Acoustic Levitation Research Articles

Single droplets to particles - size, shape, shell thickness and porosity analyses using X-ray computed tomography

The drying kinetics of single droplets of crystallizing and skin-forming species were investigated via acoustic levitation to provide a well-controlled environment mimicking spray drying operations. Particles manufactured by drying aqueous solution droplets containing varying ratios of D-mannitol and polyvinylpyrrolidone (PVP) were analysed using a range of techniques to elucidate their morphology and microstructure. Size, porosity and surface roughness of the manufactured particles increased with D-mannitol concentration in water, while the sphericity decreased, a consequence of the unique morphologies. The change in particle shell thickness was found to be minimal. Considering varying proportions of PVP led to a variety of fascinating structures, contrasting to those manufactured from pure D-mannitol. Temperature effects were further considered, and the results were matched by that from a lab and pilot scale spray dryer. Analysis of the solid form further demonstrates the opportunity to prepare different polymorphic forms of D-mannitol.

Validation of a diffusion-based single droplet drying model for encapsulation of a viral-vectored vaccine using an acoustic levitator

Development of thermally stable spray dried viral-vectored vaccine powders is dependent on the selection of a proper excipient or excipient blend for encapsulation, which can be a time and resource intensive process. In this work, a diffusion-based droplet drying model was developed to compute droplet drying time, size, and component distribution. The model predictions were validated using an acoustic levitator to dry droplets containing protein-coated or fluorescently labelled silica nanoparticles (as adenoviral vector analogues) and a range of excipient blends. Surface morphology of the dried particles was characterized by atomic force microscopy and the distribution of silica nanoparticles was quantified by confocal microscopy. The modelled distributions of adenovirus agreed with the microscopy results for three mannitol/dextran excipient blends with varying molecular weight dextrans, verifying the equations and assumptions of the model. Viral vector activity data for adenovirus in a range of (poly)saccharide/sugar alcohol formulations were also compared to the model outputs, suggesting that viral activity decreases when the model predicts increasing adenovirus concentrations near the air–solid interface. Using a validated model with excipient property inputs that are readily available in the literature can facilitate the development of viral-vectored vaccines by identifying promising excipients without the need for experimentation.

Dynamic Acoustic Levitator Based On Subwavelength Aperture Control.

Acoustic levitation provides a means to achieve contactless manipulation of fragile materials and biological samples. Most acoustic levitators rely on complex electronic hardware and software to shape the acoustic field and realize their dynamic operation. Here, the authors introduce a dynamic acoustic levitator that is based on mechanically controlling the opening and (partial) closing of subwavelength apertures. This simple approach relies on the use of a single ultrasonic transducer and is shown to permit the facile and reliable manipulation of a variety targets ranging from solid particles, to fluid and ferrofluidic drops. Experimental observations agree well with numerical simulations of the Gor'kov potential. Remarkably, this system even enables the generation of time‐varying potentials and induces oscillatory and rotational motion in the levitated objects via a feedback mechanism between the trapped object and the trapping potential. This is shown to result in long distance translation, in‐situ rotation and self‐modulated oscillation of the trapped particles. In addition, dense ferrofluidic droplets are levitated and transformed inside the levitator. Controlling subwavelength apertures opens the possibility to realize simple powerful levitators that nevertheless allow for the versatile dynamic manipulation of levitated matter.

Dynamic Invariance in Near-Field Acoustic Levitation

Abstract It is possible to levitate a mass by vibrating a flat disk located under the mass. This near-field acoustic levitation is particularly useful for eliminating friction between moving objects. This paper presents an experimental and theoretical study of the dynamic behavior of a levitating mass for different magnitudes of oscillation of the flat disk. The magnitude of the vibration of the mass appears to be independent of the amplitude of the vibration of the disk over a range of two orders of magnitude. This unusual behavior is due to the simultaneous changes of the stiffness of the air film and the natural frequency of the system as the plate vibration is changed. As the plate oscillation is reduced, the distance to resonance decreases, allowing an increase of the ratio of the output to input signals in such a way that the output remains constant. This result can be useful for improving the energy efficiency of the levitation.

