Acoustic Levitation Research Articles

Drop Dissolution Intensified by Acoustic Levitation.

Acoustic levitation can provide significant benefits for many fundamental research questions. However, it is important to consider that the acoustic field influences the measurement environment. This work focuses on the dissolution of immobilised drops using acoustic levitation in liquid-liquid systems. Previous work demonstrated that the acoustic field of standing waves impacts mass transfer by affecting the spread of dissolved substances in the continuous phase in two distinct ways: (I) solutes may either pass through nodal planes of the standing waves or (II) not pass. The binary systems examined for case (I) are 1-hexanol-water and 1-butanol-water, and for case (II), n-butyl acetate-water and toluene-water. This work quantifies the intensification effect of acoustic levitation on dissolution for the two types of behaviour, by comparing them with reference measurements of mechanically attached dissolving drops. The system was designed to ensure minimal intensification. The minimum intensification of mass transfer for levitating drops in the used setup of case (I) was 25%, and for case (II), it was 65%, both increasing with decreasing surface-equivalent diameter. With this understanding, acoustic levitation can be used more accurately in the field of mass transfer studies.

Effects and selection of update rates in acoustic levitator

Abstract Acoustic manipulation holds excellent potential for applications in life sciences, medicine, physics, and contactless measurement with non-contact, versatility, and safety advantages. The update rate (control frequency) plays a critical role in determining the performance of acoustic manipulation. However, few studies have investigated this aspect. To address this gap, this paper investigated the effects and selection of the update rate in acoustic manipulation by analyzing the dynamic characteristics of the levitated object and discussing the hardware constraints. The results revealed that the update rate significantly impacts manipulation performance. It is closely related to the rise time, defined as the duration for a system response to rise from zero to its final value. Simulations and physical experiments verified this conclusion. Furthermore, we found that when the update rate is less than the reciprocal of the rise time, an increase in the update rate leads to a significant improvement in performance, with a monotonically increasing relationship. This implies that the update rate can be selected according to the rise time. It is recommended that the update rate be chosen beyond the reciprocal of the rise time, for optimal performance. These findings will help optimize acoustic manipulation performance and facilitate further development and application of acoustic manipulation technology.

Non-Contact Multi-Degree-of-Freedom Motor Based on Hybrid Electromagnetic-Piezoelectric Drive Mode

A new non-contact ultrasonic motor consisting of a Langevin transducer, an electromagnetic device, and a spherical rotor is presented, and the designed motor is theoretically analysis and experimentally verified. The designed motor is driven by a mixture of near-field acoustic levitation and electromagnetism, and the electromagnetic platform is controlled by three stacked piezoelectric actuators to control the deflection direction, thus driving the spherical rotor to achieve the same angle of deflection and self-propagation. By exciting the Langevin transducer under the rotor, the high-frequency vibration of the stator disc causes the air between the stator disc and the rotor to be squeezed periodically, and when the air pressure in the gap is larger than the external atmospheric pressure, the levitation force generated by the stator is larger than the gravity of the rotor, thus levitating the rotor, and when the rotor deflects, it can still achieve stable levitation because of its special geometry. The proposed new motor is expected to be used in applications requiring high output torque and micro-displacement.

Exploring Colloidal Phase Transitions of Imogolite Nanotubes by Evaporation Induced Self‐Assembly in Levitation

AbstractEvaporation‐induced self‐assembly (EISA) is a versatile method for generating organized superstructures from colloidal particles, offering diverse design possibilities through the manipulation of colloid size, shape, substrate nature, and environmental conditions. While some work highlighted the potential of EISA to investigate phase transitions of inorganic liquid crystals, the influence of sample environment to determine their phase diagrams is often overlooked. In this work, the self‐assembly of lyotropic liquid crystals is compared by EISA on substrates, and by acoustic levitation (absence of substrate). The focus is on imogolite nanotubes, a model colloidal system of 1D charged objects, due to their tunable morphology and rich liquid‐crystalline phase behavior. It demonstrates the feasibility to obtain phase transitions in levitating droplets and on soft hydrophobic substrates, whereas self‐assembly is limited on rigid hydrophilic supports. Moreover, the aspect ratio of the nanotubes proves to be a pivotal factor, influencing both transitions and the resulting materials shape and surface. Besides material shaping, acoustic levitation emerges as a promising method for studying phase transitions by EISA, toward the rapid establishment of phase diagrams from diluted to highly concentrated states using a limited volume of sample.

