Acoustic Levitation System Research Articles

Ultrasonic Levitation as a Handling Tool for In-Space Manufacturing Processes

Abstract 3D printing is one of the key technologies in space exploration. The disparity in gravitational forces between Earth and space presents both challenges and opportunities with regard to material handling. This article examines the potential of employing ultrasonic levitation as a handling tool for substrate-free additive manufacturing processes in microgravity environments. Through preliminary experiments, we demonstrate the feasibility of manipulating polymer powders using acoustic fields while concurrently melting the levitated material. Subsequent experiments conducted in our drop tower facility confirm our ability to manipulate particles with acoustic traps under microgravity conditions. Building upon these findings, we outline plans to further advance our research using an expanded acoustic levitation system capable of three-dimensional object manipulation. Our objectives include moving and orienting large components beyond the wavelength limit in microgravity, manipulating granular raw material while melting it in proximity to the print part, and achieving a semi-continuous fusion of print material with the print part. Therefore, we present an intelligent control strategy based on the results of a digital twin simulation. Furthermore, we utilize a stereo camera combined with computer vision as feedback for the control system to ensure precise handling of the manipulated objects and particles. This study represents a significant advance toward the realization of efficient substrate-free additive manufacturing processes in microgravity environments, with potential applications for in-space manufacturing. Ultimately, this could result in long-term space missions becoming less reliant on supply deliveries, thus reducing cost and additionally enabling faster response to unforeseen issues.

On the instability of single-axis acoustic levitation under radial perturbations

Acoustic levitation is widely used in non-container measurement and non-contact manipulation. Particles in the single-axis acoustic levitation are easily unstable in the radial direction under external perturbations. In order to explore the instability in the acoustic levitation during radial perturbations, a nonlinear acoustic levitation model considering the coupling of radial and axial vibration is proposed to analyze the dominant factors influencing the levitation stability, an acoustic levitation system consisting of a transducer and a plane reflector is established, and high-speed photography is used to observe the vibration behavior of the particle with large radial vibration and the levitation stability. The simulation results are compared and verified with the experiments, which indicate that the reduction in axial trapping stiffness due to radial vibration plays a vital role in the levitation instability. The present model can characterize the radial anti-interference ability of different levitators as well as predict the movement trajectories of levitated particles after being disturbed, which is helpful to optimize the design of acoustic levitators and provide guidance for acoustic manipulation.

Contactless weighing method based on deep learning and acoustic levitation

Abstract Acoustic weighing is a promising contactless method for screening the mass of micro-nano objects as it avoids contact contamination and losses. Existing acoustic weighing methods determine the mass of an object by detecting its oscillation trajectory with a laser sensor. However, this method suffers from several limitations, such as short measurement distance, poor accuracy in measuring transparent objects, and inducing damage to photosensitive samples. To solve these issues, this work proposes a contactless weighing method based on location-aware neural network (LANet) and acoustic levitation. The proposed LANet is a deep learning-based image processing method that detects object bit oscillation trajectories completely contactless, regardless of the color, shape, and oscillation distance of the levitated object. We employ a cross-stage aggregation module and cross-mixed feature pyramid strategy to build LANet network depth for enhanced feature extraction. In addition, to create a contactless environment, we built an acoustic levitation system, which drives the oscillation of objects. Finally, we verified the accuracy and effectiveness of the method. The results show that the proposed network can accurately detect the oscillation trajectories of various objects with high detection performance, even for small objects in low-contrast backgrounds. Meanwhile, the proposed method can accurately measure the mass of objects with a percentage error of no more than 7.83%.

