We study the electrophoretic mobility of spherical charged colloids in a low-salt suspension as a function of the colloidal concentration. Using an effective particle charge and a reduced screening parameter, we map the data for systems with different particle charges and sizes, including numerical simulation data with full electrostatics and hydrodynamics and experimental data for latex dispersions, on a single master curve. We observe two different volume fraction-dependent regimes for the electrophoretic mobility that can be explained in terms of the static properties of the ionic double layer.

We report on comprehensive measurements of the electrophoretic mobility µ of a highly charged spherical colloid in deionized or low salt aqueous suspensions, where fluid and crystalline order develops with increased packing fraction . We propose the existence of a 'universal' shape of the µ( ) showing three distinct regimes: a logarithmic increase, a plateau and a logarithmic decrease. The position and the height of the plateau are found to be influenced by the particle surface properties and the electrolyte concentration. In particular, it starts once the counter-ion concentration becomes equal to the concentration of background electrolyte. This coincides only loosely with the range of where fluid order is developing. Also the better defined first order freezing transition is observed to be uncorrelated to the shape of the µ( ) curve.

We studied the motion of polycrystalline solids comprising of charged sub-micron latex spheres suspended in deionized water. These were subjected to a low frequency alternating square wave electric field in an optical cell of rectangular cross section. Velocity profiles in X and Y direction were determined by Laser Doppler Velocimetry. The observed complex flow profiles are time dependent due to the combined effects of electro-osmosis, electrophoresis, crystal elasticity, and friction of the crystals at the cell wall. On small time scales elastic deformation occurs. On long time scales channel formation is observed. At intermediate times steady state profiles are dominated by a solid plug of polycrystalline material moving in the cell center. At large field strengths the plug shear melts. Mobilities in the shear molten state are on the order of (6.5Ϯ0.5) 10 Ϫ8 m 2 V Ϫ1 s Ϫ1 and connect continuously with those of the equilibrium fluid. The apparent mobility of the plug is much larger than of the fluid and like the mobility of the fluid decreases with increasing particle number density. We qualitatively attribute the accelerated motion of the plug to an incomplete exposure to the electro-osmotic flow profile.

We have measured the electrophoretic mobility µ = 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 µm −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 µ to be constant at low n while at large n we observe an approximately logarithmic decrease in n. However, the descent of µ 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.

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Microgel particles of cross-linked poly(NIPAM-co-acrylic acid) with different acrylic acid contents are investigated in solution and in the adsorbed state. As a substrate, silicon with a poly(allylamine hydrochloride) (PAH) coating is used. The temperature dependence of the deswelling of the microgel particles was probed with atomic force microscopy (AFM). The inner structure of the adsorbed microgel particles was detected with grazing incidence small angle neutron scattering (GISANS). Small angle neutron scattering (SANS) on corresponding microgel suspensions was performed for comparison. Whereas the correlation length of the polymer network shows a divergence in the bulk samples, in the adsorbed microgel particles it remains unchanged over the entire temperature range. In addition, GISANS indicates changes in the particles along the surface normal. This suggests that the presence of a solid surface suppresses the divergence of internal fluctuations in the adsorbed microgels close to the volume phase transition.

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.

We here present a new device based on dynamic light scattering (DLS) for measuring kinetics in turbid and nonergodic systems. This flat cell light scattering instrument has been developed in our laboratory and is based on an original flat cell instrument employing cells of varying thickness in order to measure the static structure and dynamics of a system. The smallest cell thickness is 10 microm. To this original instrument, we have integrated the three-dimensional (3D)-DLS technology as well as the echo method, and in comparison with other 3D-DLS instruments, ours show the best performance; the maximum intercept was 0.6 as opposed to 0.15 for regular 3D-DLS devices (recently we reached beta=0.75). This was made possible by using crossed polarization filters for the two laser beams, thereby allowing the scattered light from both laser beams to be decoupled and the intercept to no longer be limited at the theoretical value of 0.25. The maximum weight fraction of the sample that is measurable with such a setup is more than ten times higher than with a standard 3D-DLS setup or with the flat cell instrument without the 3D technology. Consequently, with the 3D-DLS flat cell instrument presented here, it truly becomes possible to investigate turbid systems. Moreover, the echo method was integrated to enable measurements of nonergodic systems. Here, a new mechanical design of the echo-DLS component was necessary due to the different geometries of the flat cell in comparison with that of a standard cylindrical cell. The performance of our echo device was compared to that of our multispeckle instrument, and the results were in good agreement for correlation times up to 30,000 s and more. The main limitation of this instrument in its current version is the maximum scattering angle of about 50 degrees (or 30 degrees if echo is used).