Particle separation with acoustic forces

The acoustic radiation force Facrx in thn x-direction experienced by particles of densities pp suspended in a medium of density pf has already been expressed by equation (3.1.48)  

Consider two reservoirs connected by a narrow duct with the whole volume filled with a 50-50 mixture of He-Ar. If each reservoir is connected to a bellows-sealed piston, and the pistons are driven at, say, a low frequency of 10 Hz with independent phase and amplitude control, the oscillating pressure wave creates a composition difference of as much as 6% between the two chambers at the ends of the narrow connecting duct (Spoor and Swift, 2000). The periodic motion of the pistons produces density fluctuations in the gas this sound wave creates the composition difference via a complex interaction of thermal diffusion (compression leads to heating), ordinary diffusion, convective motion, etc. (Geller and Swift, 2002a,b). 

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Ultrasonic standing wave (USW) manipulatiOTi is a simple and useful method for handling, separating, and concentrating large groups of cells. A USW creates a pressure node that will attract particles or cells. As with DEP, a cell can experience either an attractive or repulsive acoustic force depending on its material parameters. This can be used either to trap objects locally over an ultrasonic transducer, concentrate them within a fluidic channel, or separate different types of objects from each other. Successful separation of human erythrocytes from human lipid vesicles has been reported (Fig. 5). 

Two particle types positioned, by acoustic forces, in the pressure nodal and antinodal planes of a standing wave. (Cross sectirai of the channel in (b), dashed line.) (b) Top view of a continuous separation of two particle types fiom each other and/OT a fraction of their medium (Adapted from Ref [15]). (b) Human lipid particles separated from human erythrocytes at the trifurcation of 350 mm separation chip with ultrasound turned on (Adapted from Ref [16])... 

Applications have been developed whereby particles are driven acoustically from one fluid to another, making use of the laminar flow inherent in microfluidic systems. This precludes turbulence so fluids can be in physical contact with minimal mixing. Figure 4 shows the principle. A fluid containing particles is fed in through fluid 1 inlet, and the acoustic force drives the particles from fluid 1 to fluid 2. Fluid 2 can then be withdrawn from outlet 2 with the particles. The ability to wash cells and particles and to move them from one medium to another is a critical element of many forms of analysis and hence an important component of a lab-on-a-chip toolbox. A microfabricated device that works in a manner similar to that shown in Fig. 4 has been developed by Peterson et al. with applications including lipid and erythrocyte separation in... 

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