample is an important process in container-less processing of materials; however, the mechanism of evaporation in multicomponent droplets in a single acoustic levitator is still unclear. Thus, we evaluate and understand the evaporation of an acoustically levitated multicomponent droplet and manipulate the evaporation process of the sample in this study. Specifically, we investigate the evaporation process of pure and multicomponent droplets using container-less processing experimentally. The evaporation processes and temporal evolution of the surface temperature of a multicomponent droplet were evaluated using a high-speed camera and radiation thermometer, respectively. We used water, ethanol, methanol, hexane, acetone, pentane, and binary solutions (solution of 25 wt%, 50 wt%, and 75 wt% ethanol, methanol, and acetone, respectively) as test samples to study the effect of saturated vapor pressure on evaporation. Ethanol, methanol, and acetone droplets evaporate in two different stages. It was observed that the water vapor in the air condensed during the evaporation process of these water-soluble droplets; hence, our experimental data did not agree with the theoretical prediction in accordance with the d2 law. Nevertheless, the evaporation behavior in the first stage of evaporation was consistent with the theoretical prediction. Furthermore, for binary droplets, as the concentration of the resultant solution increased owing to evaporation, the transition time from the first to the second stage of evaporation also increased. Based on these observations, estimation equations for binary droplets were developed to ensure that the experimental and theoretical values were in good agreement.

Study on effects of ultrasonic vibration on sliding friction properties of PTFE composites/phosphor bronze under vacuum

This paper is a tribological problem extracted from the application of the ultrasonic motor under the vacuum. Nine frictional states were designed. Under the atmosphere and vacuum environment, the effects of longitudinal and torsional ultrasonic vibration modes on the sliding friction pair of phosphorus bronze and Polytetrafluoroethylene (PTFE) composites were studied. The results show that under vacuum, applying vertical or horizontal ultrasonic vibration can reduce the friction coefficient. However, comparing with sliding friction under ultrasonic vibration in atmosphere, the friction coefficient increases. With the increase of vacuum degree, the effect of friction reduction by ultrasonic vibration becomes weaker, which leads to the increases of the friction coefficient under high vacuum. The friction coefficient is almost no change under low excitation voltage. It is revealed that the increase of friction coefficient caused by ultrasonic vibration in vacuum is due to the decrease of air medium density, the reduction of acoustic levitation force and the increase of contact opportunity of friction pair.

Optimization of coating processes of mini tablets with polyvinylpyrrolidone solutions in an acoustic levitator

The aim of the current study is to optimize the bed fluidized coating process of mini tablets by means of an acoustic levitator. The optimized coating parameters, which have to be set in a fluidized bed reactor, can be determined by means of contactless levitation of a single mini tablet in an acoustic levitator equipped with an ultrasonic atomizer. Mini tablets were positioned in a contactless way in the levitator and coated with polyvinylpyrrolidone (PVP) solution. The influences of process parameters on the coating, such as temperature, mass fraction, volumetric flow rate and spray time, are analyzed in an acoustic levitator in a cost- and time-saving manner. The results from the acoustic levitator were comparable to those performed in a fluidized bed reactor. A design of experiments was performed to analyze the effects of those parameters on the mass of coating thickness. The surfaces of the samples coated in the acoustic levitator at 40 °C were comparable to samples coated in a fluidized bed reactor at 40 °C by using scanning electron microscopy (SEM).

PEG Acoustic Levitation Treatment for Historic Wood Preservation Investigated by Means of FTIR Spectroscopy and Wavelets

Background: The combination of Fourier Transform InfraRed and levitation techniques, both on levitated water mixtures of Polyethylene Glycols (PEG)s and on wood finds, allows to follow the polymeric drying process as a function of time through the O-H and C-H stretching contributions and to investigate the wood thermal response. Objective: The aim of this paper is to report the FTIR investigations on wood fines, extracted from a 700’ dated wooden crucifix. Methods: At first, different acoustically levitated Polyethylene Glycol (PEG) aqueous solutions were investigated by FTIR in order to characterize the levitation induced dehydration process; then the wood fines, after having been treated by immerging them into the PEG aqueous solutions, were acoustically levitated and investigated; finally the treated fines were again studied by IR spectroscopy through a thermic cycle in the 25 ÷ 350 °C temperature range. Levitation technique furnishes an innovative approach to sample treatment allowing, in the case of solutions, to obtain highly concentrated mixtures starting from diluted solutions while, in the case of wood find treatments, to increase the polymer solution penetration within the wood matrix. Results: It clearly emerges that, in the PEG200/PEG600 comparison, the lighter polymer shows a higher effectiveness in terms of thermal restraint. Conclusion: Acoustic levitation, in combination with IR spectroscopy, reveals to be a very powerful technique in many applications concerning wood preservation and conservation. It clearly emerges that, in the comparison with PEG600, PEG200 shows a higher effectiveness in terms of thermal restraint.

