  The acoustic levitation of small particles provides a versatile platform to
investigate the collective dynamical properties of self-assembled many-body
systems in the presence of hydrodynamic coupling. However, acoustic scattering
forces can only generate attractive interactions at close range in the
levitation plane, limiting self-assembly to rafts where particles come into
direct, dissipative, contact. Here, we overcome this limitation by using
particles small enough that the viscosity of air establishes a repulsive
streaming flow at close range. By tuning the size of particles relative to the
characteristic length scale of the viscous flow, we control the interplay
between attractive and repulsive forces. In this novel granular raft, particles
form an open lattice with tunable spacing. Hydrodynamic coupling between
particles gives rise to spontaneous excitations in the lattice, in turn driving
intermittent particle rearrangements. Under the action of these fluctuations,
the raft transitions from a predominantly quiescent, crystalline structure, to
a two-dimensional liquid-like state. We show that this transition is
characterized by dynamic heterogeneity and intermittency, as well as
cooperative particle movements, that produce an effectively `cageless' crystal.
These findings shed light on fluid-coupling driven excitations that are
difficult to isolate and control in many other hydrodynamic systems.
