Colloidal systems and electrokinetic phenomena

A colloidal particle — a clay flake, a protein, a droplet in an emulsion — almost always carries some surface charge, picked up by ionising a surface group or by preferentially adsorbing one type of ion from solution. That charge doesn't sit alone: it pulls a cloud of oppositely charged counter-ions in close, forming an electric double layer — a thin, tightly bound Stern layer right at the surface, surrounded by a more loosely held diffuse layer that thins out into the bulk solution.

This double layer is what decides whether a dispersion survives. Two similarly charged particles drifting close together repel each other electrostatically, and that repulsion competes against the ever-present van der Waals attraction pulling them together. As long as the repulsive barrier is high enough, particles bounce apart before they can stick — the dispersion is kinetically stable. Add enough electrolyte, though, and the extra ions compress the diffuse layer, shrinking the repulsive barrier until attraction wins and particles clump together irreversibly: coagulation. Every electrolyte has a critical coagulation concentration below which the dispersion is essentially untouched and above which it collapses fast — it isn't an all-or-nothing effect at any concentration.

None of this double layer moves as a rigid unit when the particle and liquid slide past each other. Only the layer beyond a certain slipping plane stays with the bulk fluid; the potential right at that plane, not the bare surface charge, is the zeta potential — the quantity that actually predicts electrophoretic mobility and dispersion stability. The same relative sliding of charged surface against liquid shows up in two mirror-image experiments: in electrophoresis the particles move through a fixed field while the liquid stays still, and in electroosmosis the liquid moves through a fixed, charged porous medium (or capillary) under an applied field while the solid stays put. They share the same physics of the double layer sliding past the slipping plane, but they are genuinely different setups, not restatements of one experiment.