Diversity of solitonic wave structures to the M‐truncated dynamical system in ultrasound imaging

The utilization of ultrasound imaging has become extensively prevalent and well‐established in clinical practice. The fundamental technologies that serve as the foundation for various applications in the field include transducers, beam shaping, pulse compression, tissue harmonic imaging, contrast agents, methodologies for quantifying blood flow and tissue motion, and three‐dimensional imaging. This article focuses on the examination of ultrasonic propagation, which involves the transmission of mechanical vibrations within the molecules or particles of a material. It quantifies the velocity of sound propagation in the medium of air. The third‐order nonlinear M‐fractional Westervelt model has been used as a governing model in the imaging process for securing the different wave structures. The recently developed computational methods have been applied in this study. The different wave structures are secured in various forms of solitary wave solutions including bright, dark, and combo solitons. In the domains of medical imaging and therapy, the investigation of sound wave propagation and high‐amplitude phenomena is facilitated by the utilization of wave structures. The effectiveness of these solutions extends to acoustic cavitation, acoustic levitation, underwater acoustics, and facilitating the process of ultrasonic propagation in tissue. Ultrasound imaging technologies currently find application in the medical field, enabling the visualization and examination of internal human tissue. This technology exhibits a wide array of applications in the fields of industry and medicine. A representation of the graphs is produced using the appropriate parametric values. The results suggest that the chosen approaches exhibit effectiveness, viability, and adaptability when implemented in complex systems in various fields, with particular emphasis on ultrasonic imaging.

Inertial mixing of acoustically levitated droplets for time‐lapse protein crystallography

AbstractVarying the chemical consistency of acoustically levitated droplets opens up an in situ study of chemical and biochemical reactions in small volumes. However, the optimization of the mixing time and the minimization of the positional instability induced by solution dispensing are necessary for practical applications such as the study of the transient state of macromolecules crystallography during the ligand binding processes. For this purpose, we study the inertial mixing in a configuration compatible with the room‐temperature crystallography using the acoustic levitation diffractometer, therein solution drops ejected at high velocity collide and coalesce with droplets dispensed on acoustically levitated and rotating polymer thin‐film sample holders. With the proposed method, we are able to achieve the mixing time of ∼0.1 s for sub‐micro and a few microliter droplets. The observed short mixing time is ascribed to the rapid penetration of the solution into the droplets and confirmed by a computational fluid dynamic simulation. The demonstrated accelerated solution mixing is tested in a pilot time‐lapse protein crystallography experiment using the acoustic levitation diffractometer. The results indicate the detection of transient ligand binding state within 2 s after the solution dispensing, suggesting the feasibility of the proposed method for studying slow biochemical processes.

Acoustic levitation combined with laboratory-based small-angle X-ray scattering (SAXS) to probe changes in crystallinity and molecular organisation.

Single particle levitation techniques allow us to probe samples in a contactless way, negating the effect that surfaces could have on processes such as crystallisation and phase transitions. Small-angle X-ray scattering (SAXS) is a common method characterising the nanoscale order in aggregates such as colloidal, crystalline and liquid crystalline systems. Here, we present a laboratory-based small-angle X-ray scattering (SAXS) setup combined with acoustic levitation. The capability of this technique is highlighted and compared with synchrotron-based levitation-SAXS and X-ray diffraction. We were able to follow the deliquescence and crystallisation of sucrose, a commonly used compound for the study of viscous atmospheric aerosols. The observed increased rate of the deliquescence-crystallisation transitions on repeated cycling could suggest the formation of a glassy sucrose phase. We also followed a reversible phase transition in an oleic acid-based lyotropic liquid crystal system under controlled humidity changes. Our results demonstrate that the coupling of acoustic levitation with an offline SAXS instrument is feasible, and that the time resolution and data quality are sufficient to draw physically meaningful conclusions. There is a wide range of potential applications including topics such as atmospheric aerosol chemistry, materials science, crystallisation and aerosol spray drying.

Laser Doppler Vibrometer measurements of the 3D pressure field for the estimation of the radiation force produced by an acoustic standing-wave levitator

Acoustic levitation devices use powerful ultrasonic standing waves to levitate objects in mid-air. We have created a system and method to measure the full harmonic content of the acoustic field accurately. Our study revealed that levitated particles alter the pressure field in a way that is not immediately apparent from the first harmonic alone, highlighting the importance of precise and non-invasive measurement techniques. Our method uses computational tomography and optical sensing with the acousto-optic effect and a 3 DOF mechanical scanner. Our experiment results in a detailed 3D pressure distribution map for each harmonic of the ultrasonic acoustic field that generates the levitation forces.