Direct photodegradation of internally mixed sodium chloride and malonic acid single aerosols: Impact of the photoproducts on the hygroscopic properties of the particles

Sea-salt aerosols (SSA) are one of the key natural aerosols in our atmosphere, consisting predominantly of sodium chloride (NaCl). Throughout their atmospheric transport, these aerosols undergo complex internal mixing, giving rise to a rich variety of inorganic and organic species, including dicarboxylic acids. This study investigates firstly the composition and deliquescence properties of coarse particles containing pure malonic acid (MA2, CH2(COOH)2) and internally mixed NaCl and MA2, by means of an acoustic levitation system coupled with a Raman microspectrometer. Secondly, we report here the first experimental observation and characterization of the products arising from photochemical reactions under UV–Visible irradiation (338 ≤ λ ≤ 414 nm) in the absence of an oxidant under acoustic levitation conditions in MA2 and NaCl/MA2 aerosols. Furthermore, the impact of photodegradation on the hygroscopic properties of these particles is examined. We confirmed the irreversible formation of monosodium malonate (NaMA, HOOCCH2COONa), which coexists with NaCl or MA2 on non-irradiated particles. We also demonstrated the formation of oxalic acid (OA2, HOOC–COOH) within irradiated MA2 droplets and the appearance of glyoxylic acid (GlyA, HCOCOOH) in NaCl containing droplets. The photolysis process exerts a marked effect on the hygroscopic properties of the particles, resulting in a shift in deliquescence transitions toward higher relative humidity (RH) values. This study contributes to the understanding of the intricate physicochemical processes involved in SSA during their atmospheric transport. Likewise, this work sheds light on the impacts of these types of aerosols on cloud formation and climate change.

Behaviour of Acoustically Levitated Drops in Mid-Water.

A low-impact acoustic levitation system has been developed to study immobilised single drops in liquid-liquid systems. The ability to observe liquid drops several millimetres in diameter for days enables fundamental research into a wide range of mechanisms. Non-invasive optical measurements with excellent optical accessibility are possible. This experimental work provides the basis for mass transfer studies, emphasizing the precise volume determination, signal noise, reproducibility, and the impact of the acoustic field on the drop and its surrounding environment. The setup can be effectively controlled and proves beneficial for research objectives provided that all liquid phases are entirely degassed, and there are no compressible voids present within the liquids. In addition to the precise, uniform, and reliable measurement conditions, we observed no acoustic streaming in the proximity of the drop and there was no significant vibration of the drop. Qualitative observations using rainbow schlieren deflectometry indicate that the nodal or anti-nodal planes of the standing waves can act as barriers to the dispersion of inhomogeneous dissolved substances in the continuous phase.

Finite Difference Study to Evaluate the Resonance Condition of the Standing Wave Acoustic Levitation System

Finite Difference Study to Evaluate the Resonance Condition of the Standing Wave Acoustic Levitation System

Tractor beam acoustic levitation for time resolved crystallography experiments

Proof of concept of serial crystallography is carried out through the translation of samples delivered by tractor beam levitation. This is achieved using arrays of low powered transducers, focused to produce acoustic traps. Contrary to traditional Langevin Horn levitators, power requirement remains in the region of 10W, limiting the acoustic pressure on the levitated samples and hence the risk of damage to them. Automation is achieved by controlling the phase of the transducers. The traps and associated samples steadily translate with the controlled acoustic field. A translation speed of 2.8mms −1 between the nodal distance of the traps is achieved. This results in sequential delivery of sample containing droplets performed with 1.5s between each delivery. The results demonstrate the ability to capture automated measurements of diffraction from lysozyme micro-crystals. Our study points in the direction of an automated, acoustic levitation system for time-resolved crystallography.

