Acoustic Levitation Research Articles

Pattern formation in acoustically levitated particle systems with competing near-field interactions

Acoustic levitation in air provides a containerless, gravity-free platform for investigating driven many-particle systems with nonconservative interactions and underdamped dynamics. In prior work the interactions among levitated particles were limited to attractive forces from scattered sound and repulsion from hydrodynamic microstreaming. We report on experiments in which contact cohesion provides a third type of interaction. When particle size and separation are both much smaller than the sound wavelength, this interplay of three interactions results in forces that are attractive over several particle diameters, become repulsive at close approach, and are again attractive at contact. In the presence of sound-induced athermal fluctuations that generate particle collisions, the interplay of these three forces enables the formation of particle chains with anisotropic interactions that depend on chain size and shape due to multibody effects. With the control of the kinetic pathways and the strength of the contact cohesion, different patterns can be assembled, from triangular lattices to labyrinthine patterns of chains to lacelike networks of interconnected rings. These results shed light on the multibody character of acoustic interactions and can be utilized to direct the self-assembly of particles. Published by the American Physical Society 2025

Understanding the Salt Crystallizations from Droplets under Various Gravity and Pressure Environments: Display of the Marangoni Effect?

Understanding the drying mechanisms, crystallization, and creep dynamics in salt solutions relies on a thorough understanding of the evaporation kinetics of individual droplets. These processes are of significant interest due to their implications in fields such as astrochemistry, environmental science, and material science. However, under nonequilibrium conditions like reduced pressure (<1 atm) and microgravity (<1g), they remain poorly understood, creating a pressing need for studies addressing these gaps. Our study investigates the evaporation of droplets from saturated salt solutions under nonequilibrium conditions, with experiments conducted in low-pressure and microgravity environments using a vacuum chamber and an acoustic levitation setup. We focused on the in situ crystallization dynamics of sodium chloride, potassium chloride, and ammonium chloride droplets, hypothesizing that the poleward migration of salt within levitating droplets is driven by the Marangoni effect, which arises from concentration gradients.1 Additionally, we explored the droplet evaporation mechanism under low pressure, examining factors contributing to the reduced "ring effect" in these conditions. Our findings indicate that the surface tension of the substrate under low pressure plays a crucial role in crystallization mechanisms, influencing the size of the central ring. These insights not only enhance our understanding of salt crystallization dynamics but also address the demand for innovative approaches to studying these processes in extraterrestrial and extreme environments. Furthermore, they provide a refined physical analysis and suggest potential applications in astrophysics, space exploration, and sustainable source management.

Near-field acoustic levitation generated by dual-frequency ultrasound

Near-field acoustic levitation generated by dual-frequency ultrasound

Spherical Nucleic Acids Meet Acoustic Levitation: A Breakthrough in Synthesis and Application.

Spherical nucleic acids (SNAs), with their densely packed nucleic acid shells and programmable functionalities, have become indispensable in nanomedicine and biosensing. Developed synthesis methods, including salt aging, pH modulation, freeze-thaw cycling, n-butanol dehydration, evaporation drying, and microwave heating, have enabled foundational advances but are constrained by slow kinetics, compromised structural uniformity and especially harsh reaction conditions, making them unsuitable for in situ tracking of biological events. This concept article introducesacoustic levitation synthesisas a groundbreaking alternative, uniquely addressing these limitations through a rapid, green, and highly controllable process. By leveraging non-contact acoustic radiation forces, this method enables the synthesis of ultrahigh-density SNAs within minutes under ambient conditions, eliminating the need for toxic reagents or energy-intensive steps. The resulting SNAs exhibit superior homogeneity and stability compared to conventional approaches. We critically evaluate the conceptual novelty and limitations of this technique. Potential applications in surface-enhanced Raman spectroscopy (SERS) and targeted therapeutics are highlighted, positioning acoustic levitation as a transformative tool for next-generation nanobiotechnology.

