zeta potential

Two magnets with the same pole facing each other push apart. Many dispersed particles behave similarly: they carry like electric charges, so they repel one another and stay separated instead of clumping. Zeta potential is the practical measure of how strong that repulsion is.

When a charged particle sits in a liquid, it gathers a cloud of oppositely charged ions around it. Part of that cloud moves with the particle and part stays behind; the boundary between them is the slipping (shear) plane. Zeta potential is the electric potential measured right at that shear plane, in millivolts, and it is what governs the electrostatic push between particles as they approach.

As a rule of thumb, a zeta potential more positive than about +30 mV or more negative than about −30 mV indicates strong repulsion and a stable, deflocculated dispersion, whereas values near zero allow particles to approach and aggregate. Formulators therefore adjust pH and electrolyte content to tune zeta potential — sometimes maximising it for stability, sometimes deliberately lowering it to produce a controlled, easily redispersible flocculated suspension.