An acoustic levitation test bench was built to successfully suspend a small ball of a certain density. It was found that the smaller the diameter and density of the suspended sample, the easier it is to suspend. T he diameter of the sample should not be larger than 1/4 - 1/3 of the wavelength of the acoustic wave. In addition, as the sound pressure increases, the sound flow also affects the stability of the acoustic levitation. Comsol was used to nu-merical simulation the spatial sound field, pressure field and particle ve-locity distribution of different suspension cavity heights, and the factors affecting the acoustic levitation capacity were analyzed.
Acoustic levitation technology is mainly used to simulate various effects in the space environment, and better solve people’s increasingly high demands on the experimental environment. Although the space resources that people have obtained so far are still very limited, the ultrasonic levitation technology is a good direction to create a vacuum environment similar to gravity-free, can also solve many chemical pharmaceuticals, no container processing, liquid solidification, precision instrument processing and other aspects. Regarding suspension, people may hear the most maglev train. In fact, the physical methods for achieving object suspension are various, such as: pneumatic suspension, electromagnetic suspension, acoustic suspension, optical suspension, electrostatic suspension, superconducting suspension, etc. Acoustic suspension is the use of the force generated by sound waves to resist gravity and achieve object suspension. The acoustic suspension does not need to consider the nature of the suspended object. Theoretically, it can suspend particles of any density. It is the first time in the world to suspend tungsten with a density of up to 18.9 g/cm3 [
Ultrasonic standing wave suspension is a high-frequency piston vibration generated by the radiation end of the ultrasonic transducer [
It is usually required that the size of the suspended object ( R s ) is much smaller than the half wavelength (λ/2) [
position, there is a tendency to be pulled back to the original position. These positions are the floating position of the sample. Usually, the sound is fluctuated. The direction is parallel to the direction of gravity to overcome the gravity of the sample. For heavier objects, the suspension position will be slightly below the sound pressure node. Experimental setup:
This paper selects 28 khz ultrasonic transducer and generator circuit, 10 mm thickness reflection end, variable height longitudinal expansion platform, caliper, the particles used in the experiment are 2 - 3 mm polyethylene particles, the experimental results are as follows: white small particles uniform Suspended in the resonant cavity between the reflective end face and the ultrasonic transducer, the distance of the resonant cavity is 60 mm, which is equal to 5 times the wavelength. The wavelength is calculated as the following Formula (1):
λ = c f (1)
The experimental results can be seen as shown in
U = 2 π R s 3 ( P 2 ¯ 3 ρ f c 2 − ρ f v 2 ¯ 2 ) (2)
In this paper, the acoustic levitation force is simulated by the comsol software.
The selected model is a two-dimensional simplified model of an acoustic levator driven at a constant frequency. Small elastic particles are translated in the acoustic standing wave field, and their motion paths are detected, which are affected by acoustic radiation force, viscous force, and gravity. The model requires particle tracking module parameters as shown in
We want to study the distribution of acoustic levitation force, so we choose air particles model as the medium to characterize its distribution. In the calculation, we use dimensionless parameters. The dimensionless levitation force and restoring force coefficient are the quantities related only to the geometric parameters of the reflecting end face and the emitting end face. Therefore, the relationship between them and the geometric parameters are studied. To help us optimize the simulation analysis of the uniaxial suspension device, we calculated the potential energy of the air particle suspension position suspended at an integer multiple of half wavelength. The sound pressure equation used is as follows:
∇ ⋅ ( − 1 ρ c ( ∇ ρ t − q d ) ) − k e q 2 p t ρ c = Q m (3)
p t = p + p b (4)
k e q 2 = ( ω C c ) 2 − k z 2 (5)
C c = C , ρ c = ρ (6)
− n ⋅ ( − 1 ρ c ( ∇ p t − q d ) ) = 0 (7)
The simulation results are as follows: the sound pressure in the middle position is the largest, which is in line with our experimental results, that is, the position
name | expression | value | description |
---|---|---|---|
f0 | 28 [kHz] | 28000 Hz | Driving frequency |
c0 | 343 [m/s] | 343 m/s | Speed of sound |
lambda0 | c0/f0 | 0.01225 m | Wavelength |
Dt | 2 * lambda0 | 0.0245 m | Transducer diameter |
Dr | 3 * lambda0 | 0.03675 m | Reflector diameter |
H | 5 * lambda0/2 | 0.030625 m | Height |
dvisc | 0.22 [mm] * sqrt(100 [Hz]/f0) | 1.3148E−5 m | Viscous boundary layer thickness |
d_p | lambda0/10 | 0.001225 m | Particle diameter |
rho_p | 500 [kg/m3] | 500 kg/m3 | Particle density |
rho_f | 1.2 [kg/m3] | 1.2 kg/m3 | Fluid density (air) |
mu_f | 1.8e−5 [Pa * s] | 1.8E−5 Pa・s | Fluid viscosity (air) |
a0 | 1.5e6 [m/s2] | 1.5E6 m/s² | Normal acceleration of transducer |
of the suspended ball is in the blue position of the simulation, and the small ball particles suspended at the position have a pressure difference between the upper and lower sides. 4. Due to the powerful effect of the acoustic pressure [
Sound pressure level analysis [
caused by the influence of particle vibration on the reflective end face, and the sound pressure level changes from high to low and then from low to high. The pressure level is the smallest, and the ball acting at this position will achieve suspension.
P ( y , t ) = P max ⋅ cos ( k ⋅ y ) ⋅ sin ( ω ⋅ t ) (8)
The acceleration of the simulated air particle plum movement is calculated by the acceleration formula of the sound pressure sound pressure level particle. The motion trajectory simulation is shown in
negative directions. Offset each other, layered at half wavelength. Since the simulation calculates a dynamic process, the example reaches a stable stratification state when 0.3 s, so 0.3 s is selected as the schematic diagram.
It is found through experiments that the suspended sample is affected by its own diameter and density. The diameter of the sample should not be larger than 1/4 - 1/3 of the wavelength of the acoustic wave. In addition, through simulation, it is concluded that with the increase of sound pressure, the sound flow will also affect the stability of sound suspension. The distribution of sound pressure level will change from large to small and then from small to large. The distribution trajectory provides the basis for future research.
The authors declare no conflicts of interest regarding the publication of this paper.
Zhang, F.Q. and Jin, Z.L. (2018) The Experiment of Acoustic Levitation and the Analysis by Simulation. Open Access Library Journal, 5: e4948. https://doi.org/10.4236/oalib.1104948