Measurement of Water Mole Fraction from Acoustically Levitated Pure Water and Protein Water Solution Droplets via Tunable Diode Laser Absorption Spectroscopy (TDLAS) at 1.37 µm

In order to understand the evaporation and particle formation processes of sprays in technical applications such as fuel injectors or drying processes in the food and pharmaceutical industries in detail, single droplet drying experiments, for example, acoustic levitation, are widely used as model experiments. We combined acoustic levitation and tunable diode laser absorption spectroscopy (TDLAS) to measure the absolute H2O concentration in the exhaust gas of a levitation chamber to investigate drying and particle formation processes from single droplets of pure water and protein–water solutions. To that end, we designed and developed a non-invasive, calibration-free TDLAS-based hygrometer to analyze the 1.4 µm overtone band. To increase the detection range of the developed hygrometer and to track the complete drying process of protein solution droplets even after the critical point of drying, the absorption length was extended to a path length of 18 m using an astigmatic multipass cell of the Herriott type. The setup was validated by drying pure water droplets, resulting in a determination of the water mole fraction in a range from 73 ppm to 1314 ppm, with a single scan resolution of 1.7 ppm. For protein solution droplets, the entire drying process, even beyond the critical point of drying, can be tracked and the different phases of the drying process can be characterized at different drying temperatures.

Spray Drying of Cellulose Nanofibers: Drying Kinetics Modeling of a Single Droplet and Particle Formation

AbstractMicroparticles embedded with cellulose nanofibers (CNFs) are produced via spray drying. The microdroplets are created in the spray dryer by atomization. The solvent evaporates from the droplet, and the dried particles are formed. The drying kinetics on the scale of one droplet is still not well understood. Drying of a single droplet CNF suspension was investigated in an acoustic levitator. The drying kinetics and morphology evolution during the drying process were obtained. The reaction engineering approach (REA) model provided a reasonable prediction of the drying kinetics. An absolute relative error of 2.5 % between the measured initial mass of the droplet and the REA model's estimated value was reported.

Stiffness modeling for near field acoustic levitation bearings

The dynamic characteristics of near field levitation bearings have been investigated in this study. Through theoretical analysis, two different types of system stiffness are defined and derived analytically. The dynamic stiffness relates the excitation amplitude to the dynamic force amplitude, while the effective stiffness governs the time-averaged force–displacement relationship. The results indicate two non-linear and asymmetric spring constants that can effectively predict levitation force and height. The models are verified with a carefully designed experimental setup to eliminate the structural resonance effect. Besides, some unique dynamic behaviors are investigated and predicted based on the proposed stiffness model.

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.

Development of multi-degree-of-freedom noncontact transportation using holding force in near-filed acoustic levitation and consideration of factors of unstable operation

A non-contact transport method using near-field acoustic levitation has been studied. In this research, the levitated object is transported by multiple stators in steps. This transport method requires a large number of stators, but it has the advantage of precisef positioning because the levitated object is stopped and held on each stator. In addition, by arranging stators in two dimensions, transportation in two-dimensional direction is realized, but stable operation is not obtained, such as unintended rotation in transportation. Through experiments and analysis, we clarified under what conditions the transported object rotates.

Self-organization and culture of Mesenchymal Stem Cell spheroids in acoustic levitation

In recent years, 3D cell culture models such as spheroid or organoid technologies have known important developments. Many studies have shown that 3D cultures exhibit better biomimetic properties compared to 2D cultures. These properties are important for in-vitro modeling systems, as well as for in-vivo cell therapies and tissue engineering approaches. A reliable use of 3D cellular models still requires standardized protocols with well-controlled and reproducible parameters. To address this challenge, a robust and scaffold-free approach is proposed, which relies on multi-trap acoustic levitation. This technology is successfully applied to Mesenchymal Stem Cells (MSCs) maintained in acoustic levitation over a 24-h period. During the culture, MSCs spontaneously self-organized from cell sheets to cell spheroids with a characteristic time of about 10 h. Each acoustofluidic chip could contain up to 30 spheroids in acoustic levitation and four chips could be ran in parallel, leading to the production of 120 spheroids per experiment. Various biological characterizations showed that the cells inside the spheroids were viable, maintained the expression of their cell surface markers and had a higher differentiation capacity compared to standard 2D culture conditions. These results open the path to long-time cell culture in acoustic levitation of cell sheets or spheroids for any type of cells.

