Suspended particles have an electric surface charge that is greatly influenced by surface-adsorbed species and is subject to an electrostatic Coulomb force from an external electric field. According to the double layer theory, a diffuse layer of ions that has the same absolute charge but the opposite sign from the surface charge screens out all surface charges in fluids. The electric field also exerts a force on the ions in the diffuse layer which has direction opposite to that acting on the surface charge.
The ions in the diffuse layer, which are located some distance from the particle surface, are the ones who truly experience this latter force, but some of it is transported all the way to the particle surface via viscous stress. The Electrophoresis retardation force, or ERF for short, is another name for this component of the force. When the electric field is provided and the charged particle to be studied is at steady travel through the diffuse layer, the total resulting force is zero:
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