Salt concentration and particle density dependence of electrophoretic mobilities of spherical colloids in aqueous suspension
ABSTRACT 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.
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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.Colloids and surfaces B: Biointerfaces 02/2014; 116C:568-575. DOI:10.1016/j.colsurfb.2014.01.039 · 4.28 Impact Factor
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ABSTRACT: The motion of dilute and concentrated dispersions of colloids by external electric or magnetic fields is discussed. Electrokinetics is studied for colloids in confinement, where the confining walls can be flat or rough. As an example for a rough wall superhydrophobic surfaces are chosen. It is shown that the reduced friction at the water-air interface is insufficient to enhance electro-osmosis. Magnetic particles are pulled through a crystalline matrix formed by nonmagnetic colloids to investigate local melting and recrystallization of a crystalline matrix. The average strain field is calculated and the reorganization processes are compared to those induced by shear fields. Using single domain, magnetically blocked particles of different shape and surface characteristics, the interplay between particles, their environment and an external field is investigated.The European Physical Journal Special Topics 11/2013; 222(11):2881-2893. DOI:10.1140/epjst/e2013-02064-1 · 1.76 Impact Factor
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ABSTRACT: Motivated by both still open fundamental theoretical questions as well as novel applications, the electro-kinetics of highly charged model colloids have attracted considerable interest in the last few years. The present paper reviews corresponding new developments and trends emerging mainly from novel instrumentations for both strongly interacting and extremely dilute systems. We also highlight recent uses of local electric fields and electro-kinetic currents to realize complex micro-swimmers or prescribe crystal micro-structures.The European Physical Journal Special Topics 11/2013; 222(11):2835-2753. DOI:10.1140/epjst/e2013-02061-4 · 1.76 Impact Factor