  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 one 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.
