The electrophoretic mobility of charged colloidal spheres suspended in deionized water was measured as a function of the packing fraction ⌽ increasing from about 10 Ϫ6 to 2ϫ10 Ϫ3 . With increasing packing fraction, the mobility first increases linearly with the logarithm of the packing fraction and then saturates at a high value unaffected by the freezing transition. The electrostatic potential ⌿(r) was calculated numerically by solving the nonlinearized Poisson-Boltzmann equation in a cell model under conditions of charge regulation. The potential first is constant at low packing fraction and then decreases roughly linearly with log(⌽). In both cases the qualitative change in packing fraction dependence occurs once a significantly ͑typically 10-20 %͒ increases above 0 a given by the residual small ion concentration. Qualitatively similar behavior was found for particles of different size and surface chemistry and also under conditions of added salt. None of the theoretical approaches presently available is able to capture this interesting and complex behavior observed under low salt conditions.

We report on sytematic measurements of the low frequency conductivity σ σ σ σ in aequous supensions of highly charged colloidal spheres. System preparation in a closed tubing system results in precisely controlled number densities of 10 16

We report on extensive measurements in the low-frequency limit of the ac conductivity of colloidal fluids and crystals formed from charged colloidal spheres suspended in de-ionized water. Temperature was varied in a range of 5 degrees C < Theta < 35 degrees C and the particle number density n between 0.2 and 25 microm(-3) for the larger, respectively, 2.75 and 210 microm(-3) for the smaller of two investigated species. At fixed Theta the conductivity increased linearly with increasing n without any significant change at the fluid-solid phase boundary. At fixed n it increased with increasing Theta and the increase was more pronounced for larger n. Lacking a rigorous electrohydrodynamic treatment for counterion-dominated systems we describe our data with a simple model relating to Drude's theory of metal conductivity. The key parameter is an effectively transported particle charge or valence Z(*). All temperature dependencies other than that of Z(*) were taken from literature. Within experimental resolution Z(*) was found to be independent of n irrespective of the suspension structure. Interestingly, Z(*) decreases with temperature in near quantitative agreement with numerical calculations.

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Abstract. The electrophoretic mobility ~t of charged 0.3~tm Polystyrene spheres suspended in water was measured over a wide range of salt concentrations c and packing fractions tg. To observe isolated spheres at packing fractions (9 < 2× 10 -7 a newly developed optical tweezing electrophoresis (OTE) apparatus was used. At deionised conditions ~t = 2.5x10 -8 mZV~s~, it decreases further upon increasing the salt concentration c. Measurements at larger @ generally show much larger la and a qualitatively different dependence on c. At no added salt ~t increases in the unordered state but a saturation at la = 6.8x10 8 m2V-~s ~ is observed, as the system develops fluid order. The fluid-c~stal phase transition is of no influence on ~t and the conductivity c. In the ordered regime ~t is extremely sensible to the addition of spurious amounts of salt. Qualitatively similar behaviour is reported for particles of different size and surface chemistry and also under conditions of large amounts of added salt. Most of the presently observed trends cannot be understood in terms of the standard electrokinetic model. Possible extensions are discussed.