Acoustic Levitation Device Research Articles

Acoustic levitation facilitates contactless movement of millimetric objects

A self-orienting, self-centering near-field acoustic levitation device controls and moves fragile electronic components without contact.

Numerical study of droplet evaporation in an acoustic levitator

We present a finite element method for the simulation of all relevant processes of the evaporation of a liquid droplet suspended in an acoustic levitation device. The mathematical model and the numerical implementation take into account heat and mass transfer across the interface between the liquid and gaseous phase and the influence of acoustic streaming on this process, as well as the displacement and deformation of the droplet due to acoustic radiation pressure. We apply this numerical method to several theoretical and experimental examples and compare our results with the well-known d2-law for the evaporation of spherical droplets and with theoretical predictions for the acoustic streaming velocity. We study the influence of acoustic streaming on the distribution of water vapor and temperature in the levitation device, with special attention to the vapor distribution in the emerging toroidal vortices. We also compare the evaporation rate of a droplet with and without acoustic streaming, as well as the evaporation rates in dependence of different temperatures and sound pressure levels. Finally, a simple model of protein inactivation due to heat damage is considered and studied for different evaporation settings and their respective influence on protein damage.

Analysis and experimental study on the effect of a resonant tube on the performance of acoustic levitation devices

The influence of a resonant tube on the performance of acoustic standing wave-based levitation device (acoustic levitation device hereinafter) is studied by analyzing the acoustic pressure and levitation force of four types of acoustic levitation devices without a resonance tube and with resonance tubes of different radii R using ANSYS and MATLAB. Introducing a resonance tube either enhances or weakens the levitation strength of acoustic levitation device, depending on the resonance tube radii. Specifically, the levitation force is improved to a maximum degree when the resonance tube radius is slightly larger than the size of the reflector end face. Furthermore, the stability of acoustic levitation device is improved to a maximum degree by introducing a resonance tube of R=1.023λ. The experimental platform and levitation force measurement system of the acoustic levitation device with concave-end-face-type emitter and reflector are developed, and the test of suspended matters and liquid drops is conducted. Results show that the Φ6.5-mm steel ball is suspended easily when the resonance tube radius is 1.023λ, and the Φ5.5-mm steel ball cannot be suspended when the resonance tube radius is 1.251λ. The levitation capability of the original acoustic levitation device without a resonance tube is weakened when a resonance tube of R=1.251λ is applied. These results are consistent with the ANSYS simulation results. The levitation time of the liquid droplet with a resonance tube of R=1.023λ is longer than without a resonance tube. This result is also supported by the MATLAB simulation results. Therefore, the performance of acoustic levitation device can be improved by introducing a resonant tube with an appropriate radius.

Acoustic levitation device for probing biological cells with high-frequency ultrasound

High-frequency ultrasound has been shown to be sensitive to changes in the cell cytoskeletal makeup during a variety of biological processes. It can therefore be used to study cellular responses such as T-cell activation and cancer metastasis. To date, obtaining cytoskeletal properties with high-frequency ultrasound has required the use of monolayer cell cultures. Ultrasonic signals from these cultures can be problematic due to the interference of reflections from the culture-plate well. The cell structure is also often deformed from its native state due to adhesion to the culture plate. The objective of this study was to develop an acoustic levitation device that creates a monolayer of cells suspended in a fluid in order to simulate their environment in vivo. By using a 250 kHz transducer, standing waves were generated in a suspension of polyethylene microspheres (53–63μm diameter) in distilled H2O. Microspheres formed layers at the standing-wave nodes. Varying the shape of the transducer voltage waveform had a significant effect on layer thickness, with a square waveform creating thinner, more distinct layers than a sine waveform due to steeper pressure gradients at the nodes. Future work will entail creating a monolayer suspension of biological cells that can be probed with high-frequency ultrasound.

A case study of real-time monitoring of solid-state phase transformations in acoustically levitated particles using near infrared and Raman spectroscopy

The objective of this study was to monitor the amorphous-to-crystalline solid-state phase transformation kinetics of the model drug ibuprofen with spectroscopic methods during acoustic levitation. Chemical and physical information was obtained by real-time near infrared (NIRS) and Raman spectroscopy measurements. The recrystallisation kinetic parameters (overall recrystallisation rate constant β and the time needed to reach 50% of the equilibrated level t50), were determined using a multivariate curve resolution approach.The acoustic levitation device coupled with non-invasive spectroscopy enabled monitoring of the recrystallisation process of the difficult-to-handle (adhesive) amorphous sample. The application of multivariate curve resolution enabled isolation of the underlying pure spectra, which corresponded well with the reference spectra of amorphous and crystalline ibuprofen. The recrystallisation kinetic parameters were estimated from the recrystallisation profiles. While the empirical recrystallisation rate constant determined by NIR and Raman spectroscopy were comparable, the lag time for recrystallisation was significantly lower with Raman spectroscopy as compared to NIRS. This observation was explained by the high energy density of the Raman laser beam, which might have led to local heating effects of the sample and thus reduced the recrystallisation onset time.It was concluded that acoustic levitation with NIR and Raman spectroscopy combined with multivariate curve resolution allowed direct determination of the recrystallisation kinetics of amorphous drugs and thus is a promising technique for monitoring solid-state phase transformations of adhesive small-sized samples during the early phase of drug development.

