Properties of Colloids: Tyndall Effect, Brownian Movement and Electrophoresis

Three characteristic physical properties, taken together, provide direct experimental evidence for the size, ceaseless motion, and electric charge of colloidal particles.

The Tyndall effect is the scattering of light observed when a beam of light is passed through a colloidal dispersion: the beam becomes visible from the side, as an illuminated cone or path, because colloidal particles are large enough to scatter visible light effectively in all directions, some of which reaches an observer positioned at an angle to the original beam. A true solution shows no such effect, since its dissolved ions or molecules are far too small (well below the wavelength of visible light) to scatter light appreciably, so a beam passed through a true solution remains essentially invisible from the side. Everyday examples of the Tyndall effect include a sunbeam becoming visible as it slants through a dusty room or a forest canopy (scattering off suspended dust or aerosol colloidal particles in air) and the visible cone of light from a car's headlights in fog (scattering off colloidal water droplets).

Brownian movement is the continuous, random, zig-zag motion of colloidal particles, first observed under a microscope by the botanist Robert Brown (originally for pollen grains suspended in water, a somewhat larger particle but exhibiting the identical phenomenon). Its physical cause is the constant, random bombardment of each colloidal particle by the much smaller, invisibly fast-moving molecules of the dispersion medium, striking the particle unequally from different directions at any given instant and so imparting a net, randomly-changing kick that keeps the particle in continuous erratic motion. This ceaseless agitation is of real practical importance: it works continuously against the tendency of the (comparatively heavy) colloidal particles to settle out of the dispersion under gravity, and is one of the principal reasons colloidal dispersions remain kinetically stable — that is, remain dispersed indefinitely without settling — even though a colloid is not, thermodynamically, a true solution. …