Colloidal systems are a kind of dispersed system in which the size of dispersed phase particles is less than 1 micron. In freely dispersed colloidal systems particles of the dispersed phase in gas or liquid freely and independently participate in intense Brownian motion, uniformly filling the entire volume of the dispersion medium; such systems include, for example, sols and aerosols. In the process of Brownian motion and during stirring colloidal particles collide. If this does not result in the enlargement of particles due to their adhesion (coagulation) or fusion (coalescence), the colloidal systems are called aggregately stable. There are also structured (bonded dispersed) systems that are characterised by the presence of an irregular spatial grid formed by particles of the dispersed phase. The mechanical properties of such systems, which primarily include gels, are determined not only by the properties of the dispersion medium, but also by the properties and number of contacts between the particles of the dispersed phase.

Colloidal systems are formed by condensing a substance in a homogeneous medium (supersaturated solution, vapour, supercooled liquid), if the nucleation centres of a new dispersed phase, i.e. tiny droplets or crystals cannot grow to a size exceeding 10–5–10–4 cm. Condensation often accompanies chemical reactions that produce hardly soluble compounds. Another way of obtaining colloidal systems is spontaneous (in case of lyophilic systems) or forced (in case of lyophobic systems) dispersion. The existence of liquid aggregately stable lyophobic colloidal systems is always due to the presence in the dispersion medium of surface-active substances, i.e. stabilizers. These substances produce an adsorption-solvate protective layer on the particle surfaces and prevent the close approach and coagulation of particles under the action of short-range forces of molecular attraction. The close approach of particles may be impeded by the disjoining pressure of the liquid dispersion medium, which is solvated by molecules or ions of the stabilizer in the adsorption layer; by electrostatic repulsion of like-charged ions adsorbed on the particle surfaces; or by enhanced structural viscosity of the surface protective layer, the so-called structural-mechanical barrier. Various types of colloidal systems are presented in the Table.

Dispersed phase	Dispersion medium	Type of colloidal system	Examples
Liquid	Gas	Aerosol of liquid particles	Fog, haze Solid
Solid	Gas	Aerosol of solid particles	Smoke, dust
Gas	Liquid	Foam	Soap bubbles
Liquid	Liquid	Emulsion	Milk, mayonnaise
Solid	Liquid	Sol, suspension	Ink
Gas	Solid	Solid foam	Polyurethane
Liquid	Solid	Gel	Jelly
Solid	Solid	Solid dispersion	Ruby glass, pearl