  The properties of small clusters depend dramatically on the interactions
between their constituent particles. However, it remains challenging to design
and tune the interactions between macroscopic particles, such as in a granular
material. Here, we use acoustic levitation to trap macroscopic grains and
induce forces between them. Our main results show that particles levitated in
an acoustic field prefer to make contact along sharp edges. The radius of
curvature of the edges directly controls the magnitude of these forces. These
highly directional interactions, combined with local contact forces, give rise
to a diverse array of cluster shapes. Our results open up new possibilities for
the design of specific forces between macroscopic particles, directing their
assembly, and actuating their motion.