The dynamics of vertical coalescence of acoustically levitated droplets

Mobility manipulation of liquid droplets is an important task of digital imicrofluidics. Acoustic levitation has revolutionised the contactless manipulation of liquid droplets for various applications. Acoustic levitation technique can be effectively used to manipulate droplets to obtain their coalescence. This paper reports a unique, versatile, and material-independent approach for the vertical coalescence of the droplets suspended in an acoustic levitator. The acoustic power of the levitator is carefully engineered to obtain vertical coalescence of two liquid droplets. Water, 20% and 40% glycerol–water solutions are used as the working liquids. The results of the experiments revealed three outcomes during the coalescence. The outcomes are analysed and discussed.

Enhancing acoustic levitation capacity through array geometry optimization

The contactless micromanipulation capability and microgravity environment provided by the acoustic levitation technology have excellent application prospects in many fields, such as droplet dynamics, solidification state physics, life sciences, etc. However, the applications are severely constrained by the limitation of the transducer power. In this paper, we explore an array geometry optimization framework to improve the levitation capability. First, the acoustic pressure attenuation and directional characteristics of commercial transducers are analyzed to uncover the mechanism of the array geometry affecting the acoustic radiation force (ARF). Second, based on the Gor'kov potential theory, the array geometry form of N-elements is derived, and the arrangement in the spherical ring (ASR) method is proposed to obtain the element coordinates. Then, spherical radius and array height are adjusted to obtain the optimal array geometry. On this basis, the directional angle of each transducer is further optimized. Finally, simulation and physical verification are carried out. The simulation results indicate that the ARF increases and then decreases with the spherical radius and array height, which are the existing maximum values. Directional angle optimization (DAO) can improve the ARF, particularly when the array height exceeds the sphere diameter. These findings are consistent with the analytical results. Notably, we built a prototype physical system, and the physical experiments showed that the optimized array improved the axial ARF by 201.2% and the radial ARF by 115.9% over the conventional array. The works presented in this paper offer a valuable design approach for acoustic levitation arrays, thereby contributing to the advancement and application of ultrasonic array levitation technology.

Primary dendrite growth within binary Fe71Ge29 eutectic alloy under duplex levitation states

The primary β-Fe3Ge2 dendrite growth kinetics within liquid Fe71Ge29 eutectic alloy was studied by both acoustic levitation and electrostatic levitation techniques, with maximum experimental undercoolings of 130 and 143 K, respectively. At small undercoolings, (α1 + β-Fe3Ge2) eutectic growth proceeded and then transformed to lamellar (ε-Fe3Ge + β-Fe3Ge2) microstructure by peritectoid reaction. Once liquid undercooling reached 56 K, β primary phase started to nucleate preferentially and its maximum growth velocity attained 13.5 mm/s at 143 K undercooling. By acoustic levitation processing, β dendrites were distributed inside the alloy droplet. Under electrostatic levitation state, β dendrites were distributed both at the periphery and within the interior of alloy droplet, and their volume fraction was significantly higher than that under acoustic levitation. Numerical simulation results indicated that a duplex flow was induced by alloy droplet shape oscillation and acoustic streaming. The flow exhibited maximum intensity near the alloy surface, which inhibited the achievement of larger undercoolings during acoustic levitation.

Customized and high-performing acoustic levitators for contact-free experiments

Acoustic levitators are becoming increasingly common research instrumentation for contact-free, lab-in-a-droplet studies. Recently, levitators that employ multiple, small, ultrasonic transducers have gained popularity, given their low price, temperature and spatial stability, low voltage, and accessibility. Yet, the current state-of-the-art device, TinyLev, presents limitations for certain applications in terms of stability, strength, and compactness. Herein, we developed three new levitators and evaluated the effect of the construction parameters (e.g., distance of opposing arrays, number and arrangement of transducers, etc.) on their performance. The best performing levitator from this work had half the number of transducers, compared to TinyLev, though presented 1.7 and 3.5 times higher levitation capacity along the horizontal and vertical configurations, respectively, and 4.7 and 2.0 times higher horizontal and vertical stability of a levitated object, respectively. Additionally, we present a direct means to evaluate the acoustic radiation net force acting on a deformable object for uniaxial levitators, without the use of a microphone or a schlieren deflectometer for this type of levitators. The theoretical and experimental observations provide insights for adapting the acoustic levitator design for specific applications. Finally, we developed an open-source software which allows the evaluation of the acoustic pressure field generated by customized designs and provides the necessary files for 3D printing the scaffold of the levitator. This study aims to increase accessibility and promote further developments in contact-free experiments.

