Measurement of the amplitude and phase of the electrophoretic and electroosmotic mobility based on fast single‐particle tracking

Abstract

The electrophoretic mobility of micron-scale particles is of crucial importance in applications related to pharmacy, electronic ink displays, printing, and food technology as well as in fundamental studies in these fields. Particle mobility measurements are often limited in accuracy because they are based on ensemble averages and because a correction for electroosmosis needs to be made based on a model. Single-particle approaches are better suited for examining polydisperse samples, but existing implementations either require multiple measurements to take the effect of electroosmosis into account or are limited in accuracy by short measurement times. In this work, accurate characterization of monodisperse and polydisperse samples is achieved by measuring the electrophoretic mobility on a particle-to-particle basis while suppressing electroosmosis. Electroosmosis can be suppressed by measuring in the middle of a microchannel while applying an AC voltage with a sufficiently high frequency. An accurate measurement of the electrophoretic mobility is obtained by analyzing the oscillating particle motion for 1.5 s per particle with a highspeed camera measuring at 850 Hz, synchronized to the applied electric field. Attention is paid to take into account the effect of the rolling shutter and the non-uniform sampling in order to obtain the accurate amplitude and phase of the electrophoretic mobility. The accuracy of method is experimentally verified and compared with a commercial apparatus for polystyrene microspheres in water. The method is further demonstrated on a range of particle materials and particle sizes and for a mixture of positively and negatively charged particles.

Single Elementary Charge Fluctuations on Nanoparticles in Aqueous Solution

“…To estimate the contribution of electroosmosis to the mobility, we apply a previously developed model, , with the following equation u eo = prefix− cosh ( 2 π i f E ρ / η false( z − h / 2 false) ) cosh ( 2 π i f E ρ / η h / 2 ) with water density ρ and distance from the bottom surface z . Supplementary Figure S6 shows the calculated electroosmosis factor as a function of distance z for three different electric field frequencies.…”

Single‐particle electrophoresis for studying the adsorption of cationic polymers onto anionic particles

“…Post‐processing of the recorded particle oscillation allows to calculate the electrophoretic mobility. A detailed explanation of the single‐particle electrophoresis technique is provided in [44].…”

“…The applied peak voltage was 80 V, resulting in a field in the x ‐direction $$E_x \approx 5.1 \times 10^3$$ V/m, and the frequency of the AC electric field was $$f=300$$ Hz. The acquired images of individual particles (one particle in the field of view) were analyzed using a customized MATLAB code described in [44]. In short, a custom particle tracking algorithm was used to determine the centroid of each particle.…”

“…To eliminate the effect of electroosmosis and to ensure that just the electrophoretic mobility is measured, the measurements were performed in the middle of the microchannel (at about 200 µm in the z ‐direction for a 400 µm high channel) and at a sufficiently high frequency (i.e., 300 Hz) for the applied field. For further details regarding the AC electrophoresis theory, the method and the analysis of the SPE technique, the reader is referred to [44].…”

“…To measure the electrophoretic mobility of individual particles as a function of time, the same single‐particle electrophoresis (SPE) technique is used as described by Cid et al. [44]. The electrophoresis experiments are used to characterize the kinetics of polymer adsorption onto model particles as well as the equilibrium between adsorption and desorption.…”