Acoustic Levitation Research Articles

Using an electrically charged mist to visualize the potential wells in “sonomaglev,” a combined diamagnetic-acoustic levitator

We use a water mist to experimentally visualize the wells in the potential energy of material levitated by a combined diamagnetic-acoustic levitator. The levitator consists of an 18.5 T superconducting magnet, which can levitate diamagnetic material, such as water, plastics, and organic materials, by applying a magnetic body force counteracting the force of gravity. Low-power ultrasound transducers operated at 37.5 kHz generate an acoustic field that spatially modulates the net force acting on the diamagnetically levitated material, making “sonomaglev” capable of levitating multiple objects in stable equilibrium. In these experiments, we levitate a mist of water droplets that are electrically charged so that they repel each other, preventing them from coalescing as a single drop in each of the local potential minima. The shapes of the potential wells are revealed by the shapes of clusters of droplets, which conform to the isosurfaces of the sum of the magnetic, gravitational, and acoustic potentials. The spacing of the droplets in a cluster is shown to depend on their charge, volume, and the force constant of the well in a simple model. Compared to acoustic levitation alone, the combination of diamagnetic and acoustic levitation allows more scope for the manipulation of levitated objects, since the acoustic field is not constrained by the requirement to balance the force of gravity. The method demonstrated here allows the influence on the potential energy of switching on the acoustic field to be observed directly.

Atomization by Acoustic Levitation Facilitates Contactless Microdroplet Reactions.

Microdroplet chemistry is now well-known to be able to remarkably accelerate otherwise slow reactions and trigger otherwise impossible reactions. The uniqueness of the microdroplet is attributable to either the air-water interface or solid-liquid interface, depending on the medium that the microdroplet is in contact with. To date, the importance of the solid-liquid interface might have been confirmed, but the contribution from the air-water interface seems to be elusive due to the lack of method for generating contactless microdroplets. In this study, we used a droplet atomization method with acoustic levitation. Upon manipulation of the acoustic field, the levitated parent droplet can be further atomized into progeny microdroplets. With this method, only the air-water interface was present, and a large variety of reactions were successfully tested. We anticipate that this study can be an advance toward the understanding of the air-water interfacial processes of microdroplet chemistry.

Analysis of dynamic levitation process of the particle chain in a nonlinear standing wave field.

This research delves into the dynamic behavior of acoustic levitation of the particle chain in a nonlinear standing wave field. Experimental acoustic levitation control tests reveal bifurcation and jump phenomena during dynamic adjustments to resonant cavity height. Employing the 10-particle chain experiments and the COMSOL simulation models, the Sine-Gordon 2D vibration model is established to study the dynamic deformation process of the particle chain. The study uncovers the nonlinear interaction of particle lateral vibrations, horizontal acoustic radiation force, and conical wave fields that generate the jumping standing wave field. Notably, the fourth particle acts as a prominent jumping critical point in the secondary standing wave field, facilitating the derivation of the particle chain's nonlinear levitation dynamics. This discovery provides us with a new method to regulate the particle chain system.

Contactless Manipulation and Raman Analysis of Cometary Analogs and Micrometeorites by Acoustic Levitation

Extraterrestrial material collected during space missions is highly exposed to contamination issues during on-Earth analysis. Although high-protection-level protocols were developed, to minimize the contamination due to sample manipulation and the substrate contribution an optimal strategy is to perform in situ analysis with contactless techniques. Optical and acoustic trapping represent ideal candidates for contactless manipulation and analysis of nanometer-to-millimeter-sized particles. Here, we show results of the manipulation of cometary analogs and micrometeorite samples using a single-axis acoustic levitator. The investigation of the particle dynamics in the trap allows the calculation of the trap spring constants that are found in the mN/m range. In addition, we collect the Raman spectra of two levitated fragments of Saratov meteorite, demonstrating that acoustic levitation can be effectively used for the contactless and low-contamination characterization of samples of interest in astrophysics.

MightyLev: An acoustic levitator for high-temperature containerless processing of medium- to high-density materials.

"MightyLev," a new multi-emitter ultrasonic acoustic levitation device capable of extremely stable levitation of materials of density up to at least 11.3g cm-3, is described. The exceptional stability of medium- to high-density samples levitated in MightyLev makes the device highly suitable for chemical and structural analysis using micro-focused spectroscopic and x-ray scattering techniques. In combination with mid-infrared laser heating, MightyLev is capable of levitating metallic and oxide materials during high-temperature cycling and melting above 1500K. Instabilities in particle confinement during heating were investigated by directly visualizing the acoustic field using schlieren imaging. The results reveal jets of hot-air directed along the anti-nodes of the acoustic field. The reaction force on the sample from the jet, coupled with the restoring force of the acoustic trap, generates a parametric lateral oscillation of the sample. This result provides valuable insight for future optimization and wider application of acoustic levitation for high-temperature containerless material processing.