Sound field optimization and particle trapping of confocal ultrasonic transducer

The nonlinear effect of high-intensity sound waves produces the acoustic radiation force (ARF), which are used for acoustic levitation and manipulation practical. With no special requirement for the physical and chemical properties of the controlled objects, acoustic levitation owns a promising application prospect. The common levitation scheme includes the standing-wave system and phased-array levitation system. The standing-wave system has poor performance in the aspects of the degree of spatial freedom, the ARF along the non-axial direction, and the levitation stability. The phased-array system requires a complex control system and a high production cost. Here, we propose a single-side acoustic levitation system based on the paired confocal focused transducers. By driving the transducer pairs with reverse phase mode, two anti-phase focused spherical waves interfere with each other, resulting in constant sound pressure of 0 Pa at the focus. The resulting potential well can achieve stable particle capturing and levitating. First, we verifed the theoretical feasibility of the system according to Huygens' principle. Then, using the finite element method, we analyzed the influences of structural and driving parameters on the sound field distribution, such as the angle between the transducer axis and the central axis of the structure and the excitation phase modes. Finally, we demonstrated the particle trappings under two kinds of excitation phase modes of the levitation system experimentally. The results show that, 1) the intensity of the dominating potential well reaches a strongest value when the structural angle is 45°; 2) as the excitation phases are 0, 0, π, and π, the sound field owns three potential wells which can capture three clusters of quartz sands, the primary potential well is stronger than the secondary one; 3) as the excitation phases are 0, π/2, π, and 3π/2, the sound field owns one potential well and captures one cluster of quartz sands. The isosurface of wave intensity around the potential well is more comprehensive than in the previous phase mode. The four-phase excitation improves the levitation stability better. The proposed levitation scheme can realize stable single- or multi-position capture of high-density objects in the fluid. Moreover, it has the advantages of low cost and a high degree of freedom.

Induction of Microstreaming by Nonspherical Bubble Oscillations in an Acoustic Levitation System.

When located near biological barriers, oscillating microbubbles may increase cell membrane permeability, allowing for drug and gene internalization. Experimental observations suggest that the temporary permeabilization of these barriers may be due to shear stress that is exerted on cell tissues by cavitation microstreaming. Cavitation microstreaming is the generation of vortex flows which arise around oscillating ultrasound microbubbles. To produce such liquid flows, bubble oscillations must deviate from purely spherical oscillations and include either a translational instability or shape modes. Experimental studies of bubble-induced flows and shear stress on nearby surfaces are often restricted in their scope due to the difficulty of capturing shape deformations of microbubbles in a stable and controllable manner. We describe the design of an acoustic levitation chamber for the study of symmetry-controlled nonspherical oscillations. Such control is performed by using a coalescence technique between two approaching bubbles in a sufficiently intense ultrasound field. The control of nonspherical oscillations opens the way to a controlled cavitation microstreaming of a free surface-oscillating microbubble. High-frame rate cameras allow investigating quasi-simultaneously the nonspherical bubble dynamics at the acoustic timescale and the liquid flow at a lower timescale. It is shown that a large variety of fluid patterns may be obtained and that they are correlated to the modal content of the bubble interface. We demonstrate that even the high-order shape modes can create large-distance fluid patterns if the interface dynamics contain several modes, highlighting the potential of nonspherical oscillations for targeted and localized drug delivery.

Adjusting single-axis acoustic levitators in real time using rainbow schlieren deflectometry.

Acoustic levitation uses focused high-intensity airborne ultrasound to hold particles in mid-air. It is becoming an important tool for experiments in spectrometry, lab-on-a-droplet, and display technologies. Nowadays, arrays of multiple small transducers can be used to build acoustic levitators; however, their performance depends on the optimal alignment. This work describes a simple method capable of visualizing a 2D projection of the acoustic field in real time using rainbow schlieren deflectometry. Good agreement was found between the images obtained with this technique and simulations of the acoustic pressure. It was also found that the maximum amplitudes of the field were obtained with the levitator aligned so that the power consumption was minimum, showing another simple and affordable way to adjust the levitators. As a result of the alignment optimization, it was possible for the first time to levitate steel and mercury in a levitator constructed with off-the-shelf components. The schlieren technique was applied to the TinyLev acoustic levitation system, but it can be applied to visualize the acoustic potential produced by different types of levitation systems.

Impact of the particle mixing state on the hygroscopicity of internally mixed sodium chloride-ammonium sulfate single droplets: a theoretical and experimental study.

