  Here we present an experimental observation of the self-organization effect
of the polystyrene particles formed by acoustically-induced interaction forces.
Two types of stable configurations are observed experimentally: one is
mechanically equilibrium and featured by nonzero inter-particle separations,
and the other corresponds to a close-packed assembly, which is formed by strong
attractions among the aggregated particles. For the former case involving two
or three particles, the most probable inter-particle separations (counted for
numerous independent initial arrangements) agree well with the theoretical
predictions. For the latter case, the number of the final stable configurations
grows with the particle number, and the occurrence probability of each
configuration is interpreted by a simple geometric model.