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.

LeviLabs: learning about sound through acoustic levitation

Abstract Sound waves are often studied as part of mechanical waves in the upper-secondary and undergraduate physics curricula. Acoustic levitation is an application of standing-wave phenomena, growing in popularity through the recent adoption of low-cost ultrasound transducers. Nevertheless, acoustic levitation has not yet been included in the standard phenomena investigated in undergraduate and upper-secondary physics courses. The objective of this Frontline is to introduce acoustic levitation in the classroom. This is done with LeviLab, a low-cost and easy-to-use experiment, accompanied by three hands-on procedures (LeviLabs): modifying the cavity length, visually representing standing waves with multiple levitated particles, and a tactile experience by moving a single particle between nodes. The first procedure yields the most accurate speed of sound measurements, while the second and third offer valuable qualitative insights into standing wave properties. This work demonstrates the educational potential of acoustic levitation to investigate sound waves. It aims to make the phenomenon accessible as a learning resource for upper-secondary and undergraduate physics laboratories by providing engaging hands-on experiences for qualitative and quantitative exploration.

Improved functionality of hepatic spheroids cultured in acoustic levitation compared to existing 2D and 3D models

Hepatic spheroids are of high interest in basic research, drug discovery and cell therapy. Existing methods for spheroid culture present advantages and drawbacks. An alternative technology is explored: the hepatic spheroid formation and culture in an acoustofluidic chip, using HepaRG cell line. Spheroid formation and morphology, cell viability, genetic stability, and hepatic functions are analyzed after 6 days of culture in acoustic levitation. They are compared to 2D culture and non-levitated 3D cultures. Sizes of the 25 spheroids created in a single acoustofluidic microphysiological system are homogeneous. The acoustic parameters in our system do not induce cell mortality nor DNA damage. Spheroids are cohesive and dense. From a functional point of view, hepatic spheroids obtained by acoustic levitation exhibit polarity markers, secrete albumin and express hepatic genes at higher levels compared to 2D and low attachment 3D cultures. In conclusion, this microphysiological system proves not only to be suitable for long-term culture of hepatic spheroids, but also to favor differentiation and functionality within 6 days of culture.

Ultraviolet laser-induced fragmentation of hydrocarbon fuel droplets

The present study aims to understand droplet fragmentation due to instabilities from the radiative heating process. We focus on the plasma-induced breakup mechanism of iso-octane and n-hexane droplets from the nucleation of holes through sheets and instabilities. The plasma generation inside the droplets leads to an intense explosion followed by fragmentation of the droplets. A droplet is suspended in the air using an acoustic levitator and exposed to ultraviolet nanosecond laser pulses. The droplet sizes used are in the range of 0.5–2 mm in diameter, and laser energies are 10–20 mJ per pulse. Initially, plasma formation followed by shock wave emission is observed during the impact of the laser pulse. Afterward, the droplet opens at one side and is stretched vertically to form a liquid sheet. The hole formation and its rapid growth result in the breaking of the thin liquid sheet. Small droplets and ligaments emerge from the circular edge of the sheet, which follows the well-known ligament distribution. This study reveals the detailed analysis of expansion dynamics, evolution of single and multiple holes, and instabilities associated with the liquid sheet. The results observed are categorized into four different mechanisms: (1) plasma formation followed by shock wave propagation, (2) droplet deformation, (3) sheet breakup through nucleation of holes followed by rapid growth, and (4) complete atomization.

The resonant behavior of airborne standing-wave acoustic levitators based on arrays of ultrasonic transducers

Recently airborne standing-wave acoustic levitation has seen great advances, and its applicability has been broadened due to the development of cavities constructed with arrays of compact ultrasonic sources. Yet, the numerical methods employed to study and predict the pressure distributions inside these cavities do not consider the effect of multiple reflections on the boundaries, hiding their resonant effects. This work presents an analytical, numerical, and experimental study of the effect of multiple reflections inside ultrasonic cavities based on arrays of transducers exhibiting their influence on the pressure amplitudes of focused standing waves. Our numerical results come from a modified version of the Matrix Method to numerically compute the multiple wave reflections of cavities constructed by two opposite arrays of multiple compact sources as boundaries. The correlation between numerical and experimental results reveals that intra-cavity reflections are relevant in focused axisymmetric cavities based on two arrays of multiple ultrasonic sources having a considerable impact on the amplitude of the standing waves and consequently, on the acoustic levitation performance. Thus, intra-cavity reflections must be considered for optimal cavity designs.

