Experimental study of the oscillation of spheres in an acoustic levitator

Abstract

The spontaneous oscillation of solid spheres in a single-axis acoustic levitator is experimentally investigated by using a high speed camera to record the position of the levitated sphere as a function of time. The oscillations in the axial and radial directions are systematically studied by changing the sphere density and the acoustic pressure amplitude. In order to interpret the experimental results, a simple model based on a spring-mass system is applied in the analysis of the sphere oscillatory behavior. This model requires the knowledge of the acoustic pressure distribution, which was obtained numerically by using a linear finite element method (FEM). Additionally, the linear acoustic pressure distribution obtained by FEM was compared with that measured with a laser Doppler vibrometer. The comparison between numerical and experimental pressure distributions shows good agreement for low values of pressure amplitude. When the pressure amplitude is increased, the acoustic pressure distribution becomes nonlinear, producing harmonics of the fundamental frequency. The experimental results of the spheres oscillations for low pressure amplitudes are consistent with the results predicted by the simple model based on a spring-mass system.

Automatic contactless injection, transportation, merging, and ejection of droplets with a multifocal point acoustic levitator

“…This oscillation persists for few seconds, until it is damped by the drag forces of air and the droplet reaches the equilibrium position (i.e., the position of minimum potential). As described in the literature, [40][41][42] the acoustic radiation force acting on the droplets can be approximated by the restoring force of a spring and the oscillations can be described by making an analogy with a damped harmonic oscillator. The merging operation was achieved by a procedure similar to that described by Watanabe et al, 28 but instead of generating the focal points by rapidly switching the phases of all the transducers, our focusing scheme divided the transducers into two sets to obtain two simultaneous and independent focus points; i.e.…”

Acoustic levitation of an object larger than the acoustic wavelength

“…In this device, an acoustic standing wave is established between a sound emitter and a concave reflector, in which a small particle is trapped around the pressure node due to the action of the acoustic radiation force 3,10 . The acoustic force behavior in the neighborhood of a pressure node is similar to the restoring force of a spring 22,23 . For small distances from the pressure node, the acoustic radiation force Frad acting on a small particle is proportional to its distance from the pressure node.…”

“…(c) H = 8.25 mm with the object located at y = 0. (d) H = 8.25 mm with object located at y = 2 mm.For small lateral displacements, the lateral acoustic restoring force that acts on the object is similar to the restoring force of a spring22,23 . For our levitated object of 2.3 g, the levitation height predicted by the simulation is H = 8.25 mm.…”

Nonlinear Behavior of a Suspended Particle in Single-Axis Acoustic Levitators

“…Recent studies have related that samples in acoustic levitation do not remain static but oscillate in a particular trajectory around the equilibrium point 9,10 . Under low pressure amplitudes, usually located near the transducer or reflector in a single -axis levitator, the experimental results show that sphere oscillations can be described by the simple spring-mass system 9 . However, this model cannot be applied for samples levitating at the central node, because of prominent nonlinear effects in this region.…”