Colloidal mill:
A colloidal mill is a machine that is used to reduce the particle size of a solid in suspension in a liquid, or to reduce the droplet size of a liquid suspended in another liquid. It consists of two metallic discs nearly touching each other and rotating in opposite directions to very high speed.
The space between the discs of the mill is so adjusted that coarse suspension is subjected to great shearing force giving rise to particles of colloidal size. Colloidal solutions of black ink, paints, varnishes, dyes etc are obtained by this method.