Sodium chloride (NaCl) is the main constituent of sea-salt aerosols. During atmospheric transport, sea-salt aerosols can interact with gases and other particles including secondary aerosols containing ammonium sulfate ((NH4)2SO4). This paper reports on the deliquescence relative humidity (DRH) of internally mixed sodium chloride-ammonium sulfate (NaCl/(NH4)2SO4) coarse particles by means of an acoustic levitation system fitted with a confocal Raman microscope (CRM). The chemical composition and physical state of individual levitated particles of different initial NaCl mole fractions were monitored during the deliquescence cycle by CRM. Experimental results were compared to the data predicted by the thermodynamic model E-AIM (Extended-Aerosol Inorganics Model). We demonstrated that NH4Cl, Na2SO4 and NH4NaSO4·2H2O are formed in recrystallized particles and coexist with NaCl and (NH4)2SO4. All these products are randomly distributed within the particles. Deliquescence curves described two or three-stage phase transitions depending on the initial composition of the droplet. Significant discrepancies between the model and the laboratory experiments were observed for NaCl mole fractions varying between 0.40 and 0.77 due to a divergence between the predicted and the truly present products in the particles' solid fraction during the humidification cycle.

Non-Contact Universal Sample Presentation for Room Temperature Macromolecular Crystallography Using Acoustic Levitation

Macromolecular Crystallography is a powerful and valuable technique to assess protein structures. Samples are commonly cryogenically cooled to minimise radiation damage effects from the X-ray beam, but low temperatures hinder normal protein functions and this procedure can introduce structural artefacts. Previous experiments utilising acoustic levitation for beamline science have focused on Langevin horns which deliver significant power to the confined droplet and are complex to set up accurately. In this work, the low power, portable TinyLev acoustic levitation system is used in combination with an approach to dispense and contain droplets, free of physical sample support to aid protein crystallography experiments. This method facilitates efficient X-ray data acquisition in ambient conditions compatible with dynamic studies. Levitated samples remain free of interference from fixed sample mounts, receive negligible heating, do not suffer significant evaporation and since the system occupies a small volume, can be readily installed at other light sources.

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.

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.

Automatic contactless injection, transportation, merging, and ejection of droplets with a multifocal point acoustic levitator.

We present an acoustic levitation system that automatically injects, transports, merges and ejects liquid droplets in mid-air. The system consists of a phased array operating at 40 kHz on top of a plane reflector. The phase array generates multiple focal points at independent positions that form standing waves between the array and the reflector. In the reflector there is an inlet for a piezoelectric droplet injector which automatically inserts liquid droplets at the lower pressure nodes of the standing waves, and a hole that serves as an outlet for ejecting the processed droplets out of the system. Simulations of the acoustic radiation potential acting on the levitating droplets are in good agreement with the experiments. High-speed footage captured the functioning of the system in four fluidic operations: injection, transport, merging and ejection of liquid droplets. Having these operations integrated reliably into a single automatic system paves the way for the adoption of mid-air acoustophoretic processing in biological, chemical and pharmaceutical applications.

Noncontact Transportation of Planar Object in an Ultrasound Waveguide.

This paper investigates acoustic levitation and noncontact transportation techniques for use with planar objects. An acoustic levitation system was developed which consists of a 1-mm-thick and 400-mm-long bending plate along with two bolt-clamped Langevin-type transducers (BLTs) that have stepped horns. A plane reflector was installed parallel to the vibrating plate to generate an ultrasound standing wave between the reflector and the plate. The sound pressure distribution in the ultrasound waveguide was calculated via finite-element analysis to investigate the effects of levitation of a planar object in the standing-wave field. A 1-mm-thick polystyrene plate was levitated along the nodal line of the acoustic standing wave in the waveguide. By controlling the driving phase difference between the two BLTs, the position at which flexural vibration occurs on the vibrating plate could be shifted along the length direction, and the trapped planar object could be moved by 9 mm along the same direction when the phase difference was varied from 0° to 360°.

Chemical elemental analysis of single acoustic-levitated water droplets by laser-induced breakdown spectroscopy.

