Soft condensed-matter physicists study colloids. These 10 nanometer - 10 micron particles, typically consisting of some plastic like polystyrene or polymethyl methacrylate, are suspended in water or some other solvent. They are small enough that Brownian motion is significant, and large enought to be probed with a light microscope or using light scattering. For these reasons they make an ideal model of of atomic behavior.

The interaction between colloidal particles ranges from "hard sphere" systems, in which the particles are only aware of each other when they are in contact, to highly charged systems with long-range repulsion. Investigations into the dynamics of crystals, as well as glasses and gels have been conducted using colloids.