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.
Highlights
The simplest and most commonly used acoustic levitator is comprised of a transmitter and an opposing reflecting surface
With the advent of acoustic metamaterials[39], which allow phase engineering on impinging sound waves using unit elements with a surface area smaller than the one of the corresponding transducers, the sound-field can be controlled with greater spatial resolution than when traditional sources are used alone[40,41]
To maintain the most generic case, we will consider a generic levitator to be comprised of one or more ultrasonic sources, distributed in a surface, and of an opposing acoustic metamaterial, which acts as a reflector
Summary
The simplest and most commonly used acoustic levitator is comprised of a transmitter and an opposing reflecting surface. With the advent of acoustic metamaterials[39], which allow phase engineering on impinging sound waves using unit elements with a surface area smaller than the one of the corresponding transducers, the sound-field can be controlled with greater spatial resolution than when traditional sources (e.g. transducers) are used alone[40,41]. Using this method, higher control on the field can be achieved between the source and a reflecting metamaterial, resulting in effects like self-bending beams[42], broadband extraordinary reflection[43] and even unidirectional transmission[44,45]. Image created using Autodesk 3ds Max 2018 and Adobe Illustrator CC 2017.0.2
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