Acoustic Levitation Synthesis of Ultrahigh‐Density Spherical Nucleic Acid Architectures for Specific SERS Analysis

AbstractControllably regulating the electrostatic bilayer of nanogold colloids is a significant premise for synthesizing spherical nucleic acid (SNA) and building ordered plasmonic architectures. We develop a facile acoustic levitation reactor to universally synthesize SNAs with an ultra‐high density of DNA strands, which is even higher than those of various state‐of‐the‐art methods. Results reveal a new mechanism of DNA grafting via acoustic wave that can reconfigure the ligands on colloidal surfaces. The acoustic levitation reactor enables substrate‐free three‐dimentional (3D) spatial assembly of SNAs with controllable interparticle nanogaps through regulating DNA lengths. This kind of architecture may overcome the plasmonic enhancement limits by blocking electron tunneling and breaking electrostatic shielding in dried aggregations. Finite element simulations support the architecture with 3D spatial plasmonic hotspot matrix, and its ultrahigh surface‐enhanced Raman scattering (SERS) capability is evidenced by in situ untargeted tracking of biomolecular events during photothermal stimulation (PTS)‐induced cell death process. For biomarker diagnosis, the conjugation of adenosine triphosphate (ATP) aptamer onto SNAs enables in situ targeted tracking of ATP during PTS‐induced cell death process. Particularly, the CD71 receptor and integrin α3β1 protein on PL45 cell membrance could be well distinguished by label‐free SERS fingerprints when using specific XQ‐2d and DML‐7 aptamers, respectively, to synthesize SNA architectures. Our current acoustic levitation reactor offers a new method for synthesizing SNAs and enables both targeted and untargeted SERS analysis for tracking molecular events in living systems. It promises great potentials in biochemical synthesis and sensing in future.

Acoustic Levitation Synthesis of Ultrahigh-Density Spherical Nucleic Acid Architectures for Specific SERS Analysis.

Controllably regulating the electrostatic bilayer of nanogold colloids is a significant premise for synthesizing spherical nucleic acid (SNA) and building ordered plasmonic architectures. We develop a facile acoustic levitation reactor to universally synthesize SNAs with an ultra-high density of DNA strands, which is even higher than those of various state-of-the-art methods. Results reveal a new mechanism of DNA grafting via acoustic wave that can reconfigure the ligands on colloidal surfaces. The acoustic levitation reactor enables substrate-free three-dimentional (3D) spatial assembly of SNAs with controllable interparticle nanogaps through regulating DNA lengths. This kind of architecture may overcome the plasmonic enhancement limits by blocking electron tunneling and breaking electrostatic shielding in dried aggregations. Finite element simulations support the architecture with 3D spatial plasmonic hotspot matrix, and its ultrahigh surface-enhanced Raman scattering (SERS) capability is evidenced by in situ untargeted tracking of biomolecular events during photothermal stimulation (PTS)-induced cell death process. For biomarker diagnosis, the conjugation of adenosine triphosphate (ATP) aptamer onto SNAs enables in situ targeted tracking of ATP during PTS-induced cell death process. Particularly, the CD71 receptor and integrin α3β1 protein on PL45 cell membrance could be well distinguished by label-free SERS fingerprints when using specific XQ-2d and DML-7 aptamers, respectively, to synthesize SNA architectures. Our current acoustic levitation reactor offers a new method for synthesizing SNAs and enables both targeted and untargeted SERS analysis for tracking molecular events in living systems. It promises great potentials in biochemical synthesis and sensing in future.

Model-based feedforward control for an optimized manipulation of acoustically levitated spheres

We present a simple dynamic model for predicting the manipulation behavior of an acoustically levitated sphere. The model allows for the calculation of the sphere position over time, which is demonstrated for two manipulation strategies: a straight motion with a constant manipulation velocity and a straight motion in which the sphere acceleration follows a cosine function. The dynamic model as well as the manipulation strategies is verified experimentally in an acoustic levitator system consisting of an array of 16 by 16 ultrasonic transducers emitting at 40 kHz and an opposing reflector. In this system, a glass sphere of a diameter of 2 mm is manipulated horizontally by controlling the phases of the transducers. The sphere motion is recorded using a high-speed camera, and a tracking algorithm is used for capturing the sphere position over time. Moreover, a model predictive control algorithm is applied on a path-following problem to move the sphere along a given reference trajectory by means of a model-based optimal feedforward control. The proposed dynamic model as well as the methodology presented in this paper enables faster manipulation speeds with reduced oscillations during object movement.