Acoustic levitation and high-resolution synchrotron X-ray powder diffraction: A fast screening approach of niclosamide amorphous solid dispersions

The levitation of samples in an acoustic field has been of interest in the preparation and study of amorphous solid dispersions (ASD). Here, niclosamide-polymer solutions were levitated in a multi-emitter single-axis acoustic levitator and analyzed for 10 min at a High-resolution synchrotron X-ray powder diffraction beamline. This assembly enabled high-quality and fast time-resolved measurements with microliter sample size and measurement of solvent evaporation and recrystallization of niclosamide (NCL). Polymers HPMCP-55S, HPMCP-50, HPMCP-55, Klucel®, and poloxamers were not able to form amorphous dispersions with NCL. Plasdone® and Soluplus® demonstrated excellent properties to form NCL amorphous dispersions, with the last showing superior solubility enhancement. Furthermore, this fast levitation polymer screening showed good agreement with results obtained by conventional solvent evaporation screening evaluated for five days in a stability study, carried out at 40 °C/75% RH. The study showed that acoustic levitation and high-resolution synchrotron combination opens up a new horizon with great potential for accelerating ASD formulation screening and analysis.

Acoustic levitation of a rigid nano-sphere at non-continuum conditions

Abstract

Design and Performance of an Acoustic Levitator System Coupled with a Tunable Monochromatic Light Source and a Raman Spectrometer for In Situ Reaction Monitoring.

The design and performance of a custom-built reaction chamber combined with an acoustic levitator, a tunable monochromatic light source, and a Raman spectrometer are reported. The pressure-compatible reaction chamber was vacuum-tested and coupled with the acoustic levitator that allows contactless sample handling, free of contingent sample requirements such as charge and refractive index. The calibration and performance of the Raman spectrometer was studied utilizing gated detection and three different gratings that can be interchanged within seconds for a desired resolution and photon collection range. A wide range of 186–5000 cm–1 Raman shift, with a small uncertainty of ±2 cm–1, can be recorded covering a complete vibrational range in chemical reaction monitoring. The gating of the detector allowed operation under the room light and filtration of unwanted sample fluorescence. The in situ reaction perturbation and monitoring of physical and chemical changes of samples by the Raman system were demonstrated by degradation of polystyrene by monochromatic UV light and photobleaching of a potato slice by visible light. This instrument provides a versatile platform for in situ investigation of surface reactions, without external support structures and under controlled pressure and radiation conditions, relevant to various disciplines such as materials science, astrochemistry, and molecular biology.

Quantitative Analysis of Pharmaceutical Drugs Using a Combination of Acoustic Levitation and High Resolution Mass Spectrometry.

A combination of acoustic levitation, laser vaporization, and atmospheric pressure chemical ionization mass spectrometry (APCI-MS) is presented in this study that enabled sensitive analysis of pharmaceutical drugs from an aqueous sample matrix. An unfocused pulsed infrared laser provided contactless sample desorption from the droplets trapped inside an acoustic levitator by activation of the OH stretching band of aqueous and alcoholic solvents. Subsequent atmospheric pressure chemical ionization was used between the levitated droplet and the mass spectrometer for postionization. In this setup, the unfocused laser gently desorbed the analytes by applying very mild repulsive forces. Detailed plume formation studies by temporally resolved schlieren experiments were used to characterize the liquid gas transition in this process. In addition, the role of different additives and solvent composition was examined during the ionization process. The analytical application of the technique and the proof-of-concept for quantitative analysis were demonstrated by the determination of selected pharmaceutical drugs in aqueous matrix with limits of quantification at the lower nanomolar level and a linear dynamic range of 3-4 orders of magnitude.

Design approach of perforated labyrinth-based acoustic metasurface for selective acoustic levitation manipulation

This paper proposes a metasurface design approach with perforated labyrinthine path coil structure to manipulate the acoustic transmission with inexpensive materials. The medium in the labyrinthine path coils in this design is air, but not limited to air. A systematic approach has been proposed for the unit cell design of acoustic metamaterials with adjustable resonance peak frequencies and bandgap width. The theory demonstrates that the length of pipe segments determines resonance peak frequencies and the cross-sectional area ratio adjusts the bandgap width. The proposed design approach uses an equivalent pipe circuit based analytical model to design the high transmission (high pass) and high reflection (low pass) unit cell. The simulation and experiment has been performed to evaluate the validity of the theory. Although there exists some assumptions in the theory, the theory still has enough accuracy to guide the metasurface design illustrated by the simulation and experiment results.

