Acoustical Society of America
134th Meeting Lay Language Papers

Popular version of paper 4pPA4
Presented Thursday afternoon, December 4, 1997
134th ASA Meeting, San Diego, CA
Embargoed until December 4, 1997

Separation is an integral part of many industries, with a prominent role in everything from the manufacture of chemicals, metals, plastics, pharmaceuticals, fossil fuels, and paper, to key roles in the treatment of waste, recycling, food processing, and biotechnology. A separation process sorts materials based on differences in one or more physical properties. For example, centrifugation is commonly used to separate blood based on differences in density, while filtration is used to separate particles by size. Other common methods of separation include distillation, evaporation, and electrophoresis (separation based on electric charge differences).

Acoustophoresis is the separation of particles using high intensity sound waves. It has long been known that high intensity standing waves of sound can exert forces on particles. A standing wave has a pressure profile which appears to "stand" still in time. The pressure profile in a standing wave varies from areas of high pressure to areas of low pressure. Standing waves are produced in acoustic resonators. Common examples of acoustic resonators include many musical wind instruments such as organ pipes, flutes, clarinets, and horns (see figure 1).

The force exerted on a particle by the standing wave depends partly on the strength and frequency of the acoustic wave, as well as the size of the particle. Furthermore, the force depends on the relative elastic properties of the particle and the liquid in the resonator. For example, consider a liquid containing particles which are more "compressible" than the liquid, such as an oil drop in water. When the sound field is turned on, the particles will feel a force which tends to push them to the nearest acoustic pressure maximum. If the particles were less compressible than the fluid, they would migrate towards the nearest acoustic pressure minimum.

The particles will form "bands" and may begin to sediment out of the liquid as the particles clump together. There are commercially available devices which combine an acoustic resonator with a steady flow of liquid through the cell. The particles become trapped by the acoustic field and are collected at the bottom of the resonator as they sediment out. The liquid flowing out of the resonator has most of the particles removed.

This example illustrates how particles can be separated from a liquid, but it is also possible to separate two or more different types of solid particles based on differences in their compressibilities and densities. Say, for example, that we have a liquid containing two types of particles which differ in elastic properties. One type of particle is more compressible than the liquid, while the other type is less compressible. When the liquid in the resonator is subjected to a strong acoustic standing wave, the forces on the two types of particles will be in opposite directions, causing them to separate and migrate towards distinct positions in the standing wave.

Acoustophoresis can be integrated into a continuous process, in which a steady flow of liquid is fed through the resonator such that the motion caused by the acoustic forces will be perpendicular to that of the flow. By the time the particles reach the output end of the resonator, they will be separated and the portions of the flow containing the different particle types can merely be picked off by one or more thin separators.

Because acoustophoresis separates particles based on differences in elastic properties, it is useful in separating particles similar in size, charge, and density which can not be distinguished by other processes. In optimizing a system for separating different types of particles, there are several considerations:

1. The fluid exerts a drag force on particles opposing their motion in the fluid. This drag force (related to the viscosity of the fluid) limits the speed at which particles can be separated by the acoustic field. The force of gravity on the particles should be considered, though it plays a small role if the particles and liquid have similar densities. The density and elastic properties of the host fluid can be modified somewhat to enhance the separation process.
2. The acoustic force on particles can be increased by using higher frequencies. Typically, biomedical ultrasound frequencies are used (millions of cycles per second). The wavelength at these frequencies is on the order of millimeters, so the resonating cells must be built quite small.
3. The force on the particles can be increased by increasing the strength of the sound field. The acoustic pressure cannot be increased without limit since high acoustic pressures can cause damage to biological cells.
4. When in an acoustic field, particles exert an additional attractive force on each other, causing them to move closer together. The attraction increases as particles move closer to one another (similar to the attraction of oppositely charged particles). This force is generally much smaller than the main acoustic force unless the particles are very close together. If particles of different types come too close before they are separated, the attractive force between particles will dominate the force trying to separate them, and the separation process will be compromised. However if the attractive forces dominate only after the particles separate, the larger clumps of same particles will feel a stronger acoustic force and the speed of separation will be enhanced.
5. The net motion of the particles is a combination of the motion in the direction of the flow, and the motion perpendicular to the flow due to the acoustic force. Typically, the fluid flow through the resonator is not uniform so that different acoustic field/ fluid flow configurations lead to different particle trajectories.

Figure 2. Calculated particle trajectories in a half-wavelength resonator with parallel walls. Fluid flow is  from left to right and the acoustic standing wave propagates in the vertical direction. Red lines represent the trajectories of 20 micron oil droplets, while green lines represent the trajectories  of 20 micron plastic spheres. The cell is tuned to 1 million cycles per second, and the acoustic pressure amplitude is 1 atmosphere. Fluid flow rate is 2.77x10^-6 m²/s.

Here at the Yale Acoustics Laboratory, we are currently exploring several cell configurations in hopes of optimizing the separation process. Results will be presented and discussed at the talk in San Diego.

[Work supported by NASA through Grant NAG8-1351.]

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