Acoustic levitation of super-wavelength elastic films using ultrasound phased arrays

We present a method for levitating films with a surface size larger than the wavelength using airborne ultrasound phased arrays. A typical example is a polyimide film with a side length of 40–50 mm and a thickness of 5 μm (aspect ratio: 8–10 × 103). We verified our method by measuring the height, horizontal position, and vibration of the levitating film. The results show that the film levitates at the height of the original standing wave node and at discrete horizontal positions approximately every transducer interval. The levitated film vibrates at the same frequency as the ultrasonic transducer and cannot be regarded as rigid against ultrasonic waves. Different film materials and thicknesses were examined, including metal foils and wood papers. In this study, the maximum surface density of the films that levitated was 3.5 ×10−2 mg/mm2. Therefore, the proposed method can be used to hold film samples in the air for observation or as an aerial screen.

Freezing of an acoustically levitated drop: From liquid disk to ice ring

Drop freezing plays an important role in industries ranging from food and materials to biomedicine, but the contact between drops and substrates is unavoidable. The contact effect has great influence on the nucleation and growth of ice. To eliminate the wall effect on drop freezing, we studied the freezing of liquid drops under container-free conditions by combining the sound field and temperature field. The levitated drops exhibited varied frozen morphologies, such as ice saucer, ice cake, and ice ring, depending on their initial shapes. For a levitated liquid disk, it eventually freezes to form a levitated ice ring, which is caused by the atomization rupture of its central liquid film. In addition, we accomplished the landing–lifting manipulation of the frozen drops by controlling the sound field. Our work highlights the coupling effects between thermodynamically driven phase transitions and mechanically driven drop dynamics involved in acoustic levitation. The investigation of levitated drop freezing contributes to a in-depth understanding of the phase transition process and dynamic behavior of fluids in a container-free environment, and also inspires and expands new methods for the fabrication of ring-shaped materials.

Numerical Analysis of a Horizontal Flexible Rotor Supported by a Squeeze Film Air Bearing Based on Near-Field Acoustic Levitation

Numerical Analysis of a Horizontal Flexible Rotor Supported by a Squeeze Film Air Bearing Based on Near-Field Acoustic Levitation

Periodic gradient duplexcavity meta-lens for wide-band directional acoustic beaming

Abstract Selective multi-directional sound projection across a wide-band low mid-frequency range remains a significant challenge in acoustic applications, primarily due to the manipulation of long wavelengths within compact acoustic meta-lens. This study introduces a novel directional acoustic beaming theoretical method, specifically a duplexcavity four-lobe valve-shaped acoustic meta-lens (DFVAM). By manipulating geometric parameters, the DFVAM effectively controls downstream acoustic in a directional sound projection fashion that enhances sound wave propagation across multiple wide frequency bands. Experimental results validate substantial sound focusing at frequencies of 4500–4800 Hz, 7200–7600 Hz, 9700–10 300 Hz, and 11 400–11 600 Hz, achieving sound beam lengths between 12 and 21 λ, cross-sectional areas ranging from 0.7 to 2 λ, and FWHM values of 0.140–0.218 m. This meta-lens offers efficient acoustic modulation and focusing across various frequency ranges, presenting promising applications in acoustic directional transmission, sound energy collecting and acoustic levitation.

Anisotropic growth dynamics of liquid bridge during droplet coalescence under acoustic levitation

Anisotropic growth dynamics of liquid bridge during droplet coalescence under acoustic levitation

Accelerating particle aggregation in acoustic levitation by collective effects: Application to cost-effective ultrasonic harvesting of microalgae

Accelerating particle aggregation in acoustic levitation by collective effects: Application to cost-effective ultrasonic harvesting of microalgae

Crystal nucleation and growth kinetics of acoustically levitated liquid SCN-DC transparent alloys

