Electrokinetic optimization of a micromixer for lab-on-chip applications

This paper is concerned with the optimization of an electrokinetic micromixer suitable for Lab-on-Chip and other microfluidic applications. The mixing concept is based on the combination of an alternating electrical excitation applied to a pressure-driven base flow in a meandering microchannel geometry. The electrical excitation induces a secondary electrokinetic velocity component which results in a complex flow field within the meander bends. A mathematical model describing the physicochemical phenomena present within the micromixer is implemented in an in-house Finite-Element-Method code. We first perform simulations comparable to experiments concerned with the investigation of the flow field in the bends. The comparison of simulation and experiment reveals excellent agreement. Hence, the validated model and numerical schemes are employed for a numerical optimization of the micromixer performance. In detail, we optimize the secondary electrokinetic flow by finding the best electrical excitation parameters, i.e. frequency and amplitude, for a given waveform. The simulation results of two optimized electrical excitations featuring a discrete and a continuous waveform are compared and discussed. The results demonstrate that the micromixer is able to achieve high mixing degrees very rapidly

Streaming potential revisited: the influence of convection on the surface conductivity

Electrokinetic phenomena play an important role in the electrical characterization of surfaces. In terms of planar or porous substrates, streaming potential and/or streaming current measurements can be used to determine the zeta potential of the substrates in contact with aqueous electrolytes. In this work, we perform electrical impedance spectroscopy measurements to infer the electrical resistance in a microchannel with the same conditions as for a streaming potential experiment. Novel correlations are derived to relate the streaming current and streaming potential to the Reynolds number of the channel flow. Our results not only quantify the influence of surface conductivity, and here especially the contribution of the stagnant layer, but also reveal that channel resistance and therefore zeta potential are influenced by the flow in the case of low ionic strengths. We conclude that convection can have a significant impact on the electrical double layer configuration which is reflected by changes in the surfaces conductivity

Streaming Potential Revisited: The Influence of Convection on the Surface Conductivity

Electrokinetic phenomena play an important role in the electrical characterization of surfaces. In terms of planar or porous substrates, streaming potential and/or streaming current measurements can be used to determine the zeta potential of the substrates in contact with aqueous electrolytes. In this work, we perform electrical impedance spectroscopy measurements to infer the electrical resistance in a microchannel with the same conditions as for a streaming potential experiment. Novel correlations are derived to relate the streaming current and streaming potential to the Reynolds number of the channel flow. Our results not only quantify the influence of surface conductivity, and here especially the contribution of the stagnant layer, but also reveal that channel resistance and therefore zeta potential are influenced by the flow in the case of low ionic strengths. We conclude that convection can have a significant impact on the electrical double layer configuration which is reflected by changes in the surfaces conductivity

Zeta Potential of Poly(methyl methacrylate) (PMMA) in Contact with Aqueous Electrolyte–Surfactant Solutions

The addition of surfactants can considerably impact the electrical characteristics of an interface, and the zeta potential measurement is the standard method for its characterization. In this article, a comprehensive study of the zeta potential of poly­(methyl methacrylate) (PMMA) in contact with aqueous solutions containing an anionic, a cationic, or a zwitterionic surfactant at different pH and ionic strength values is conducted. Electrophoretic mobilities are inferred from electrophoretic light scattering measurements of the particulate PMMA. These values can be converted into zeta potentials using permittivity and viscosity measurements of the continuous phase. Different behaviors are observed for each surfactant type, which can be explained with the various adsorption mechanisms on PMMA. For the anionic surfactant, the absolute zeta potential value below the critical micelle concentration (CMC) increases with the concentration, while it becomes rather constant around the CMC. At concentrations above the CMC, the absolute zeta potential increases again. We propose that hydrophobic-based adsorption and, at higher concentrations, the competing micellization process drive this behavior. The addition of cationic surfactant results in an isoelectric point below the CMC where the negative surface charge is neutralized by a layer of adsorbed cationic surfactant. At concentrations near the CMC, the positive zeta potential is rather constant. In this case, we propose that electrostatic interactions combined with hydrophobic adsorption are responsible for the observed behavior. The zeta potential in the presence of zwitterionic surfactant is influenced by the adsorption, because of hydrophobic interactions between the surfactant tail and the PMMA surface. However, there is less influence, compared to the ionic surfactants. For all three surfactant types, the zeta potential changes to more-negative or less-positive values for alkaline pH values, because of hydroxide adsorption. An increase of the ionic strength decreases the absolute value of the zeta potential, because of the shielding effects