Instrument for the measurement of the zeta potential of particles suspended in an electrolytic fluid.

The zeta potential is the gradient of the electro-kinetic potential at a solid-liquid phase border. The surface of a suspended particle is generally electrically charged (often negative). An ionic layer that surrounds this surface is formed in the suspension liquid from dissociated molecules with opposite charges. This layer may compensate to a large degree but not completely the surface charge of the particles. Complete neutralization occurs with an additional diffuse liquid layer around the particle, which still displays a slight surplus of the respective counter ions. Charge equilibrium exists only outside this double layer. Zeta potentials can be measured by a forced tangential displacement of the mobile double layer, with values ranging typically from +40 to -50 mV.

Agglomeration or flocculation of the solid particles can intensify the hindered sedimentation (see Swarm Sedimentation). In the region of low solids concentrations an excess sedimentation velocity can be observed caused by the formation of hydrodynamic complexes. At higher concentrations the sedimentation velocity decreases strongly as a swarm is formed, in which the velocity determining particle properties like specific density, size and shape lose their influences. In this region, the resulting hindered sedimentation can be described with the Richardson & Zaki equation. The hindered sedimentation is beneficial in sedimentation equipment because it forms a sharp sedimentation front.