As an important and promising experimental method for simulating the containerless state in outer space, acoustic levitation provides excellent contact-free condition to investigate solidification process. Meanwhile, the radiation pressure and acoustic streaming caused by nonlinear effects bring various kinds of novel phenomena to crystallization kinetics. In this work, high-speed CCD, low-speed camera and infrared thermal imager were used simultaneously to observe the crystallization process of acoustically levitated SCN-DC transparent alloys. The undercooling ability and solidification process of alloy droplets with different aspect ratios were explored under acoustic levitation state. For hypoeutectic SCN-10wt%DC, eutectic SCN-23.6wt%DC and hypereutectic SCN-40wt%DC alloys, the experimental maximum undercoolings reached 22.5(0.07&lt;i&gt;T&lt;/i&gt;&lt;sub&gt;L&lt;/sub&gt;), 16(0.05&lt;i&gt;T&lt;/i&gt;&lt;sub&gt;E&lt;/sub&gt;) and 32.5K(0.1&lt;i&gt;T&lt;/i&gt;&lt;sub&gt;L&lt;/sub&gt;) respectively and the corresponding crystal growth velocities were 27.91, 0.21 and 0.45 mm/s. In SCN-10wt%DC hypoeutectic alloy, the nucleation mode of SCN dendrite changed from edge nucleation to random nucleation with the increase of undercooling. For SCN-23.6wt%DC eutectic alloy, when undercooling exceeded 12.6K, DC dendrite preferentially nucleated and grew, and then the (SCN+DC) eutectic grew attached to DC dendrite. Moreover, the growth interface of DC dendrite gradually changed from sharp to smooth within SCN-40wt%DC hypereutectic alloy as the undercooling degree rose. The undercooling distribution curve and nucleation probability variation trend were analyzed versus aspect ratio. It was found that as the aspect ratio increased, undercooling of alloy droplet increased firstly, then decreased, and finally remained almost unchanged. Further analysis showed that with the increase of aspect ratio, the cooling rate would rise and thus enhanced the undercooling. However, the increase in surface nucleation rate and the droplet oscillation inhibited deep undercooling of alloy droplet. Therefore, the coupled effects of cooling rate, surface nucleation rate and droplet oscillation determined the undercooling of the alloy. In the case of SCN-40wt%DC hypereutectic alloy, the acoustic streaming and surface oscillation arising from acoustic field were the principal factors intensifying surface nucleation.

Characterizing Hypergolic Propellants Using Impinging Jets and Droplets in Acoustic Levitation

This study presents an experimental investigation into the hypergolic ignition process of nontoxic propellants. Multispectral high-speed imaging was employed to capture the dynamic combustion process and temperature history across three distinct ignition setups. Six fuel samples containing different proportions of N,N,N′,N′-tetramethylethylenediamine (TMEDA) and N-methyldiethanolamine (MDEA), catalyzed with 1–2 wt% copper nitrate trihydrate (referred to as PAHyp 1), paired with rocket-grade hydrogen peroxide (RGHP) as the oxidizer, were selected. Formulations with higher concentrations of TMEDA containing 2 wt% catalyst paired with 95 wt% RGHP yielded ultrafast ignition delays as short as 7.6 ms in drop tests. Additionally, as binary droplet collision is a common occurrence in spray combustion, an acoustic levitator was proposed to study the ignition behaviors of droplet–droplet collision. The collision of binary droplets in the levitator, conducted at lower impact velocities, resulted in a minimum ignition delay time of about 17 ms. It was observed that depending on the size of the droplets and collision angle, ignition may not occur. This new approach to studying the dynamics of hypergolic ignition without “wall effects” has shown promise due to its ease of implementation. Finally, two fuel samples were selected to conduct ignition tests under flow conditions with an impinging jet apparatus. Remarkably, it was demonstrated that the addition of only 1 wt% is sufficient to achieve fast ignition with 90% hydrogen peroxide. Results from this new recipe indicate that samples containing at least 50% MDEA remain chemically stable for over 1 year and demonstrate competitive ignition and thermodynamic performance compared to conventional hydrazine-based fuels.

