The concept of a single-beam acoustical tweezer device which can simultaneously trap microparticles at different points is proposed and demonstrated through computational simulations. The device employs an ultrasound beam produced by a circular focus
ed transducer operating at 1 MHz in water medium. The ultrasound beam exerts a radiation force that may tweeze suspended microparticles in the medium. Simulations show that the acoustical tweezer can simultaneously trap microparticles in the pre-focal zone along the beam axis, i.e. between the transducer surface and its geometric focus. As acoustical tweezers are fast becoming a key instrument in microparticle handling, the development of acoustic multitrapping concept may turn into a useful tool in engineering these devices.
We present a theoretical expression for the acoustic interaction force between small spherical particles suspended in an ideal fluid exposed to an external acoustic wave. The acoustic interaction force is the part of the acoustic radiation force on o
ne given particle involving the scattered waves from the other particles. The particles, either compressible liquid droplets or elastic microspheres, are considered to be much smaller than the acoustic wavelength. In this so-called Rayleigh limit, the acoustic interaction forces between the particles are well approximated by gradients of pair-interaction potentials with no restriction on the inter-particle distance. The theory is applied to studies of the acoustic interaction force on a particle suspension in either standing or traveling plane waves. The results show aggregation regions along the wave propagation direction, while particles may attract or repel each other in the transverse direction. In addition, a mean-field approximation is developed to describe the acoustic interaction force in an emulsion of oil droplets in water.
