  We examine the behavior of spherical silica particles trapped at an
air-nematic liquid crystal interface. When a strong normal anchoring is
imposed, the beads spontaneously form various structures depending on their
area density and the nematic thickness. Using optical tweezers, we determine
the pair potential and explain the formation of these patterns. The energy
profile is discussed in terms of capillary and elastic interactions. Finally,
we detail the mechanisms that control the formation of an hexagonal lattice and
analyze the role of gravity for curved interfaces.
