Formation of meso-structures in colloidal monolayers

The spontaneous formation of loosely bound ordered aggregates has been observed in colloidal monolayers trapped at the air/water interface. The distance between particles in these meso-structures is of the order of the particle radius, implying that the colloidal interaction potential has a minimum at such distances. This is inconsistent with accepted theory. Possible connections with recent suggestions of hitherto unexpected attractive contributions to the pairwise interaction potential for bulk colloidal systems are discussed.