Using laser Doppler velocimetry in the superheterodyne mode, we conducted a systematic study of the electrophoretic mobility of dispersions of small silica spheres (a=18 nm) suspended in water at different salinities and particle concentrations. The concentration of NaCl was varied from 40 microM up to 16 mM, while the particle concentrations were varied between 4.2x10(18) and 2.1x10(20) m-3. We find a decrease of mobility with increasing salt concentrations and an increase with increased particle number densities. The latter observation is not backed by the standard cell model of electrophoresis with Shilov-Zharkikh boundary conditions. Rather, if the experimental data are interpreted within that model, an unexpected change of the zeta potential at constant added salt concentration results. Interestingly, all experimental data collapse onto a single master curve, if plotted versus the ratio C* of particle counterions to added salt ions. We obtain a logarithmic increase of mobility for C*<1 and a plateau for C*>1. This may indicate a change of the Stern layer structure not yet included in the theoretical model.
"These instruments are based on Laser Doppler Velocimetry (LDV) and Phase Analysis Light Scattering (PALS) techniques. Nevertheless these instruments give ensemble averaged results which have been reported to be dependent on the particle concentration  and are not adequate when the shape of particles differs significantly from that of a sphere as in the case of the bacterial spores investigated in this work. In the last three decades, optical tweezers   (OT), have revealed as a formidable tool in many areas of science. "
[Show abstract][Hide abstract] ABSTRACT: In this work we report on the simultaneous measurement of the hydrodynamic coefficient and the electric charge of single Bacillus subtilis spores. The latter has great importance in protein binding to spores and in the adhesion of spores onto surfaces. The charge and the hydrodynamic coefficient were measured by an accurate procedure based on the analysis of the motion of single spores confined by an optical trap. The technique has been validated using charged spherical polystyrene beads. The excellent agreement of our results with the expected values demonstrates the quality of our procedure. We measured the charge of spores of B. subtilis purified from a wild type strain and from two isogenic mutants characterized by an altered spore surface. Our technique is able to discriminate the three spore types used, by their charge and by their hydrodynamic coefficient which is related to the hydrophobic properties of the spore surface.
"electro-osmotic µ eof and electrophoretic µ e mobilities  . To separate the two contributions the electrophoretic mobility has been studied in detail in cells where the contribution of electro-osmosis is either small or well known  . It has been shown that µ e depends on colloid and salt concentration. "
[Show abstract][Hide abstract] ABSTRACT: On a superhydrophobic surface a liquid is exposed to a large air-water interface. The reduced wall friction is expected to cause a higher electro-osmotic mobility. On the other hand, the low charge density of a superhydrophobic surface reduces the electro-osmotic mobility. Due to a lack of experimental data it has not been clear so far whether the reduced wall friction or the reduced charge density dominate the electrokinetic mobilities. To separate the relative contributions of electrophoresis and electro-osmosis, the mobilities of colloids on a negatively charged hydrophilic, a superhydrophobic (Cassie) and a partially hydrophilized superhydrophobic (Cassie composite) coating were measured. To vary the charge density as well as its sign with respect to those of the colloids the partially hydrophilized surfaces were coated with polyelectrolytes. We analyzed the electrokinetic mobilities of negatively charged polystyrene colloids dispersed in aqueous medium on porous hydrophilic and superhydrophobic surfaces by confocal laser scanning electron microscopy. In all cases, the external electric field was parallel to the surface. The total electrokinetic mobilities on the superhydrophobic (Cassie) and negatively charged partially hydrophilized (Cassie composite) surfaces were similar, showing that electro-osmosis is small compared to electrophoresis. The positively charged Cassie composite surfaces tend to 'trap' the colloids due to attracting electrostatic interactions and rough morphology, reducing the mobility. Thus, either the charge density of the coatings in the Cassie composite state or its slip length is too low to enhance electro-osmosis.
[Show abstract][Hide abstract] ABSTRACT: The cellulose membrane used in the present studies was prepared by impregnating cellulose acetate dissolved in acetone and mixed up with aqueous KBr, which has been added through a sintered G2 (porosity) disc. The flow of water, 1,4-dioxane and their different compositions through this membrane has been measured at different temperatures under different electric and magnetic field strengths. The results are interpreted in terms of a unit rate process. The electro osmotic permeability coefficients, enthalpy of activation (ΔH*), entropy of activation (ΔS*), free energy of activation (ΔG*), number of pores, pore radius and zeta potential have also been calculated. The flow process of various aqueous–dioxane mixtures across the membrane does not seem to be thermodynamically feasible. However, the dipolar nature of the solvent mixture does affect the membrane structure as shown by the variation in pore radius, number of pores and zeta potential.
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