Colloidal electro-phoresis in the presence of symmetric and asymmetric electro-osmotic flow

Abstract

We characterize the electro-phoretic motion of charged sphere suspensions in the presence of substantial electro-osmotic flow using a recently introduced small angle super-heterodyne dynamic light scattering instrument (ISASH-LDV). Operation in integral mode gives access to the particle velocity distribution over the complete cell cross-section. Obtained Doppler spectra are evaluated for electro-phoretic mobility, wall electro-osmotic mobility and particle diffusion coefficient. Simultaneous measurements of differing electro-osmotic mobilities leading to asymmetric solvent flow are demonstrated in a custom made electro-kinetic cell fitting standard microscopy slides as exchangeable sidewalls. Scope and range of our approach are discussed demonstrating the possibility of an internal calibration standard and using the simultaneously measured electro-kinetic mobilities in the interpretation of microfluidic pumping experiment involving an inhomogeneous electric field and a complex solvent flow pattern.freely suspended far off any walls [10,11,12].The standard method to measure the electro-osmotic mobility is the streaming potential [1, 3] but also microscopic determinations of suspension flow-profiles in standard or custom-made microelectro-phoresis cells have been performed [13,14,15]. We here adapt the latter approach but combine a suitable electro-kinetic cell with a recently introduced versatile version of LDV [16]. This is further motivated by the fact that in many technical applications and in particular contemporary micro-fluidic experiments both mobilities are needed simultaneously to interprete the experimental findings. A typical examples are micro-fluidic applications like electrophoretic [17], diffusio-phoretic [18] and osmotic trapping [19] or single particle electro-kinetic experiments of sedimented and optically trapped particles [20,21,22,23 ]. Here often inhomogeneous electric fields are employed which in addition may vary in time. In phoretic micro-swimming and electroosmotic pumping [24,25,26] typical experiments involve locally generated diffusio-electric fields. [27,28]. Also here we are relying on well known values for the electro-kinetic mobilities of substrates and transported particles. The electro-osmotic mobilities of several high purity materials are well documented in literature. However, for many standard materials like glass, quartz, PMMA or PDMS results show a large spread of values depending on cleaning and conditioning conditions. Moreover, in microfluidics and other applications, coated cell walls are frequently used, which introduces additional dependence of µeo on the preparation protocol followed. In this situation, it would be highly desirable to have a fast and reliable characterization method available, which in addition would also yield the particle mobility under exactly the same conditions.Our integral small angle super-heterodyne dynamic light scattering instrument (ISASH-LDV) was designed to determine the electro-phoretic mobilities of charged colloidal spheres in aqueous...

Characterization of active matter in dense suspensions with heterodyne laser Doppler velocimetry

“…Averaging over larger volumes will thus detect both flow types. Similar flows are well known not only from phoretic experiments in closed cells [41,42,44] but also from patterning experiments with light-sensitive swimmers [52]. Due to the illumination/observation geometry in our experiment, solvent convection in negative z-direction along the cell walls will be monitored and readily yield a spectral contribution for frequencies smaller than ω B .…”

“…The spectral shape of these two successively taken spectra coincides near quantitatively the signal has become somewhat more symmetric. Both thus isolated single scattering signals can now be compared or subjected to further evaluation [42]. Here, we stick with a qualitative comparison.…”

“…We here employ the recently introduced super-heterodyne laser Doppler velocimetry (SH-LDV) allowing for the correction of multiple scattering by an empirical post-recording correction scheme [41]. There, both velocimetry and the determination of average diffusion coefficients, D, were possible with a statistical uncertainty of σ ≤ ΔD/D ≈ 0.02 at transmissions of 40% and larger, and we studied systems undergoing phase transitions and suspensions under symmetric and asymmetric solvent flows [42]. For the competing cross-correlation techniques, similar values have been reported, with modulated 3D cross-correlation showing somewhat better marks due to the modulation-induced increase of the intercept [32].…”

Demonstration of variable angle super-heterodyne dynamic light scattering for measuring colloidal dynamics

“…This was possible in small angle SH-DLS at transmissions of 40% and larger [26]. With this approach, we studied diffusion in turbid systems including flowing suspensions, active matter, and systems undergoing phase transitions [26,40,41]. For the competing crosscorrelation techniques, similar values have been reported, with modulated 3D cross-correlation showing somewhat better marks due to the modulation induced increase of the intercept [18].…”

Numerical Study of Electro-Osmotic Fluid Flow and Vortex Formation

“…The dynamics of this system is governed by an effective conservative energy that for large separations depends on the inverse of the distance. In this way, associated investigation of pumping including electrokinetic phenomenon were performed by [25,26]. Recently Arcos et al [27], studied the behavior of a sPTT fluid subject to time dependent zeta potentials.…”