Electrophoretic mobility of electrostatically interacting colloidal spheres

We have measured the electrophoretic mobility mu = v(E)/E (where E is the electric field strength and V-E the electrophoretic velocity) of highly charged colloidal spheres in deionized aqueous suspension at particle number densities n between 0.15 and 150 mum(-3). Under these conditions the system exhibits fluid or crystalline order. We used laser Doppler velocimetry to determine the electrophoretic velocities V-E as spatially averaged particle velocities from both integral and spatially resolved measurements. With this approach we were for the first time able to extend measurements far into the crystalline region of the phase diagram. We found mu to be constant at low n while at large n we observe an approximately logarithmic decrease in n. However, the descent of mu is not affected by the phase transition. This indicates that this transport coefficient rather depends on the local structure of the ionic clouds surrounding the particles than on the long range order of the suspension.