On the acoustic levitation stability behaviour of spherical and ellipsoidal particles

Abstract

We present here an in-depth analysis of particle levitation stability and the role of the radial and axial forces exerted on fixed spherical and ellipsoidal particles levitated in an axisymmetric acoustic levitator, over a wide range of particle sizes and surrounding medium viscosities. We show that the stability behaviour of a levitated particle in an axisymmetric levitator is unequivocally connected to the radial forces: the loss of levitation stability is always due to the change of the radial force sign from positive to negative. It is found that the axial force exerted on a sphere of radius R s increases with increasing viscosity for R s /λ < 0.0125 (λ is the acoustic wavelength), with the viscous contribution of this force scaling with the inverse of the sphere radius. The axial force decreases with increasing viscosity for spheres with R s /λ > 0.0125. The radial force, on the other hand, decreases monotonically with increasing viscosity. The radial and axial forces exerted on an ellipsoidal particle are larger than those exerted on a volume-equivalent sphere, up to the point where the ellipsoid starts to act as an obstacle to the formation of the standing wave in the levitator chamber.

Aerosol manipulation by acoustic tunable phase-control at resonant frequency

“…Due to the size 0.08-1 μm of aerosols and the wavelength 13.12 cm of acoustic wave in our experiment, the regime of aerosol manipulation using ASW agrees with the Rayleigh regime [6]. Because the size of aerosols used by us is much smaller than the wavelength of acoustic wave [12], once the shape of aerosols is equivalent to compressible sphere, the acoustic radiation force, F, on a single, small sphere can be given by Gor'kov [28].…”

“…A number of papers [6,7,11,12,17,21,[27][28][29][30] on particle manipulation using ASW have indicated that aerosols in ASW field are exerted an external acoustic radiation force. The acoustic radiation force results from the acoustic radiation pressure [11,31].…”

“…Other relative important researches are the acoustic manipulation [9] and levitation [10][11][12] of micro-and macro scale particles in gas. Karpul et al [9] designed industrial acoustic filters to manipulate particles, and showed the advantage of optimizing operating and structure factors of acoustic filter on achieving better manipulation.…”

Acoustophoretic contactless transport and handling of matter in air

“…An additional acoustic medium volume was added to the model to avoid the influence of the nonreflecting boundary on the core of the levitator. The model was validated against the experimental values of force acting on a sphere inside an axisymmetrical levitator (30). The implemented quasistatic model suffices for our analysis.…”

“…The radiation pressure acting on a sphere in a standing wave becomes fully established after a period of τ rad = N · τ wave , with N being the number of acoustic wave cycles needed to reach steady state and τ wave = f −1 = 41 μs being the wave period. N was determined numerically to be between 50 and 100 (30,31). The characteristic time of a sample transported at an average linear speed _ x is defined as τ mov = λ= _ x , and for our case, λ∼d → τ mov ∼ T .…”