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.