Evaporation of acoustically levitated bicomponent droplets: mass and heat transfer characteristics

Evaporation of multicomponent droplets is important, albeit complex, in a wide range of applications, and requires a careful investigation. We investigate experimentally and numerically the evaporation characteristics of spherical, ethanol–water droplets with different initial concentration ratios in the acoustic levitation field. Imaging techniques and infrared thermometry are used for acquiring volume and surface temperature variations of droplets, reflecting their mass and heat transfer characteristics. Numerical simulations are conducted using modified parameters based on a theoretical model to consider the effect of the acoustic field. The calculation results show good agreement with the experimental data. The concentration and temperature distribution within the droplet is further investigated based on the numerical results.

Optical fiber-based acoustic intensity microphone for high-intensity airborne ultrasound measurement

The increasing use of airborne ultrasonic waves in daily life, driven by advances in parametric and phased arrays, has led to innovative applications like highly directional speakers, non-contact tactile feedback, 3D acoustic levitation, and medical therapies. These advancements necessitate accurate measurement of high-intensity ultrasonic waves, exceeding the capability of traditional microphones limited to around 160 dB, and highlight the growing importance of measuring the sound field not merely as scalar (sound pressure) but as vector (acoustic intensity) to accommodate future technological developments. This paper introduces an acoustic intensity microphone using optical fibers as probes to overcome these limitations. The proposed method replaces the two ordinary microphones used in the traditional acoustic intensity measurement method with thin optical fibers, minimizing sound field disturbance. Experimental validation and the structure of a practical acoustic intensity microphone are discussed, building upon foundational work presented at USE2023 with added verification and insights.

Continuous Perfusion Experiments on 3D Cell Proliferation in Acoustic Levitation.

An acoustofluidic trap is used for accurate 3D cell proliferation and cell function analysis in levitation. The prototype trap can be integrated with any microscope setup, allowing continuous perfusion experiments with temperature and flow control under optical inspection. To describe the trap function, we present a mathematical and FEM-based COMSOL model for the acoustic mode that defines the nodal position of trapped objects in the spherical cavity aligned with the microscope field of view and depth of field. Continuous perfusion experiments were conducted in sterile conditions over 55 h with a K562 cell line, allowing for deterministic monitoring. The acoustofluidic platform allows for rational in vitro cell testing imitating in vivo conditions such as cell function tests or cell-cell interactions.

Extraordinary stability of surfactant‐free bubbles suspended in ultrasound

AbstractGravity‐induced drainage is one of the main destabilizing mechanisms for soap bubbles and foams. Here we show that solely through acoustic levitation without introducing any chemical stabilizers, liquid drainage in the bubble film can be completely inhibited, therefore leading to a significant enhancement of bubble lifetime by more than two orders of magnitude and enabling the bubble to survive puncturing by a needle. Based on sound simulation and force analysis, it has been found that acoustic radiation force, exerted on both the inner and outer surfaces of the levitated bubble, acts in opposite directions, thus providing a squeezing effect to the bubble film. The hydrostatic pressure that induces drainage has been balanced by the acoustic radiation pressure exerted on both sides of the film, which is at the origin of the sound stabilization mechanism. This study provides new insights into the interplay between sound and soap bubbles or films, thus stimulating a wide range of fundamental research concerning bubble films and expanding their applications in bio/chemical reactors.

Airborne Ouzo: Evaporation-induced emulsification and phase separation dynamics of ternary droplets in acoustic levitation

We experimentally investigated the evaporation dynamics of acoustically levitated Ouzo droplets (a mixture of ethanol, water, and anise oil). Acoustic levitation has gained significant attention in various fields owing to its potential to create a lab-in-a-drop. Although evaporation is a key process in nature and industry, many studies have focused on single and binary components, and ternary droplets in acoustic levitation have rarely been experimentally investigated. In this study, the evaporation-induced spontaneous emulsification (the Ouzo effect) and phase separation process at 40–90 vol. % ethanol were visualized. We estimated the concentration change by evaporation of each component in the levitated ternary droplets based on an evaporation model to determine the experimental results. Our experimental results revealed four distinct stages of evaporation in levitated Ouzo droplets: (1) preferential evaporation of the volatile component (ethanol), (2) spontaneous emulsification (myriad micro-oil droplets generation), (3) phase separation forming a core–shell droplet, and (4) water evaporation completion resulting in the remaining oil droplets. Finally, we analyzed the emulsification process using a spacetime diagram. These findings suggest that acoustic levitation is suitable for physicochemical manipulation in mid-air.