W. J. Xie

Parametric study of single-axis acoustic levitation

Remarkable enhancement of the single-axis acoustic levitation force is achieved by properly curving the surface and enlarging the section of the reflector so as to levitate high density material like tungsten (ρs=18.92 g/cm3). A two-cylinder model incorporating the boundary element method simulations is presented for systematic study of the relationship between levitation capabilities and geometric parameters. The model proves to be successful in predicting resonant modes and explaining deviation of the levitated samples near the reflector and driver. The dependence of levitation force on resonant mode, reflector section radius Rb and curvature radius R is revealed and summarized, which agrees with the experiment in principle and suggests that a reflector with large Rb and small R (when Rb/λ⩾0.982) working under mode 1 assures better levitation capabilities.

Acoustic method for levitation of small living animals

Ultrasonic levitation of some small living animals such as ant, ladybug, and young fish has been achieved with a single-axis acoustic levitator. The vitality of ant and ladybug is not evidently influenced during the acoustic levitation, whereas that of the young fish is reduced because of the inadequacy of water supply. Numerical analysis shows that the sound pressures on the ladybug’s surface almost reach the incident pressure amplitude p0 due to sound scattering. It is estimated that 99.98% of the acoustic energy is reflected away from the ladybug. The acoustic radiation pressure pa on the ladybug’s surface is only 1%–3% of p0, which plays a compression role on the central region and a suction role on the peripheral region.Ultrasonic levitation of some small living animals such as ant, ladybug, and young fish has been achieved with a single-axis acoustic levitator. The vitality of ant and ladybug is not evidently influenced during the acoustic levitation, whereas that of the young fish is reduced because of the inadequacy of water supply. Numerical analysis shows that the sound pressures on the ladybug’s surface almost reach the incident pressure amplitude p0 due to sound scattering. It is estimated that 99.98% of the acoustic energy is reflected away from the ladybug. The acoustic radiation pressure pa on the ladybug’s surface is only 1%–3% of p0, which plays a compression role on the central region and a suction role on the peripheral region.

Acoustic levitation with self-adaptive flexible reflectors

Two kinds of flexible reflectors are proposed and examined in this paper to improve the stability of single-axis acoustic levitator, especially in the case of levitating high-density and high-temperature samples. One kind is those with a deformable reflecting surface, and the other kind is those with an elastic support, both of which are self-adaptive to the change of acoustic radiation pressure. High-density materials such as iridium (density 22.6 gcm(-3)) are stably levitated at room temperature with a soft reflector made of colloid as well as a rigid reflector supported by a spring. In addition, the containerless melting and solidification of binary In-Bi eutectic alloy (melting point 345.8 K) and ternary Ag-Cu-Ge eutectic alloy (melting point 812 K) are successfully achieved by applying the elastically supported reflector with the assistance of a laser beam.

Microstructural characterization of cobalt–antimony eutectic alloy droplets solidified in drop tube

Abstract Co–Sb alloy droplets have been solidified during free fall in a 3 m drop tube. The cooling rate and undercooling are estimated versus droplet diameter, both of which play a dominant part in dendritic and eutectic growth. The eutectic coupled zone has been calculated on the basis of TMK and LKT/BCT theories, which covers a composition range from 22.7% to 49%Sb. A ‘lamellar-anomalous’ eutectic transition occurs to Co-41.4%Sb droplets with the increase of undercooling. For Co-20%Sb hypoeutectic alloy, primary α-Co phase experiences a ‘dendritic to equiaxed’ structural morphology transition and an abrupt grain refinement at a critical undercooling of about 88 K under drop tube conditions. The remaining interdendritic liquid does not solidify in a mode of eutectic growth but forms into a divorced structure under drop tube conditions. The primary β-CoSb intermetallic compound of Co-45%Sb hypereutectic alloy grows in a dendritic way inside droplet and forms as a spherical shell on droplet surface. Most Co-45%Sb droplets undercooled by 80∼130 K exhibit complete eutectic microstructures. Significant solute trapping takes place in primary α-Co and β-CoSb dendrites.

