Colloidal

Colloidal systems are mixtures in which a dispersed phase consists of particles intermediate in size between a true solution and a suspension, typically about 1 nanometer to 1 micrometer, distributed through a continuous medium. The dispersed phase may be solid, liquid, or gas, and the continuous phase may be liquid or gas.

Common colloidal states include sols (solid in liquid), gels (liquid in a solid network), emulsions (liquid droplets in another immiscible liquid), foams (gas bubbles in a liquid), and aerosols (liquid or solid in a gas).

Stability is governed by surface forces. Brownian motion counteracts sedimentation, while electrostatic or steric stabilization prevents coalescence of particles. Colloids often exhibit the Tyndall effect, scattering light and appearing cloudy or milky, which helps distinguish them from true solutions.

Formation methods include dispersion of bulk materials into finely divided particles in a liquid, and condensation from a supersaturated solution; precipitation and mechanical breakup are common routes. Control of particle size and surface properties is important for achieving desired stability and rheology.

Applications span food, cosmetics, pharmaceuticals, paints and inks, ceramics, and nanomaterials; colloidal systems also play key roles in biology, such as blood plasma and cellular suspensions. The term colloid was introduced by Thomas Graham in 1861 to describe substances with glue-like behavior, with subsequent advances refining understanding of stability, aging, and interfacial phenomena.