kolloid

Kolloid, or colloid, refers to a heterogeneous system in which a dispersed phase consists of microscopic particles distributed through a continuous medium. The dispersed particles are larger than most molecules but small enough to remain suspended rather than quickly settling. Typical particle sizes range from about 1 nanometer to 1000 nanometers. Colloids exhibit distinct behaviors such as the Tyndall effect, where light is scattered by the particles, and Brownian motion, the random movement caused by collisions with the surrounding medium.

Colloidal systems are commonly classified by the phase of the dispersed and continuous components. Sols are solid particles dispersed in a liquid; gels are liquids dispersed in a solid network; emulsions are liquids dispersed in another liquid; foams contain gas dispersed in a liquid or solid; aerosols are liquids or solids dispersed in a gas. Examples include milk (an emulsion), fog (an aerosol), ink (a colloidal suspension), and gelatin (a gel).

Stability and control of colloids rely on forces that prevent coalescence or sedimentation. Electrostatic repulsion or steric hindrance can stabilize dispersed particles; zeta potential is a key indicator. The DLVO theory describes the balance between attractive van der Waals forces and repulsive electrostatic forces. Colloids can be prepared by dispersion (breaking up larger materials) or condensation (forming particles through chemical reactions), and stabilized with surfactants, polymers, or charge.

Historically, the term colloid was coined in the 19th century by Thomas Graham. Today colloids are central in food science, medicine, materials and paints, cosmetics, ceramics, environmental remediation, and nanotechnology, where colloidal particles serve as carriers or functional building blocks. Common everyday examples include milk, blood, fog, ink, and mud suspensions.