Acoustic force field

Figure 4.2.8. (a) Schematic illustration of a particle mixture in a closed chamber. No acoustic force field is present in the chamber. The suspended particles have negative (white) and positive (dark) -factors. 

Acoustic force An acoustic field used to enhance the evaporation, coagulation, or condensation of particulate matter. 

The soot formation and its control was studied in an annular diffusion flame using laser diagnostics and hot wire anemometry [17, 18]. Air and fuel were independently acoustically forced. The forcing altered the mean and turbulent flow field and introduced coherent vortices into the flow. This allowed complete control of fuel injection into the incipient vortex shedding process. The experiments showed that soot formation in the flame was controlled by changing the timing of fuel injection relative to air vortex roll-up. When fuel was injected into a fully developed vortex, islands of unmixed fuel inside the air-vortex core led to... 

Dense-phase fluidization can also be conducted in the presence of force fields other than a gravitational field. Such force fields include vibrational, acoustic, centrifugal, and magnetic fields. Operations with applications of these fields are known, respectively, as vibrofluidized beds [Mori et al., 1992], acoustic fluidized beds [Montz et al., 1988 Chirone et al., 1992],... 

Recently Maret et al. (JL) have observed the longitudinal acoustic mode in oriented DNA fibres and films in a Brillouin scattering experiment. They observed the largest acoustic velocity for the driest samples, smallest for wet samples and at all times the observed velocity was larger than that for water itself. We have assumed that the velocities for propagation parallel to the helix axis are characteristic of acoustic modes in the DNA double helix and have used these values along with previously refined valence force field parameters (2,3) to fit the non-bonded force constants for the double helix. 

Acoustic-FFF was proposed and experimentally verified by preliminary measurements by Semyonov and Maslow applying a standing acoustic wave field as external force [286]. The particles can be pressed against the wall or be focused within the channel depending on the sign of the adiabatic compressibility difference between sample and solvent. The difference between the sample adiabatic compressibilities or alternatively the particle size can be determined from this FFF technique. 

The calculation of vibration spectra in terms of force constants is similar to the calculation of energy bands in terms of interatomic matrix elements. Force constants based upon elasticity lead to optical modes, as well as acoustical modes, in reasonable accord with experiment, the principal error being in transverse acoustical modes. The depression of these frequencies can be understood in terms of long-range electronic forces, which were omitted in calculations tising the valence force field. The calculation of specific heat in terms of the vibration spectrum can be greatly simplified by making a natural Einstein approximation. 

Force Modulation Mode Contact Electric Force Microscopy (EFM) Young s Modulus Microscopy (YMM) Scanning near-field acoustic Force-distance measurements... 

There is a clear trend today within the bioanalytical and biomedical fields toward more frequent use of cell-based studies. The dimensions of microfluidic systems are well matched to meet the demand on cell-based systems. Still, new methods are needed that can efficiently handle and manipulate cells in those formats. Examples have already been given in this chapter where acoustic forces are used to trap and manipulate cells. The device in Figure 44.22 has been forther developed for use in cell-based bioassays. The temperature characteristics of the device have been examined to be able to control the temperature during the cell experiments. The major source of heat in the acoustic resonance systems presented here is the power dissipation in the transducer itself. The power dissipation follows a... 

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. 

Particle Positions Under Gravity and Other Forces Yosioka and Kawasima also considered the direction in which an acoustic force will act on a particle in suspension. The equilibrium position of a particle within the field will be determined by the resolution of this acoustic force and any other forces acting on the particle. Typically this would be gravity, although it applies to any force. In the case of a planar resonant field in which the positive x-axis points vertically upwards, the particle will be in equilibrium when... 

Secondary Radiation Forces In addition to the axial and lateral radiation forces attributable to the primary acoustic field, secondary acoustic forces are produced between particles themselves. These particle-particle interactions, known as Bjerknes forces, aid the formaticai of aggregates within a standing wave, but are negligible until the particles are in close proximity. 

Figure 3-3. Dispersion curves of crystalline orthorhombic polyethylene with two molecules per unit cell (from [49]). Comparison with Figure 3-1 shows the splitting of the frequency branches and the shape of the acoustical branches at p 0 (see text). Attention should be paid to the fact that the frequencies and the shape of the dispersion curves shown in Figure 3-1 and in this figure may differ because they have been calculated with two different force fields.

Figure 3-6. Example of lattice dynamical calculations on ir-bonded tridimenional crystals with short range interactions. Dispersion curves for cubic diamond along the F — (K) —> X symmetry direction. Experimental points from neutron-scattering experiments dispersion curves from least squares frequency fitting of a six parameters short range valence force field (from [60]). The Raman active phonon is the triply degenerate state indicated with F j near 1300 cm Notice that at k 0 the degeneracy at F is removed because of the lowering of the symmetry throughout the whole BZ. Notice also the three acoustic branches for which v 0 at k F.

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