What Are Colloids? The Physics of Suspended Particles

A colloid is not a substance but a state: microscopically small particles of a material float finely dispersed in a carrier medium, neither dissolving in it nor sinking to the bottom. The particles are larger than single molecules yet far too small for the naked eye — usually between one and a few hundred nanometres. A nanometre is a billionth of a metre.

Nature loves this state: fog is a colloid of water in air, milk one of fat in water — and blood and lymph are colloidal fluids too. In colloidal minerals, a pure element such as silver, gold or boron floats in purest water: only the element, only water, nothing else.

Two forces keep a colloid suspended. The first is Brownian motion: water molecules ceaselessly knock the tiny particles about, keeping them in a perpetual dance. The second is electric charge: true colloidal particles all carry the same surface charge and repel one another like matching magnetic poles — they cannot clump together and cannot settle.

This charge is also the Achilles heel of every colloidal solution: like a battery it can weaken over time — through light, heat or electromagnetic fields. A well-made, well-stored solution, however, holds its suspension for many months.

Many products sold as “colloidal” are in truth mostly ionic solutions: instead of elemental particles they contain dissolved ions, electrically charged single atoms that react chemically and bond with whatever they meet. A true colloid, by contrast, consists of neutral, elemental particles made of many atoms. The difference shows in colour and laser light: true colloids tint the solution and make the beam visible as a band.