A finely divided, solid material, which when dispersed in a liquid medium, scatters a light beam and does not settle by gravity; such particles are usually less than 2 microns in diameter. Some drilling fluid materials become colloidal when used in a mud, such as bentonite clay, starch particles and many polymers. Oil muds contain colloidal emulsion droplets, organophilic clays and fatty-acid soap micelles.