Fluid acoustic forces

Another method recently developed for manipulating small particles uses the forces created by a two- or three-dimensional sound field that is excited by a vibrating plate, the surfaces of which move sinusoidally and emit an acoustic wave into a layer of fluid. Such a wave is reflected by a rigid surface and generates a standing sound field in the fluid, the forces of which act on particles by displacing them in one, two or three dimensions. In this way, particles of sizes between one and several hundred microns can be simultaneously manipulated in a contactless manner. Equations describing this behaviour have been reported [63]. 

An original method involves quadrupole oscillations of drops K The drop (a) in a host liquid (P) is acoustically levitated. This can be achieved by creating a standing acoustic wave the time-averaged second order effect of this wave gives rise to an acoustic radiation force. This drives the drop up or down in p, depending on the compressibilities of the two fluids, till gravity and acoustic forces balance. From then onwards the free droplet is, also acoustically, driven into quadrupole shape oscillations that are opposed by the capillary pressure. From the resonance frequency the interfacial tension can be computed. The authors describe the instrumentation and present some results for a number of oil-water interfaces. 

Dynamic pumps infuse energy to the fluid in a manner that increases either its momentum (centrifugal pumps) or its pressure (electroosmotic and electrohydro-dynamic pumps) as shown in Table 4. They involve centrifugal or hydrodynamic actuations and, more specifically, are driven by electro-/magneto-hydrodynamic [257], electroosmotic [254-256], electrokinetic [251-253], electrowetting [258], and acoustic forces [259]. Centrifugal pumps are typically less effective for fluids with low Reynolds numbers and have limitation in miniaturization. 

Gould R.K. and Coakley W.T., The effects of acoustic forces on small particles in suspension. In Proceeding of the 1973 Symposium of Finite Amplitude Wave Effects in Fluids, Bjprno L. (Ed.), IPC, Guilford, UK, pp. 252-257. 

Other aspects of the drop oscillation problem, such as oscillation of liquid drops immersed in another fluid [17-21], oscillations of pendant drops [22, 23], and oscillations of charged drops [24, 25], have also been considered. In particular, there are numerous works on the oscillation of acoustically levitated drops in acoustic field. In such studies, high-frequency acoustic pressnre is required to levitate the droplet and balance the buoyancy force for the experimental studies performed on the Earth. As a result of balance between buoyancy and acoustic forces, the equilibrium shape of the droplet changes from sphere to a slightly flattened oblate shape [26]. Then a modulating force with frequency close to resonant frequencies of different modes is applied to induce small to large amplitude oscillations. Figure 5.4 shows a silicon oil droplet levitated in water and driven to its first three resonant modes by an acoustic force and time evolution for each mode. 

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... 

For most applications of interest, including cells in aqueous solution, the acoustic force will act towards the pressure node however, in the case of bubbles that are smaller than resmiant size, and certain two-phase fluid mixtures, the bubble, or second phase fluid, will experience... 

When the first edition was published in 1992, the resolution of the acoustic microscope techniques used at the time was controlled by the wavelength. In practice the frequency-dependent attenuation of the acoustic wave in the coupling fluid sets a lower limit to the wavelength, and therefore to the resolution, of about 1 pm for routine applications. Since then scanning probe techniques with nanometre scale resolution have been developed along the lines of the atomic force microscope. This has resulted in the development of the ultrasonic force microscopy techniques, in which the sample is excited by... 

In82) the authors describe the behavior of a viscoelastic fluid on the surface of an acoustic vibrator. The diagram shows that the fluid located on a horizontal surface of acoustic radiator, in the area of viscoelasticity, is acted upon by forces normal for this surface the fluid swells above the radiator, takes a shape close to a spheric drop, then a thin neck is formed through which the fluid flows into the drop until it sinks to the surface under the action of gravity then this process is repeated. These phenomena had not been described earlier in literature. 

Because of the high amplitudes of particle motion in the fluid due to (1) and (2), nonlinear acoustic effects can be important. In particular, acoustic streaming can occur, so that a propagating sinusoidal wave produces a steady ( zero frequency ) force in the direction of wave propagation. This steady force causes fluids in contact with the membrane to move. 

Hydrodynamic mechanisms are those which produce particle interactions through the surrounding fluid due to hydrodynamic forces and the asymmetry of the flow field around each particle. These mechanisms, which are not dependent on the relative differences in acoustic particle entrainments, can act from distances larger than the acoustic displacement and have to be considered as the main mechanism in the agglomeration of monodispersed aerosols, where particles are equally entrained. There are two main types of hydrodynamic mechanisms, namely mutual radiation pressure [50] and the acoustic wake effect [51,52]. The radiation pressure is a second-order effect which produces a force on a particle immersed in an acoustic field due to the transfer of momentum from the acoustic wave to the particle. This force moves the particles towards the pressure node or antinode planes of the applied standing wave, depending on the size and density of the particles. The mutual radial pressure can be computed from the primary wave as well as from other wave fields of nearby scatters. In fact, it gives rise to particle interactions as the result of forces produced on two adjacent particles by a non-linear combination of incident and scattered waves. 

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