Acoustic levitation

Containerless processing may also be used for applications requiring very-high-purity materials or chemical reactions too rigorous to happen in a container.

[7] More recently the development of phased array transducer boards have allowed more arbitrary dynamic control of multiple particles and droplets at once.

The "TinyLev" is an acoustic levitator which can be constructed with widely available, low-cost off-the-shelf components, and a single 3D printed frame.

The experiment in a resonant chamber demonstrated that the particles could be gathered at the nodes of a standing wave by the acoustic radiation forces.

[14] Taylor Wang was the leader of a team which made significant use of acoustic radiation forces as a containment mechanism in zero gravity, taking a device up on the Space Shuttle Challenger mission STS-51-B to investigate the behaviour of levitated droplets in micro-gravity.

[7] A new generation of acoustic levitators employing a large number of small individual piezoelectric-transducers have recently become more common.

The use of multiple small sources was initially designed as a cost saving measure, but also opened the door for phased array levitation, discussed below.

[27] This new approach also led to significant developments using Phased Array Ultrasonic Transducers[9][8] (often referred to as PATs) for levitation.

Unlike their counterparts in the non-destructive testing or imaging fields, these arrays will use a continuous output, as opposed to short bursts of energy.

They are often favoured for research into the dynamics of levitated objects due to the simplicity of their geometry and subsequent ease of simulation[36] and control of experimental factors.

[52] There is also a significant amount of work into combining these techniques with 3D printed phase shifting components for advantages such as passive field forming[29][31][32] or higher spatial resolution.

Whilst PATs are common it has also been shown that Chladni Plates can be used as a single standing wave source to manipulate levitated objects by changing the frequency.

Acoustic levitation provides a container-less environment for droplet drying experiments to study liquid evaporation and particle formation.

The levitation of surface mount electrical components has been demonstrated[12][46] as has micro-assembly with a combination of acoustic and magnetic fields.

A Langevin horn type standing wave acoustic levitator at the Argonne National Laboratory
A drawing of the Kundt's tube experiment. The movement of the particles due to the acoustic radiation forces were the first demonstration of the possibility of acoustic levitation.
A TinyLev acoustic levitator including the electronics and a diagram of the peak pressure field.
A single beam acoustic levitator using a vortex trap to levitate an expanded polystyrene particle approximately twice the size of the wavelength. The vortices are alternated rapidly in direction to avoid spinning the particle to the point of instability. [ 53 ] Here 450 transducers at 40kHz are used.
A selection of acoustically Levitated Objects in a TinyLev including solids, liquids, an ant and an electrical component. All in the size range of 2 mm-6 mm. [ 12 ]
(Left) Images of acoustically levitated droplets during liquid evaporation and particle formation. (Right) X-ray microtomography gives insights into the final particle 3D structure. [ 58 ]
An acoustophoretic volumetric display where a small expanded polystyrene particle is rapidly moved with light projected onto it to produce the image of a 'stop sign'. This is a composite image taken over 20 seconds. [ 68 ]