Colloidal particles at fluid interfaces: Effective interactions, dynamics and a gravitation-like instability

Colloidal particles of micrometer size usually become irreversibly trapped at fluid interfaces if they are partially wetted by one phase. This opens the chance to create two-dimensional model systems where the effective interactions between the particles are possibly influenced by the presence of the interface to a great extent. We will review recent developments in the quantitive understanding of these effective interactions with a special emphasis on electrostatics and capillarity. Charged colloids of micrometer size at an interface form effective dipoles whose strength sensitively depends on the double layer structure. We discuss the success of modified Poisson-Boltzmann equations with regard to measured colloidal dipole moments. On the other hand, for somewhat larger particles capillary interactions arise which are long-ranged and analogous to two-dimensional screened Newtonian gravity with the capillary length lambda as the screening length. For colloidal diameters of around 10 micrometer, the collective effect of these long-ranged capillary interactions will dominate thermal motion and residual, short-ranged repulsions, and results in an instability towards a collapsed state for a finite patch of particles. Such long-ranged interactions with the associated instability are also of interest in other branches of physics, such as self-gravitating fluids in cosmology, two-dimensional vortex flow in hydrodynamics, and bacterial chemotaxis in biology. Starting from the colloidal case we develop and discuss a dynamical phase diagram in the temperature and interaction range variables which appears to be of more general scope and applicable also to other systems.