Specially synthesized colloidal suspensions consisting of particles uniform in size and shape and dispersed in a solvent can be used as colloidal model systems. The particle interaction can be designed by the particle synthesis and can be varied over a wide range including short and long ranged repulsions as well as attractions. Spherical and directed interactions can realized. The particle interaction is generated by either entropic, electrostatic, dipolar or magnetic forces and is analytically tractable.
The colloidal particles experience thermal fluctuations of the solvent which makes them move randomly (Brownian motion). A large number of colloidal particles has to be described as an statistical ensemble. Therefore colloidal suspensions are used both experimentally and theoretically as model systems for statistical physics. If one identifies the colloid particles as "macro atoms", the ensemble of the dispersed colloidal particles behaves like an image of an atomic system at a different time and length scale.
In contrast to their atomic counterparts, colloids show typical length scales in the range of the wavelength of visible light. This make them readily accessible for optical experiments like microscopy and light scattering and offers the unique possibility to obtain complementary information from real and reciprocal space with excellent spatial resolution. Because of their large size compared to atoms, both the spatial structure and dynamics of colloids are experimentally much more accessible than it is the case for atomic and molecular systems. For example, phase transition processes are very slow compared to atomic systems, which allows to obtain data with very high temporal resolution.
Colloidal model systems are used to study the physics of complex liquids, solids and bio-systems. Non-equilibrium phenomena such as the self-organization of particles into extended structures. An exciting richness of phase behaviors, crystalline structures and a corresponding wealth of solidification scenarios can be observed. Not only the macroscopic phase behavior but also the phase transition kinetics and the microscopic particle dynamics on the single particle scale is of great scientific interest. Colloidal particles ca be steered by external forces like gravitational forces, electromagnetic forces, phoretic forces and hydrodynamic forces. This leads to a further enrichment of phenomena under equilibrium and also under non-equilibrium situations.