Colloids at Interfaces
Dr. Martin Oettel (Max-Planck-Institut für Metallforschung, Stuttgart)

Colloidal particles can be irreversibly trapped at fluid interfaces and are generic model systems to study solidification and melting in 2d owing to their mutual repulsion at short distances. Additionally, for particles with sizes ranging from nano- to micrometers spontaneous formation of mesoscopic patterns has been observed which is consistent with surprisingly long--ranging attractive forces between the colloids. A candidate for such a long--ranging force is the capillary interaction caused by deformations of the fluid interface, and by analogy to the gravity driven force observed for cereal flakes in a milk bowl a capillary force (~ 1/distance) caused by colloidal charges has been speculated to be the source of attractions. We argue that in "ideal" experimental systems such a long-ranging capillary force can never occur. However, for strongly charged colloids on water we find an attractive potential minimum at intermediate distances (far beyond the van-der-Waals attraction) if the colloid radius and the Debye screening length of water are roughly equal. "Nonideal" experimental systems may be used to tune the attractions, examples involving water and nematic surfaces are dicussed.