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RESEARCH PRODUCT

Electrophoretic mobility of charged spheres

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description

The electrophoretic mobility μ of charged 0.3 µm Polystyrene spheres suspended in water was measured over a wide range of salt concentrations c and packing fractions Φ. To observe isolated spheres at packing fractions Φ ≤ 2 × 10−7 a newly developed optical tweezing electrophoresis (OTE) apparatus was used. At deionised conditions μ=2.5 × 10−8 m2V−1s−1, it decreases further upon increasing the salt concentration c. Measurements at larger Φ generally show much larger μ and a qualitatively different dependence on c. At no added salt μ increases in the unordered state but a saturation at μ=6.8 × 10−8 m2V−1s−1 is observed, as the system develops fluid order. The fluid-crystal phase transition is of no influence on μ and the conductivity σ. In the ordered regime μ 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.