Reflective acoustic metamaterial for dynamic axial sub-wavelength field control in acoustic levitation

Acoustic levitation is a non-contact manipulation techniques used in a wide range of fields from biology, physics to medical research. The manipulation of levitated objects can be achieved by controlling the phase of each transducer in a phased array or by using reflective metamaterial. Reflective metamaterial is the acoustic analogy of spatial light modulators, and horizontal movement of the levitated object may be achieved by physical translation of the metamaterial itself. However, axial movement relative to the reflective metamaterial has not been enabled. Here, we propose a reflective metamaterial for dynamic sub-wavelength field control to realize the axial movement of the levitated object, using the property that levitation points are created at distances multiple of half of a wavelength relative to the reflector in a sound field with sufficient sound pressure. It was shown that by mounting the triangular prisms with multiple of half of a wavelength on a reflector and moving them horizontally, the object would rise/fall by the height of the prism. The simple design of reflectors using the properties of acoustic levitation, paves the way to expand the capability of reflective acoustic metamaterial. [Funded by Strategic Research Platform towards Digital Nature Powered by Pixie Dust Technologies, Inc.]

Acoustic levitation and the acoustic radiation force

We investigate acoustic levitation in a vertical standing wave in an attempt to understand the basic physical mechanism responsible for this phenomenon. We find that a description in terms of a simple pressure force leads to the prediction of stable equilibria that occur slightly below the anti-nodes of the standing pressure wave. We then demonstrate that such a prediction is at odds with experimental data, which show that levitating particles come to rest slightly below the nodes of the standing pressure wave. Finally, we outline a theoretical approach based on fluid dynamics that correctly predicts the locations of the levitating particles, which leads to a simple qualitative description for this fascinating phenomenon.

Automatic differentiation approach for acoustic holograms: Performance and potential applications

Acoustic holography underpins the number of modern acoustics applications. The usage of multi-focal acoustic holograms with phased array transducers (PATs) in ultrasonic tactile displays and acoustic levitation are increasing, and achieving a high-quality hologram are of significant interest. Inspired by the latest optical hologram optimizer, we developed a novel acoustic hologram optimizer for PATs called Diff-PAT [T. Fushimi, K. Yamamoto, and Y. Ochiai, “Acoustic hologram optimisation using automatic differentiation”]. Diff-PAT is based on a gradient-descent algorithm and automatic differentiation. The performance of Diff-PAT was numerically evaluated using three array configurations and it achieved superior accuracy over the conventional optimizers. For example, when two focal points are generated with 196 ultrasonic (40 kHz) transducers; the state of art optimizer (with phase and amplitude modulation) based on Eigensolver has an average error of 4.07 % where Diff-PAT (with only phase modulation) achieve average error of 0.0174%. These results show that amplitude and phase modulation may not be necessary for PATs to be successful. Furthermore, we demonstrate the versatility of Diff-PAT by applying it to binary acoustic holograms, simultaneous modulation of acoustic amplitude and phase at the target plane, and phase plates. [Work supported by Pixie Dust Technologies, Inc., Diff-PAT is patent pending (JP2020-167367A).]

Special topic on Drops and Flows in Acoustic Levitation

Special topic on Drops and Flows in Acoustic Levitation

Broadband, frequency-independent acoustic metasurfaces through inverse design

The recent development of acoustic metasurfaces has broadened the capabilities of wave engineering devices with a compact profile. However, the application of acoustic metasurfaces in many scenarios has been limited by their bandwidths. In this work, we report the realization of broadband acoustic metasurfaces that deliver frequency-independent functionalities. Three examples are demonstrated, namely beam steering, focusing and acoustic levitation with fixed refraction angle, constant focal depth and stable levitation. This is achieved by analyzing the requirement of each functionality and engineering the dispersion of the unit cells. The architecture of the unit cells is obtained through inverse design and each unit cell displays a unique dispersive response outlined by the theoretical requirements. The designed metasurface is experimentally verified and we demonstrate a fractional bandwidth of around 100% for all three functionalities. We also attempt to reveal the mechanism for such broadband by characterizing the behavior of representative unit cells.