CO2 induced phase transition on a self-standing droplet studied by X-ray scattering and magnetic resonance

HypothesisAcoustic levitation is a suitable approach for studying processes occurring at the gas–liquid interfaces, as it allows its investigation in a contact-free manner while providing control over the gas phase. Here, we hypothesize that phase transitions induced by a CO2 rich atmosphere can be examined, at different length scales, in a contact-free manner. ExperimentalA system consisting of 12-hydroxysteric acid (HSA) soaps mixed with different ratios of monoethanolamine (MEA) and choline hydroxide, was prepared. Microliter droplets of the samples were acoustically levitated and monitored with a camera, while exposed to CO2 to modify the pH through diffusion at the air–liquid interface and inside the droplet. The phase transition and water mobility in the levitated droplets were evaluated through X-ray scattering (SAXS/WAXS) and magnetic resonance studies, in real-time. Finally, the droplets were collected and examined under the microscope. FindingsThe introduction of CO2 gas induced a phase transition from micelles to multi-lamellar tubes, resulting in a gel-like behavior both in the bulk and at the interface. The high stability of the acoustic levitator allowed the investigation of this dynamic phenomenon, in real-time, in a contact-free environment. This study showcases the suitability of acoustic levitation as a tool to investigate complex chemical processes at interfaces.

Direct measurement of forces in air-based acoustic levitation systems.

Acoustic levitation is frequently used for non-contact manipulation of objects and to study the impact of microgravity on physical and biological processes. While the force field produced by sound pressure lifts particles against gravity (primary acoustic force), multiple levitating objects in the same acoustic cavity interact via forces that arise from scattered sound (secondary acoustic forces). Current experimental techniques for obtaining these force fields are not well-suited for mapping the primary force field at high spatial resolution and cannot directly measure the secondary scattering force. Here, we introduce a method that can measure both acoustic forces in situ, including secondary forces in the near-field limit between arbitrarily shaped, closely spaced objects. Operating similarly to an atomic force microscope, the method inserts into the acoustic cavity a suitably shaped probe tip at the end of a long, flexible cantilever and optically detects its deflection. This makes it possible to measure forces with a resolution better than 50 nN and also to apply stress or strain in a controlled manner to manipulate levitated objects. We demonstrate this by extracting the acoustic potential present in a levitation cavity, directly measuring the acoustic scattering force between two objects, and applying tension to a levitated granular raft of acoustically bound particles in order to obtain the force-displacement curve for its deformation.

The Effect of the Movement of a Passive Reflector on the Atomization of a Levitating Drop

Abstract This study investigates the instability of distilled water drops under acoustic levitation induced by the movement of a reflector toward the transducer. The experimental setup consisted of a passive concave reflector and a Langevin transducer in a uniaxial-resonance configuration. The experiment consists of achieving a stable levitation of the droplet and then moving the reflector constantly toward the transducer, shortening the distance h between them (reflector-transducer). All experiments were recorded using a high-speed camera to analyze the drop’s behavior. The results revealed three distinct instabilities: radial atomization occurred when the drop volume (VD ) was ≤ 3.74 μl, beyond which central atomization was observed. A third instability is observed when a low reflector velocity (Vr < 0.67 mm/s) is used, where the droplet exhibited modes of oscillation with pulsed central atomization.

Micro container made of levitated liquid bead

Liquid bead, an emerging digital microfluidic platform offers a wide range of applications in research and industry. Making liquid beads using acoustic levitation technology allows for the fabrication of micro container. Extensive insight into levitated liquid beads is essential for determining their performance. Firstly, this paper unfolds the unique interface of the levitated liquid bead, which has thin membrane of shell material on top and thus serves as a micro container. Next, we characterised the puncture toughness of liquid beads to assess the capability of releasing the liquid content. Liquid bead with larger corresponding shell volume and lower core volume recorded the highest resistance against needle penetration. Finally, we observed the thermal behaviour of liquid beads at elevated temperature. The liquid beads showed no morphological change over 10-minutes period at 50 °C. At 65 °C, liquid bead exceeded the critical temperature threshold, which resulted in breaking the thin membrane and releasing the liquid from the core.

Acoustic Levitation-Controlled Droplets for Contactless Enrichment and Sensitive Detection of Volatile Organic Substances Integrated with GC-MS.

