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Moving Particles with Sound Waves

Sound waves can be powerful - powerful enough to move large groups of particles. Recently, scientists at the Chinese Academy of Sciences announced an acoustic sieve that can sift, move, trap, or align large groups of particles by size and density under water.

They began by focusing an ultrasound beam on a flat plate. The brass plate has a grating on top made of materials (phononic crystals) specifically for manipulating acoustic waves. Below the plate is a container of glass spheres of various sizes and densities. When the ultrasound beam turns on, it creates a localized field underneath the plate that can trap the particles.

The experimental setup used, where an ultrasound beam is focused on a plate to trap particles. Image Credit: Phononic-Crystal-Based Acoustic Sieve for Tunable Manipulations of Particles by a Highly Localized Radiation Force

The plate moves closer to the particles, and scientists can manipulate the particles as they wish.

Changing the voltage driving the ultrasound beam allows the plate to handle the particles differently and affect how they behave. The video below shows the scientists sifting glass and tin particles apart (PCP indicates the plate used in the experiment).

Currently, the most popular method of acoustic trapping only isolates and moves a few particles at a time using standing waves. One of the easiest ways to visualize trapping particles this way is with a Chladni plate. These are metal plates covered in sand, driven by a frequency generator. When the generator vibrates the plate, the sand settles into the nodes and antinodes created as the plate moves.

Chladni plates are a great way to visualize how nodes and antinodes are created on vibrating surfaces.
Image Credit: Wikipedia
For acoustic traps, they use the idea of nodes to capture particles between different nodes created by the sound waves. But this method can only trap a few particles at a time.

This new method is notable because it can handle larger quantities of particles, and is able to effectively sift through this larger quantity. One of the most obvious applications for this technology is 3-D printing. Printers could separate materials and deposit them in precise layers on a target. But if this technology continues to develop, it can be applied to many situations like delivering targeted drug therapy or sifting debris out of fluids necessary in drilling.

The paper and supplemental materials are published in Physical Review Applied.


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