Acoustic levitation with optimized reflective metamaterials

Abstract

the simplest and most commonly used acoustic levitator is comprised of a transmitter and an opposing reflecting surface. This type of device, however, is only able to levitate objects along one direction, at distances multiple of half of a wavelength. In this work, we show how a customised reflective acoustic metamaterial enables the levitation of multiple particles, not necessarily on a line and with arbitrary mutual distances, starting with a generic input wave. We establish a heuristic optimisation technique for the design of the metamaterial, where the local height of the surface is used to introduce delay patterns to the reflected signals. Our method stands for any type and number of sources, spatial resolution of the metamaterial and system's variables (i.e. source position, phase and amplitude, metamaterial's geometry, relative position of the levitation points, etc.). Finally, we explore how the strength of multiple levitation points changes with their relative distance, demonstrating sub-wavelength field control over levitating polystyrene beads into various configurations. Since the first levitation of an object with sound, almost a century ago, the physics behind levitation and manipulation of objects with acoustic waves has been thoroughly studied 1-3. With the exception of a limited number of studies involving chemical analysis 4,5 , acoustic levitation has been mainly used to observe the dynamics of levitated objects 6-8 , including small animals 9. More recently, acoustic levitation in air has been used to display technical information 10,11 , to convey graphical messages 11,12 or to elicit novel interactions 13,14 and multi-sensory experiences 15. The standard acoustic levitator involves the creation of a standing wave 16-19 , either by two opposing ultrasonic transducers or a source and a reflector. In air, particles in a levitator would aggregate at the nodes of the field (i.e. the low-pressure points), which are typically arranged in a line between the elements, at distances of half of the wavelength of the emitted signal. The ability to freely position objects in mid-air seems therefore limited to multiples of half of a wavelength (λ/2) in the vertical direction 19-21. Conversely, acoustic self-aggregation of particles into structures with mutual distances much smaller than λ/2 has been observed experimentally in horizontal standing waves, both in air 22-24 and in water 25,26. This phenomenon, due to particle-particle interactions 26,27 , requires however particles much smaller than the wavelength (λ ∼ /100): overcoming this limit in acoustic levitators-which tend to use larger particles-requires a finer control on the acoustic field. More control on the field (e.g. focusing the acoustic energy in specific locations) can be achieved using Phased Arrays of ultrasonic Transducers (PATs) 10,12,28-31. In these setups, the position of the levitated object can be modified by adjusting either the transducers' phases 21,31 or amplitudes 32-34. Multi-object levitation in predefined positions can thus b...

Physical Modeling of the Ancient Greek Wind Musical Instrument Aulos: A Double-Reed Exciter Linked to an Acoustic Resonator

“…Moreover, by clearly describing the physical phenomena, we expect to inspire researchers to: a) improve the current model by resolving the constrain of the calculated pressure point been inside the resonator and the absence of the even harmonics (as per the theory of a closed-open pipe), b) describe musical instruments as acoustic metamaterials like various apparatus are used in other fields of acoustics [45], [46] and, c) by knowing the acoustics of ancient theatres [47], [48], making them virtually sound again as they use to in their natural auditory space.…”

Generating Airborne Ultrasonic Amplitude Patterns Using an Open Hardware Phased Array

“…In recent years, optical holographic methods have been adapted to acoustics [13,[16][17][18], opening the possibility of generating arbitrary acoustic fields that can be controlled in real time. Acoustic holography is normally achieved using either passive metamaterial structures [17,19,20] or an array of ultrasonic transducers [13,16,21]. Metamaterial structures have the main advantage of allowing for the generation of acoustic fields with a higher spatial resolution, but they cannot dynamically change the field.…”

Acoustic levitation in mid-air: Recent advances, challenges, and future perspectives

“…The phases at each point of the emitter plane can be set to generate intricate acoustic fields that trap particles at the target location, using either phased-arrays [24][25][26] with dozens of emitters [Fig. 2(a)] or passive modulators [27][28][29][30] [Fig. 2(b)].…”

“…26 When phased-arrays are employed, the levitated objects can be moved in real time; however, the reduced number of emitters leads to limitations in spatial resolution and, thus, in the ability to produce complex acoustic fields. 27,31 On the other hand, passive structures can create more intricate fields, [27][28][29][30] but they cannot dynamically change the field to move the particles. 27,29 Objects larger than the acoustic wavelength have been levitated during the past decade.…”

“…27,31 On the other hand, passive structures can create more intricate fields, [27][28][29][30] but they cannot dynamically change the field to move the particles. 27,29 Objects larger than the acoustic wavelength have been levitated during the past decade. This can be achieved either by generating a standing wave field between the emitter and the object [32][33][34] or with special vortices of high apperture.…”