Analyzing trace-level volatile organic compounds (VOCs) remains challenging due to initial sampling and preconcentration limitations. Inspired by the highly reproducible and constantly renewable electrode surface of dropping mercury electrode (DME), a contactless enrichment process was first reported by using an acoustic levitation device to trap and concentrate VOCs from gas samples onto suspended droplets, which were then directly transferred into gas chromatography-mass spectrometry (GC-MS) for real-time analysis. Compared with traditional methods injection methods, this method achieves a 46-fold increase in nicotine peak area. The detection sensitivity was enhanced significantly, attributed to the high specific surface area of the droplets and the accelerating extraction vibration. Notably, the number of identified VOCs from burning cigarettes significantly increased from 17 to 212, including 22 aromatic compounds with distinct aromas. The remarkable versatility of this method was demonstrated by effectively monitoring the dynamic changes of 16 VOCs in environmental tobacco smoke (ETS) following cigarette burning, revealing the persistence of these compounds, even after 40 min. Moreover, directly analyzing human-exhaled aerosol found that nicotine rapidly decreased while its metabolite cotinine increased, showcasing the potential for tracking human metabolism and behavior in vivo. Furthermore, multivariate data analysis of VOC profiles from six cigarette brands allowed for their visual differentiation. With versatility, sensitivity, and the ability to distinguish trace-level VOCs in realtime, this method offers promising avenues for environmental monitoring, metabolic studies, and various analytical applications.

Weakly nonlinear shape oscillations of viscoelastic drops

Axisymmetric shape oscillations of a viscoelastic drop in a vacuum are studied by a weakly nonlinear analysis. The two-lobed initial drop deformation mode is studied. The Oldroyd-B model is used for characterizing the drop liquid rheological behaviour. The equations of motion and the solutions up to second order are developed, where elastic effects appear. Solutions of the characteristic equation are validated against the decay rate and frequency of damped shape oscillations of polymer solution drops in an acoustic levitator. The theory shows enhancing or dampening of the nonlinear behaviour and enhanced mode coupling, as compared with the Newtonian case. The study reveals an excess time in the prolate shape and a frequency change, together with a quasi-periodicity of the oscillations. The Fourier power spectra of traces of the drop north pole position in time show mode coupling as a nonlinear effect. At moderate stress-relaxation Deborah number, the resultant drop motion for large Ohnesorge number, suggesting aperiodic drop behaviour, may nonetheless be oscillatory, where the drop elasticity is owing to the liquid bulk elasticity instead of surface tension. A method is developed to predict the oscillatory or aperiodic behaviour of a drop of a given size from a rheologically characterized viscoelastic Oldroyd-B liquid.

Fluorescence Lifetime Measurement For Studying Evaporation Of Bi-Component Droplet

Sprays are essential in engineering applications like evaporators and combustion chambers. Predicting the evaporation of multi-component liquid droplets is complex due to changing composition and properties. Measuring droplet composition is challenging, with limited non-intrusive methods available. Raman spectroscopy faces hurdles like weak signals and interference. Intensity based Laser-Induced Fluorescence (LIF) requires optical models to interpret data due to varying droplet size and position. Fluorescent dyes in liquid mixtures are sensitive to temperature, complicating measurements. Fluorescence lifetime measurements offer a promising alternative, being an absolute quantity less affected by reabsorption. This study develops a novel method using fluorescence lifetime to analyze droplet composition, leveraging Time-Correlated Single Photon Counting (TCSPC) for precise measurements. Factors such as dye concentration, solvent polarity, and temperature sensitivity are investigated. Calibration curves for ethanol-water mixtures correlate fluorescence lifetime with volume fraction. Experimental setups use TCSPC and femtosecond lasers for dye excitation. Results show that fluorescence lifetime decreases linearly with solvent polarity and is influenced by temperature and dye concentration. High dye concentrations cause self-quenching, altering temperature sensitivity. The method was applied to study the evaporation of ethanol-water droplets under acoustic levitation. Findings indicate that evaporation rates slow as the water fraction increases, with initial measurements showing higher water content due to partial evaporation. This research introduces a novel technique for droplet compositional analysis using fluorescence lifetime, providing insights into optimizing measurement accuracy and advancing understanding in engineering and related fields.

Numerical solution and sensitivity analysis of time–space fractional near-field acoustic levitation model using Caputo and Grünwald–Letnikov derivatives

Numerical solution and sensitivity analysis of time–space fractional near-field acoustic levitation model using Caputo and Grünwald–Letnikov derivatives

Effect of array arrangement on acoustic levitation performance

Effect of array arrangement on acoustic levitation performance