Zeta potential is a useful property to know for colloids, nanoparticles, and proteins. It is a substitute for the otherwise hard to measure surface charge. The zeta potential is not equal to the surface potential but rather it is the electrostatic potential difference between a rotationally averaged point at the shear, i.e., the slip plane, and an average point out in the liquid. Particles with the same sign zeta potential repel each other and thus maintain dispersion stability. Particles with the opposite sign zeta potential attract each other and thus produce aggregation. And particles with zero or nearly zero zeta potential also aggregate because there is no force strong enough to counteract the always-present attractive forces (ignoring any steric stabilization that may be present).
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