Programmable Acoustic Metasurfaces.

Metasurfaces open up unprecedented potential for wave engineering using subwavelength sheets. However, a severe limitation of current acoustic metasurfaces is their poor reconfigurability to achieve distinct functions on demand. Here a programmable acoustic metasurface that contains an array of tunable subwavelength unit cells to break the limitation and realize versatile two-dimensional wave manipulation functions is reported. Each unit cell of the metasurface is composed of a straight channel and five shunted Helmholtz resonators, whose effective mass can be tuned by a robust fluidic system. The phase and amplitude of acoustic waves transmitting through each unit cell can be modulated dynamically and continuously. Based on such mechanism, the metasurface is able to achieve versatile wave manipulation functions, by engineering the phase and amplitude of transmission waves in the subwavelength scale. Through acoustic field scanning experiments, multiple wave manipulation functions, including steering acoustic waves, engineering acoustic beams, and switching on/off acoustic energy flow by using one design of metasurface are visually demonstrated. This work extends the metasurface research and holds great potential for a wide range of applications including acoustic imaging, communication, levitation, and tweezers.

Acoustic levitation and manipulation by a high-frequency focused ring ultrasonic transducer

Recently, acoustic levitation for non-contact micro-particle manipulation has been attracting great interest in physical, biological, and medical applications. Among the state-of-the-art manipulation technologies, single beam acoustic tweezing exhibits advantages of providing stronger trapping force and deeper penetration depth in tissues, inducing less tissue damage, and a simple configuration involving only one device. However, particle trapping by the single beam acoustic tweezer could only be operated on a smooth two-dimensional substrate, which limits the potential for real applications. Here, we report an initial attempt to acoustically levitate an individual micro-particle stably in water and manipulate the levitated micro-particle arbitrarily two-dimensionally by simply employing a 60-MHz focused ring ultrasonic transducer. The proposed working mechanism agrees well with the phenomenon. This approach could not only acoustically levitate and manipulate a micro-particle on a culture dish and on a mylar film, but could also work properly in levitating and manipulating a micro-particle placed inside the polyimide tube. This simple and low-cost approach is extremely useful for effective non-contact micro-particle manipulation without having critical concerns on the substrate properties.

Point-Spread-Function (PSF) Characterization of a 340-GHz Imaging Radar Using Acoustic Levitation

In this paper, resolution characterization and beam analysis of a 340-GHz imaging radar are demonstrated by means of a point-spread-function (PSF) study by imaging an acoustically levitated point-like target. It is shown that at terahertz frequencies, conventional PSF measurement techniques are limited by the presence of strong scattering response of background objects, such as suspension threads, within the imaging field-of-view (FOV). Using acoustic levitation, it is possible to eliminate secondary objects within the FOV and achieve a pure PSF characterization of the radar. It is shown that the PSF patterns obtained using acoustic levitation exhibit high fidelity and are free from artifacts. We demonstrate this using a small water droplet suspended in air at the focus of the 340-GHz radar. The measured PSF characteristics of the radar are in excellent agreement with physical optics (PO) simulations and analytical results.

Contactless, programmable acoustofluidic manipulation of objects on water.

Contact-free manipulation of small objects (e.g., cells, tissues, and droplets) using acoustic waves eliminates physical contact with structures and undesired surface adsorption. Pioneering acoustic-based, contact-free manipulation techniques (e.g., acoustic levitation) enable programmable manipulation but are limited by evaporation, bulky transducers, and inefficient acoustic coupling in air. Herein, we report an acoustofluidic mechanism for the contactless manipulation of small objects on water. A hollow-square-shaped interdigital transducer (IDT) is fabricated on lithium niobate (LiNbO3), immersed in water and used as a sound source to generate acoustic waves and as a micropump to pump fluid in the ±x and ±y orthogonal directions. As a result, objects which float adjacent to the excited IDT can be pushed unidirectionally (horizontally) in ±x and ±y following the directed acoustic wave propagation. A fluidic processor was developed by patterning IDT units in a 6-by-6 array. We demonstrate contactless, programmable manipulation on water of oil droplets and zebrafish larvae. This acoustofluidic-based manipulation opens avenues for the contactless, programmable processing of materials and small biosamples.