Temperature dependence of single-axis acoustic levitation

The temperature-dependent physical conditions for single-axis acoustic levitation are theoretically analyzed with consideration of the deviation of the actual acoustic field from the plane standing wave approximation. The effects of temperature variation on the resonant conditions, levitation force and threshold pressures pm (the minimum entrapping pressure) and pM (the maximum pressure to keep the integration of a liquid drop) are discussed by assuming a quasi-static heating and cooling process. The first resonant spacing H1 between the reflector and emitter is larger than that predicted for plane standing waves, and its temperature dependence comes mainly from the variation of wavelength, which is proportional to T1/2. The maximum levitation force FM has a drastic decreasing tendency with temperature rise due to its sensitivity to the ratios of the geometric parameters to wavelength. For the containerless processing of water and the Pb–Sn eutectic alloy, pm decreases whereas pM increases with the enhanc...

Interaction of acoustic levitation field with liquid reflecting surface

Single-axis acoustic levitation of substances, such as foam, water, polymer, and aluminum, is achieved by employing various liquids as the sound reflectors. The interaction of acoustic levitation field with liquid reflecting surface is investigated theoretically by considering the deformation of the liquid surface under acoustic radiation pressure. Numerical calculations indicate that the deformation degree of the reflecting surface shows a direct proportion to the acoustic radiation power. Appropriate deformation is beneficial whereas excessive deformation is unfavorable to enhance the levitation capability. Typically, the levitation capability with water reflector is smaller than that with the concave rigid reflector but slightly larger than that with the planar rigid reflector at low emitter vibration intensity. Liquid reflectors with larger surface tension and higher density behave more closely to the planar rigid reflector.

A numerical simulation of acoustic field within liquids subject to three orthogonal ultrasounds

When one beam of ultrasound propagates along a single direction in liquids, the cavitation effect is always confined to a limited volume close to the ultrasonic source. This greatly limits the application of power ultrasound in liquid processing and materials fabrication. In this study, a methodology for applying three orthogonal ultrasounds within liquids has been proposed. By solving the Helmholtz equation, the sound field distribution characteristics are investigated in 1D (one dimensional), 2D (two dimensional) and 3D (three dimensional) ultrasounds at their resonant frequencies, which show that the coherent interaction of three beams of ultrasounds is able to strikingly promote the sound pressure level and reinforce the mean acoustic energy density as compared with that in 1D case. Hence, the potential cavitation volume is enlarged remarkably. This opens new possibilities for the design and optimization of ultrasonic technology in fabricating materials.

Sound field inside acoustically levitated spherical drop

The sound field inside an acoustically levitated small spherical water drop (radius of 1mm) is studied under different incident sound pressures (amplitude p0=2735–5643Pa). The transmitted pressure ptr in the drop shows a plane standing wave, which varies mainly in the vertical direction, and distributes almost uniformly in the horizontal direction. The maximum of ptr is always located at the lowermost point of the levitated drop. Whereas the secondary maximum appears at the uppermost point if the incident pressure amplitude p0 is higher than an intermediate value (3044Pa), in which there exists a pressure nodal surface in the drop interior. The value of the maximum ptr lies in a narrow range of 2489–3173Pa, which has a lower limit of 2489Pa when p0=3044Pa. The secondary maximum of ptr is rather small and only remarkable at high incident pressures.

Multiple-lobed bifurcation of rotating liquid drops levitated by ultrasound

It is previously predicted that the equilibrium shape of a rotating liquid drop evolves from the axisymmetric to the two-, three-, and four-lobed morphologies as the angular velocity increases. Although the two- and three-lobed shape bifurcations have been observed in experiments, the four-lobed equilibrium shape is scarcely reported. Here, we investigate the multiple-lobed shape bifurcations of rotating drops by using acoustic levitation, and in particular, follow the evolution of the four-lobed equilibrium shape. A new shape family of rotating drops characterized by five-lobed bifurcation is also observed, which is unexpected in the theoretical predictions. A numerical method is employed to simulate the shape evolution of acoustically levitated and rotating drops. And the results validate the existence of bifurcation point shifts among all the lobed-shape families due to the initial drop flattening induced by the acoustic radiation pressure, which plays a decisive role in the emergence of the five-lobed...