Laser-induced breakdown spectroscopy is presented for trace element detection of liquid samples by analyzing a single droplet levitated by ultrasonic waves. A single liquid droplet is placed in the node of a standing acoustic wave produced by a uniaxial levitator for further chemical analysis. The acoustic levitator consists of a commercial Langevin-type transducer, attached to a concave mechanical amplifier, and a concave reflector. A micro-syringe was used to manually place individual liquid droplet samples in the acoustic levitation system. For chemical analysis, a laser-induced plasma is produced by focusing a single laser pulse on the levitated water droplet after it partially dries. The performance of the acoustic levitator on micron-sized droplets is discussed, and the detection of Ba, Cd, Hg, and Pb at parts per million (milligrams/liter) and sub-parts per million levels is reported. The process, starting from placing the sample in the acoustic levitator and ending on the chemical identification of the traces, takes a few minutes. The approach is particularly interesting in applications demanding limited volumes of liquid samples and relative simple and inexpensive techniques.

Simultaneous measurement of surface tension and viscosity using freely decaying oscillations of acoustically levitated droplets

Oscillations of small liquid drops around a spherical shape have been of great interest to scientists measuring physical properties such as interfacial tension and viscosity, over the last few decades. A powerful tool for contactless positioning is acoustic levitation, which has been used to simultaneously determine the surface tension and viscosity of liquids at ambient pressure. In order to extend this acoustic levitation measurement method to high pressure systems, the method is first evaluated under ambient pressure. To measure surface tension and viscosity using acoustically levitated oscillating drops, an image analysis method has to be developed and factors which may affect measurement, such as sound field or oscillation amplitude, have to be analyzed. In this paper, we describe the simultaneous measurement of surface tension and viscosity using freely decaying shape oscillations of acoustically levitated droplets of different liquids (silicone oils AK 5 and AK 10, squalane, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, 1-heptanol, and 1-octanol) in air. These liquids vary in viscosity from 2 to about 30 mPa s. An acoustic levitation system, including an optimized standing wave acoustic levitator and a high-speed camera, was used for this study. An image analysis was performed with a self-written Matlab® code. The frequency of oscillation and the damping constant, required for the determination of surface tension and viscosity, respectively, were calculated from the evolution of the equatorial and polar radii. The results and observations are compared to data from the literature in order to analyze the accuracy of surface tension and viscosity determination, as well as the effect of non-spherical drop shape or amplitude of oscillation on measurement.

Ultrasonic robotic system for noncontact small object manipulation based on Kinect gesture control

Acoustic levitation system, with the advantages of avoiding clamping force on samples, has been widely introduced into noncontact manipulation. Previous acoustic levitation systems were generally fixed and could not realize remote adjustment of manipulation stage. In this article, we develop an ultrasonic robotic system for small object manipulation which is capable of noncontact operation, remote control, and gesture-based intelligent control. The whole setup is composed of acoustic levitation platform, mobile operation platform, and Kinect sensing system which provides a humanized approach for human–machine interaction. The experimental results validate a sufficient suite of innovations for achieving remote control of this acoustic levitation system based on gesture recognition method. It will not only pave a new way for noncontact manipulation system but also greatly reduce the difficulty and cost of operation.

Application of rainbow refractometry for measurement of droplets with solid inclusions

Characterization of droplets with solid inclusions is of great research interest and has wide industrial applications. Reported here is a theoretical and experimental investigation of the measurement of droplets with solid inclusions using rainbow refractometry. A rainbow extinction model of a droplet with solid inclusions was deduced based on Beer-Lambert's Law. It takes into account the volume concentration, relative size, scattering efficiency of the solid inclusion, and liquid refractive index. An acoustic levitation system for a single droplet and a global rainbow instrumentation system for spray were integrated to study the effect of the H2O-CaCO3 suspension droplets on the rainbow signal and the measured parameters. The results showed that the rainbow encountered unusual disturbances, introduced by the solid inclusions, but its overall structure was not destroyed. Discoveries also included that for volume concentrations of 2.5% or less the CaCO3 particles with diameters below 4μm had little effect on the measured parameters of the host droplet. The extinction characteristic was also analyzed. The rainbow extinction model failed to quantity the volume concentration of CaCO3, but succeeded in its qualitative analysis.