Determination of Radiation Pressure in Acoustic Levitation by Optical Acoustic-Field Measurement

Pulsed digital holography was used to determine the acoustic radiation force on objects suspended in a single-axis acoustic levitator. Digital holography provided both the object images and the optical phase data required to determine the instantaneous acoustic pressure. The phase data were utilized in tomography calculations, converting the measured line-of-sight variations to a pressure field by assuming that the system was axisymmetric. The pressure field was used to calculate the velocity field and these together were used to calculate the acoustic radiation pressure. This pressure was integrated over the surface of the object to determine the vertical force on the object. Force measurements were completed for five different polymer spheres, of different sizes and densities, and for an evaporating water droplet. For all but the heaviest sphere, the optically measured force agreed with the measured weight of the sphere within experimental uncertainty. For the heaviest sphere, the overall average showed an upward bias but was still only 4.7% high. For the water data, the optically measured force was consistent with the droplet volume as the droplet evaporated.

Hydrate nucleation and growth on water droplets acoustically-levitated in high-pressure natural gas.

Hydrate formation was studied using water droplets acoustically levitated in high-pressure natural gas. Despite the absence of solid interfaces, the droplets' area-normalised nucleation rate was about four times faster than in steel autoclave measurements with interfacial areas roughly 200 times larger. Multiple stages of stochastic, template-free hydrate growth were observed.

3P4-7 Measurement of holding force acting on tabular object in near-field acoustic levitation

3P4-7 Measurement of holding force acting on tabular object in near-field acoustic levitation

Acoustic Standing Wave Field Measurement Using a Laser Doppler Vibrometer Based on the Hankel Fourier Algorithm

Measuring the acoustic pressure in an acoustic standing wave field is essential as a means to study acoustic levitation and other related techniques. In this work it was shown that the pressure of an acoustic standing wave field could be measured on line, in a non-contact way, using a scanning Laser Doppler Vibrometer (LDV), where the LDV output is proportional to the integral of the acoustic pressure over the laser path. In the method used to measure acoustic pressure, first, the LDV outputs, $\upsilon _{sLDV}$ and $\upsilon _{eLDV}$ , are obtained by using both COMSOL-MATLAB-based (CM) co-simulation and through experimental measurements. Next, the acoustic pressure distribution is obtained using a MATLAB-based programme and this is reconstructed by using $\upsilon _{sLDV}$ and $\upsilon _{eLDV}$ , based on the Hankel-Fourier (HF) algorithm. Further, the acoustic pressure measurements obtained by using these two methods are cross compared, in that way to demonstrate the effectiveness of the method presented.

Prediction of Load-Carrying Capacity in the Radial Direction for Piezoelectric-Driven Ultrasonic Bearings

This paper presents a theoretical modeling method on radial levitation force for ultrasonic bearings actuated by piezoelectric transducers with the aim of predicting their load-carrying capacity. The finite difference method is adapted to calculate the model, which is represented by a high-order multidimensional partial differential equation. The static levitation force testing experiments validate that the established mechanical model can interpret the experimental data well. This model also reveals that the ultrasonic bearing’s levitation mechanism under the comprehensive action of near-field acoustic levitation effect and hydrodynamic effect when the bearing operates at high speeds. Some key factors including bearing’s dynamic parameters, working media, clearance sizes, and environmental conditions that influence the ultrasonic bearing’s levitation effect are discussed. The analysis of these factors can guide the ultrasonic bearing’s structural design and the selection of the ultrasonic bearing’s working parameters.

A Review of Micromotors in Confinements: Pores, Channels, Grooves, Steps, Interfaces, Chains, and Swimming in the Bulk.

One of the recent frontiers of nanotechnology research involves machines that operate at nano- and microscales, also known as nano/micromotors. Their potential applications in biomedicine, environmental sciences and engineering, military and defense industries, self-assembly, and many other areas have fueled an intense interest in this topic over the last 15 years. Despite deepened understanding of their propulsion mechanisms, we are still in the early days of exploring the dynamics of micromotors in complex and more realistic environments. Confinements, as a typical example of complex environments, are extremely relevant to the applications of micromotors, which are expected to travel in mucus gels, blood vessels, reproductive and digestive tracts, microfluidic chips, and capillary tubes. In this review, we summarize and critically examine recent studies (mostly experimental ones) of micromotor dynamics in confinements in 3D (spheres and porous network, channels, grooves, steps, and obstacles), 2D (liquid-liquid, liquid-solid, and liquid-air interfaces), and 1D (chains). In addition, studies of micromotors moving in the bulk solution and the usefulness of acoustic levitation is discussed. At the end of this article, we summarize how confinements can affect micromotors and offer our insights on future research directions. This review article is relevant to readers who are interested in the interactions of materials with interfaces and structures at the microscale and helpful for the design of smart and multifunctional materials for various applications.

Fully automating liquid processing within an acoustic levitation system

The automated insertion and ejection of droplets has been integrated into a midair, contactless liquid manipulation device.