Volume 125, Issue 6, June 2009
Index of content:
- NONLINEAR ACOUSTICS 
125(2009); http://dx.doi.org/10.1121/1.3119625View Description Hide Description
The partial-wave series for the scattering of an acoustic helicoidal Bessel beam by a sphere centered on the axis of the beam is applied to the calculation of the acoustic radiation force by the beam on the sphere in an inviscid fluid. The term “helicoidal” refers to a type of beam having an axial amplitude null and an azimuthal phase gradient. Such a beam is known as an acoustic vortex and only the case of a vortex having a unit magnitude topological charge is considered. There is no monopole contribution to the radiation force. Radiation force examples are computed for a soft sphere, a fixed rigid sphere, a movable rigid sphere, an aluminum sphere in water, and an acrylic sphere in water. Beam parameters are found for the rigid sphere and the aluminum and acrylic cases in which the radiation force is directed opposite the propagation direction of the beam. Negative radiation forces appear to be associated with relatively weak scattering into the backward hemisphere. Some aspects of the low frequency scattering of plane waves and helicoidal Bessel beams are examined for movable rigid spheres.
125(2009); http://dx.doi.org/10.1121/1.3124769View Description Hide Description
The possibility of using acoustic radiation force in standing waves for stirring and mixing small volumes of liquids is theoretically analyzed. The principle of stirring considered in this paper is based on moving the microparticles suspended in a standing acoustic wave by changing the frequency so that one standing wave mode is replaced by the other, with differently positioned minima of potential energy. The period-average transient dynamics of solid microparticles and gas microbubbles is considered, and simple analytical solutions are obtained for the case of standing waves of variable amplitude. It is shown that bubbles can be moved from one equilibrium position to another two to three orders of magnitude faster than solid particles. For example, radiation force in a standing acoustic wave field may induce movement of microbubbles with a speed of the order of a few m/s at a frequency of and ultrasound pressure amplitude of , whereas the speed of rigid particles does not exceed under the same conditions. The stirring effect can be additionally enhanced due to the fact that the bubbles that are larger and smaller than the resonant bubbles move in opposite directions. Possible applications of the analyzed stirring mechanism, such as in microarrays, are discussed.
125(2009); http://dx.doi.org/10.1121/1.3119628View Description Hide Description
In this paper the problem of parametric sound generation in an acoustic resonator filled with a fluid is considered, taking explicitly into account the influence of the nonlinearly generated second harmonic. A simple model is presented, and its stationary solutions were obtained. The main feature of these solutions is the appearance of bistable states of the fundamental field resulting from the coupling to the second harmonic. An experimental setup was designed to check the predictions of the theory. The results are consistent with the predicted values for the mode amplitudes and parametric thresholds. At higher driving values a self-modulation of the amplitudes is observed. This phenomenon is identified with a secondary instability previously reported in the frame of the theoretical model.