Optical tweezing electrophoresis of isolated, highly charged colloidal spheres

We report on a novel optical tweezing experiment measuring the electrophoretic mobility μ of highly charged spherical particles suspended in water at volume fractions Φ≤2×10−7. At deionised conditions μ=2.5×10−8 m2 V−1 s−1, it decreases further upon increasing the salt concentration c. We compare our data to measurements at larger Φ, where generally much larger μ and a qualitatively different dependence on the salt concentration are observed. To characterise the dependence on Φ we performed measurements at no added salt. There the mobility increases in the unordered state but stays constant at μ=6.8×10−8 m2 V−1 s−1 as the system develops fluid order. In the ordered regime the salt concentration dependence becomes very pronounced for c≤10−5 M. None of the presently observed trends can be understood in terms of the standard electrokinetic model.