Analysis of the effects of the oscillations of a rigid sphere inside a cylindrical cavity containing a standing acoustic wave

Under certain driving conditions of a single‐axis acoustic levitation device, a suspended sample leaves its stability state and starts to oscillate vertically around the initial equilibrium position. A published theory on such instabilities [J. Rudnick and M. Barmatz, "Oscillational instabilities in single‐mode acoustic levitators," J. Acoust. Soc. Am., 87(1), 81‐92, 1990] predicts the occurrence of time delays between the response of the cavity of the device and the motion of the sample inside it. In this paper, a theoretical and experimental investigation on similar time delay effects will be described. A solid sphere was moved in a controlled way inside a closed cylindrical cavity by means of a rod connecting the object to the outside of the system. A standing wave was generated inside the cavity by using a speaker. In this way, oscillations of the sphere were produced and the response of the sound field to such movement was studied. The effect of the frequency of the oscillations of the sphere on the time delay between the sound pressure and the movement of that object will be reported. In addition, the relations between the obtained results and the published theory on oscillational instabilities will be discussed.

Equipment for the Spectral Characterization of High-Temperature Particles

The spectral radiant characteristics of plume particles of a solid rocket engine are important in the design of the engine specific impulse, ablative material, and plume flame hiding. These parameters are measured from tests of the engine. Some equipment has been established to realize particle heating, uniform particle distribution, and measurements based on an FTIR spectral instrument. The equipment is based on SiC heating and is divided into a warm-up chamber and a measurement chamber to improve the particle temperature stability. A special design of uniform particle distribution combined with an acoustic levitation device is used to determine the particle falling speed. The spectral characteristics and the transmission rate of the particles have been measured by using the system including a standard blackbody, an assembled optical system, and an FTIR spectrometer. The measurements of particle concentration and temperature are given in detail. The instrument specifications are as follows: temperature range – 60–1500 °C; spectral range – 0.60–25 μm; and particle dimension range – 10–500 μm.

A Portable Raman Acoustic Levitation Spectroscopic System for the Identification and Environmental Monitoring of Algal Cells

We report the coupling of a portable Raman spectrometer to an acoustic levitation device to enable environmental monitoring and the potential taxonomic identification of microalgae. Spectra of living cells were recorded at 785 nm using a fiber-optic probe coupled to a portable Raman spectrometer. The spectra exhibit an excellent signal-to-noise ratio and clearly show bands from chlorophyll a and beta-carotene. Spectra of levitated photobleached microalgae clearly show a reduction in chlorophyll a concentration relative to beta-carotene after 10 min of exposure to a quartz halogen lamp. Spectra recorded from levitated nitrogen-limited cells also show a significant reduction in bands associated with chlorophyll a, as compared to nitrogen-replete cells. To investigate the diagnostic capability of the technique, four species of microalgae were analyzed. Good quality spectra of all four species were obtained showing varying ratios of beta-carotene to chlorophyll. The combination of an acoustic levitation device and a portable Raman spectrometer shows potential as a taxonomic and environmental monitoring tool with direct application to field studies in remote environments.

Acoustic levitation device for sample pretreatment in microanalysis and trace analysis

A sample preparation device for microanalysis and trace analysis based on acoustic levitation is described. Derivatization can be performed in a single levitated drop as part of the sample pretreatment in an analytical procedure. The applicability of the experimental setup is demonstrated. First results of the preconcentration of hexachlorobenzene are given.

A new acoustic levitation device using the interference sound field generated from two opposed radiators

A new technique for acoustically levitating a specimen is described. The technique deviates from a previous single‐axis acoustic levitation method that utilizes a single‐acoustic radiator and a passive reflector to generate the interference produced sound field. The new method involves the use of two opposing focusing acoustic radiators that produce an interference field that has capabilities for the levitation and positioning of small objects or liquid drops. The theoretical predictions regarding this technique are examined and those results are compared with laboratory measurements. The nature of the apparatus and its capabilities is also addressed. [Work supported by NASA.]

Compact acoustic levitation device for studies in fluid dynamics and material science in the laboratory and microgravity

An ultrasonic levitation device operable in both ordinary ground-based as well as in potential space-borne laboratories is described together with its various applications in the fields of fluid dynamics, material science, and light scattering. Some of the phenomena which can be studied by this instrument include surface waves on freely suspended liquids, the variations of the surface tension with temperature and contamination, the deep undercooling of materials with the temperature variations of their density and viscosity, and finally some of the optical diffraction properties of transparent substances.

Parametric study of an acoustic levitation system

The fields and forces along the axis of an acoustic levitation device were measured and characterized. The device consisted of a St. Clair generator and reflector arranged in a single-axis configuration. A significant (i.e., factor of four) enhancement in the levitation forces was obtained by using concave surfaces for the driver and reflector. For bodies of various sizes and geometries, the acoustic levitation force is roughly proportional to body volume until the characteristic ’’diameter’’ reaches ∼λ/2(λ=wavelength). In addition, the behavior of materials when in the liquid state was studied. Liquid drops were found to deform and generally rupture when a characteristic size was reached. The characteristic radius appears to depend on material parameters and is from 0.2–0.5 cm for most materials on the ground.