Omnidirectional and Multi‐Material In Situ 3D Printing Using Acoustic Levitation

AbstractAdditive manufacturing is critical in modern production with advanced applications across various domains. However, achieving omnidirectional printing that can adapt to varying substrate geometries is still a challenge. Moreover, multi‐material in situ printing without cross‐contamination presents another hurdle. In this study, an acoustophoretic 3D fabrication system capable of omnidirectional and multi‐material in situ 3D printing is reported using acoustic levitation. This system harnesses a phased array of transducers (PAT) to finely tune the ultrasonic field, generating acoustophoretic forces that levitate and transport objects in mid‐air. It allows omnidirectional in situ printing of materials onto complex substrates with diverse orientations in a contactless and voxel‐by‐voxel manner. It is capable of manipulating a broad spectrum of materials, including liquids with zero‐shear viscosities from 1 to 5,000,000 mPa·s, as well as solids. AcoustoFab has successfully printed structural, conductive, and biological materials in varying directions on complex substrates. Additionally, the contactless approach enables in situ printing safely on a delicate surface, as demonstrated by printing on a human hand. The flexibility and versatility of this approach demonstrate promising applications in areas ranging from biomedical engineering to industrial manufacturing.

Effect of solution flow field on evaporation-induced NaCl crystallization in acoustically levitated droplets

Acoustically levitated droplets have exhibited great potential in crystallization studies due to their versatility with various solution types and the simplicity of the apparatus. By separating the precursor solution and the solid surface, the crystallization process could be observed and analyzed in a contactless environment. The decoupling of the crystallization system and surface physical/chemical properties greatly boosts the in situ investigation of early-stage nucleation and crystal growth mechanisms. However, the interaction between the precursor solution and applied acoustic field during the crystallization process is often neglected, which imposes significant influences on the crystal products. In this paper, visual experiments were carried out to study the NaCl crystallization process in acoustically levitated droplets. Image processing was employed to acquire the evaporation rate of the droplets, and particle image velocimetry analysis was used to characterize the flow field. Effects of droplet size and initial NaCl concentration were investigated, and the crystallization behaviors in the levitated droplet and sessile droplet were compared. The results indicate that the acoustic field introduced a forced convection of fluid within the levitated droplets, influencing the evaporation rate, supersaturation degree, and the morphology of the crystal product. The obtained mechanism is important for the application of acoustically levitated droplets and can be further applied to other crystallization research based on the acoustic levitation systems.

Calibrating position and orientation of multiple ultrasound phased array units using Bayesian optimization.

Airborne ultrasound phased arrays (AUPAs) generate non-contact tactile sensations and enable acoustic levitation with specific focus fields. Using multiple units together offers numerous advantages, such as increased stimulus intensity and the ability to overcome occlusion. The AUPA units are typically mounted on a fixed frame, with their poses manually measured using tools such as a ruler for calibration. However, to increase the degrees of freedom for these units, a more flexible calibration method is required. With a wavelength of 8.5 mm, a 4 mm deviation in propagation distance from the two phased arrays can weaken the pressure at the focus position. Hence, in this study, calibration based on pose and focus information obtained through image processing was performed. First, augmented reality markers are attached to each AUPA unit for rough estimation of pose parameters. Second, using these approximate poses, the pressure distribution generated on a specific plane is estimated through thermal imaging. Finally, Bayesian optimization is employed to efficiently explore the pose parameters to minimize the error between the desired position and generated focal point. This approach enables the efficient calibration of the relative poses of AUPA units, even when they are placed in challenging-to-measure locations.

Multi-level alterable transportation of a two-dimensional near-field acoustic levitation platform

Multi-level alterable transportation of a two-dimensional near-field acoustic levitation platform

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.