Acoustic levitation of a Mie sphere using a 2D transducer array

Abstract

Most acoustic levitation techniques are limited to objects smaller than half the wavelength. To overcome this limit, different strategies have been proposed for suspending macroscopic objects in mid-air. Two approaches to levitate spherical and non-spherical macroscopic objects have been recently presented: the acoustical virtual vortices and the boundary hologram method. However, the former approach places high demands on the available hardware due to the mandatory high switching rate while the latter uses a computationally expensive model that prevents future real-time manipulation. In the present work, we demonstrate the single-beam levitation of a Mie sphere using a 2D transducer array. To achieve this, we employ a computationally fast sound field model based on spherical harmonics expansion. To obtain a suitable array output, we formulate an optimization problem that maximizes the stability of the sphere while keeping the net force balanced. In addition, we prove the local asymptotic stability for the equilibrium position and determine a domain of attraction using Lyapunov-based methods. In experiments, we show that the macroscopic sphere is stably levitated in a twin tuning forks trap, which results from a superposition of two twin trap signatures and a bottle trap signature. This result could open up the possibility of a computationally fast and convenient non-contact manipulation of macroscopic objects by a superposition of holographic elements in future applications.

Evaporation issues of acoustically levitated fuel droplets

“…Despite the existence of advanced simulation techniques for droplet evaporation, many phenomena remain challenging to explain. Hence, depending on the study's actual targets, it may be essential to develop acoustic suspension technology, such as overcoming limits of droplet size in a levitator [54] and using advanced sensors to measure small droplet evaporation [112] , [113] , conducting the appropriate experiments, or verify the evaporation model. There are several potential applications for acoustic suspension technology, such as controlling droplet shape, size, fragmentation, coalescence, detecting droplet viscosity and surface tension, etc.…”

“…The levitated droplets are tiny and usually limited to half of the wavelength [52] , [53] , [54] , , where is the sound velocity depending on the ambient temperature, while is the frequency [55] . Therefore, high temperatures can be implicitly influenced the performance of the levitator [22] , [56] .…”

Acoustic manipulation of multi-body structures and dynamics

“…This work has shown that, for particular sizes, Mie particles can be stably levitated by plane-waves in pressure anti-nodes, but off-axis (in contrast to Rayleigh particles, which levitate on-axis and in pressure nodes). More generally, the stable levitation of Mie particles has focused, especially on the experimental side, on shaping the incident wavefront via computational methods [54][55][56][57][58].…”

“…Although a parallel analysis has been proposed and analyzed theoretically for ray acoustics [161,162], the range of particle sound absorbency and beam conditions that permit stable levitation appears to be very small. Additionally, current experimental techniques for acoustic levitation in the Mie limit have so far successfully levitated only objects of size smaller than three wavelengths, far from the regime where such an analysis would be appropriate [54,55,163,164]. Nevertheless, the principle of redirected acoustic momentum flux can be used to levitate and exert forces on particles with size of order λ.…”

Position and Orientation Control of Complex-Shaped Samples in Acoustic Levitation

“…Although objects larger than the wavelength have been levitated, controlled levitation has been limited to sub-wavelength sample size with few exceptions, such as spheres and cubes. Previously larger-than-wavelength sample levitation was achieved using the sample as reflector [6], a multifrequency high aperture vortex trap [7], or by direct optimization of the radiation force acting on a sphere [8]. Near-field levitation achieves larger-than-wavelength levitation in close proximity to the transducer.…”