Electrophoretic flow behaviour and mobility of colloidal fluids and crystals

We report on measurements of the electrophoretic mobility μ of charged colloidal spheres in the deionized state, where the suspensions show fluid or crystalline order. In the fluid state, parabolic flow profiles are observed due to electro-osmotic solvent flow. In the crystalline state, complex flow profiles occur due to additional crystal cohesion. The mobility μ then may inferred from the flow velocity averaged over the complete cell cross section as performed in our home built super-heterodyne Doppler velocimeter. For two particle species of 68 and 122 nm diameter we measured μ as a function of particle concentration. Starting from a plateau value at low concentration, μ decreases approximately logarithmically with increased concentration. Interestingly, the decrease of μ is not affected by the phase transition, indicating that electro-kinetic properties may be viewed as single particle properties even in the case of structure formation. Moving further along this line of thought, we show that this behaviour may indeed be captured near quantitatively by an ad hoc combination of charge renormalization calculations and the Standard Electro-kinetic Model originally designed for isolated, non-interacting particles. From this coincidence, we may conclude that (i) counter-ion condensation behind the hydrodynamic slip plane is the dominant effect of finite particle concentrations and leads to a decrease of the effective particle charge and (ii) an increased colloid concentration can be understood in terms of an elevated total ion concentration and that (iii) it can thus be reduced to a modification of the screening parameter, such that the problem can be mapped onto a single particle theory for an infinite electrolyte.