- Background of the invention: The present invention relates to a method and apparatus for collecting, manipulating, or separating particles from a fluid suspension. In particular, the present invention relates to an acoustically driven method and apparatus for separating fine particles from a fluid suspension. A variety of processes are used for separation of particles from fluid suspensions. None of the conventional separation processes has proved practicable for separation of fine (on the order of 1 .mu.m) particles. Conventional packed bed systems are subjected to prohibitively large pressure drops when processing fine particles. Mixer-settler contactors require extremely prolonged sedimentation steps when processing fine particles. Fluidized bed processes have proven unsuitable because fine particles are too susceptible to entrainment in the outflowing fluid. The efficiency of cyclone separation systems is too sensitive to particle size. For example, the optimum diameter of a cyclone processing 5 .mu.m mineral particles suspended in water would be about 1 cm, which is impractical for processes of any significant scale. Acoustic methods recently have shown promise for solving the problems of separating fine particles from fluid suspensions. Under appropriate conditions, a standing acoustic wave imposed on a fluid suspension containing fine particles will drive the particles to the nodes (positions of minimum range of acoustic pressure) and trap them there. An example of a separation method employing this principle is disclosed in U.S. Pat. No. 4,055,491, in which an ultrasonic generator is energized to set up a standing acoustic wave in a chamber containing blood and trap blood cells at the nodes of the standing wave. After the generator is de-activated, the blood cells settle downwardly in response to the acceleration of gravity to an outlet at the bottom of the chamber. Although acoustic waves have been shown to be effective in trapping fine particles in a fluid suspension, the methods heretofore used for trapping and removing particles have not exhibited the speed desirable for large-scale commercial processing. For example, the blood cell-separation method of U.S. Pat. No. 4,055,491 requires energizing the ultrasonic generator for twenty seconds to trap the blood cells and de-energizing the generator for five seconds to permit the trapped particles to settle out of the space subjected to the acoustic field. For the successful commercial application of acoustically aided separation methods, it is desirable both to accelerate the trapping of particles and to provide a removal scheme that does not require de-energizing the acoustic field. The present invention is intended to provide an acoustically driven particle separation method that quickly traps fine particles throughout a cell. The present invention also is intended to provide an acoustically driven particle separation method that can trap and remove fine particles simultaneously.