Acoustic Levitation

Acoustic levitation is a method for suspending matter in a medium by using acoustic radiation pressure form intense sound waves in the medium. Acoustic levitation is possible because of the non-linear effects of intense sound waves.

Today this type of levitation is usually used for containerless processing which has become more important due to the small size and resistance of microchips and other things in industry. Containerless processing may also be used for application which requires high-purity materials or chemicla reactions too rigorous to happen in container. This method is very hard to control than other methods of containerless processing such as electromagnetic levitation but has an advantage of being able to levitate nonconducting materials.

But how acoustic levitation works?

The concept of levitating small particles by acoustic radiation pressure was proposed as early as 1934 by King. He was the pioneer who made theoretical foundation of acoustic levitation. However, it was not until the 1970's that acoustic drivers wer sufficiently powerful to allow practical application of acoustic levitators. As i mentioned earlier, the acoustic levitation is ued for containerless materials processing and space shuttle experiments. In a typical levitator, shown in nex picutre, an acoustic standing wave is generated between a transmitter and a reflector separated by an integral number of half wavelengths. The particle is supported against gravity by the pressure forces and tends towards a stable equilibrium position in the vicinity of the acoustic nodal planes. The reflector needs to be curved to a point within the nodal plane that lies along the axis of the device. Thus, both verical and horizontal stability is achieved with a single device.

Acoustic levitation holds several unique advantages over conventional experimental techniques (e.g. vertical wind tunnels, free fall testing, mechnaincal suspension or in situ measurement). The principal advantage of the acoustic levitation technique is the ability to support particles in a precise manner for indefinete periods of time. The stabilty in position control for a typical vertical levitator is on the other of one droplet diameter or less. This is in marked contrast to vertical wind tunnels, where the small scale of the droplet, compared with the mean and turbulet flow scales, make percies positioning difficult. The stedy and repeatable nature of the support allows invetigation of microphysical behaviour such as phase change or hydrometeor evolution which may occur on time scales of minutes. Experiments of this type are extremly difficult in vertical wind tunnels and impossible in free fall testing.

The net force on the object which is subjected to an acoustic field arises from the nonlinear realtionship between the instantaneous pressure and velocity in an acoustic medium. Under certain condition, this nonlinearity can produce significant pressure gradients over the surface of an object, resulting in an appreciable net force on it. As a result, sufficinetly strong acoustic fields can suspend object against the force of gravity, making it possible to stady the object in a contact-free environment.