Electroosmotic Mixing Research Articles

Investigation of slip effects on electroosmotic mixing in heterogeneous microchannels based on entropy index

In this article electrokinetic mixing through heterogeneous microchannels has been studied and the effects of slip coefficient, zeta-potential, Debye–Hückel parameter and Reynolds number on mixing efficiency have been investigated. The microchannels studied here, have non-homogenous zeta-potential distribution at the wall, while other surface properties are considered to be homogenous. In order to investigate the electro-osmotic mixing, the Navier–Stokes, Nernst–Planck, Laplace and convection–diffusion equations have been solved numerically for velocity field, ions distribution, electrical potential and concentration field, respectively. The entropy of concentration distribution has been used as a quantitative index to evaluate the mixing performance. The results show that the behavior of electro-osmotic micromixers strongly depends on the amount and distribution of wall zeta-potential and in most cases the mixing efficiency increases with reduction of slip coefficient or Debye–Hückel or Reynolds number. It is found that in presence of slip, mixing efficiency decreases at low Reynolds numbers, while increases at high Reynolds numbers. Also, the accuracy of Helmholtz–Smoluchowski approximate model is investigated and it is found that the performance of the Helmholtz–Smoluchowski model in predicting the mixing efficiencies deteriorates for high wall zeta-potential or low values of Debye–Hückel parameter.

Analytical study of AC electroosmotic mixing in 2-dimensional microchannel with time periodic surface potential.

This work reported an analytic study of AC electroosmotic flows with a view to control the degree of mixing in a rectangular microchannel. Only with spatially non-uniform zeta potential distribution, fluid particles travel back and forth along a vortical flow field developed inside a microchannel. Although complex patterns of electroosmotic vortical flows can be obtained by various types of non-uniform zeta potential distributions, fluid particles always follow regular paths due to a laminar flow limit. To further facilitate the mixing of sample fluid, we propose a scheme that the zeta potential distribution was temporally non-uniform as well. General solutions for both the double layer potential distribution and the AC electroosmotic flow field are analytically determined by solving the unsteady Stokes equation with an electrostatic body force. As an illustrative example, we consider a case where two different types of non-uniform zeta potential distributions alternate with each other and the effects of both the AC frequency and the frequency of the alternation of the two zeta potential distributions on flow characteristics are examined using the Poincaré sections. Conclusively, one can either enhance or prevent mixing compared to a static electroosmotic flow, which is in line with previously demonstrated experimental works. Thus, the results presented would be an effective mean for controllable electroosmotic flow in a microfluidic platform.

Numerical investigation and simultaneous optimization of geometry and zeta-potential in electroosmotic mixing flows

Effects of zeta potential distribution and geometrical specifications on flow rate and mixing performance of electroosmotic flows in straight, converging, diverging, and converging-diverging microchannels are investigated by lattice Boltzmann method. Modified Navier-Stokes equations along with the Nernst–Planck equation for ionic distributions are solved and validated against the available electroosmotic flow solutions. The geometrical effects on flow patterns and mixing efficiencies in the presence of patched zeta potential distributions are examined in search for the simultaneously enhanced mixing performance and mass flow rate. Numerical results indicate that converging channel leads to a sizable increase in mixing efficiencies, while at the same time decreases the flow rate. In contrast, diverging channels increase the flow rate, where the mixing performance deteriorates. Thus, it is expected to achieve a balance between the mixing efficiency and mass flow rate using converging-diverging geometries. In this regard, the response surface methodology (RSM) is employed to maximize the mixing efficiency and flow rate of the converging-diverging microchannel. According to numerical results, the optimum parameters predicted by RSM are in excellent agreement with full numerical simulations.

Numerical simulation for electro-osmotic mixing under three types of periodic potentials in a T-shaped micro-mixer

Electro-osmotic process perpendicular to the mainstream direction is able to promote fluid mixing greatly under micro-scale condition. Three types of periodic potentials are applied on the electrodes fixed to the wall of a T-shaped micro-mixer. Mixing process and efficiency in different frequencies are obtained by numerical simulation. It is concluded that mixing efficiency fluctuates with the time when the frequency is relatively small and the fluctuation decreases until disappear when the frequency turns larger. Overall, mixing efficiency first rises and then drops with the frequency of periodic potential. By analyzing the concentration changes in micro-mixer under the condition of square wave function potential, the frequency and the concentration of the dart-like flow pattern observed contribute to the efficiency change in a micro-mixer. The results can offer a reference for the application of micro-mixers in Lab-On-a-Chip systems.

Effect of electroconvection during pulsed electric field electrodialysis. Numerical experiments

One of the ways of improving electrodialysis (ED) and some microfluidic processes is the optimization of current regime. It is recognised now that the use of pulsed electric fields (PEF) in ED allows enhancement of mass transfer and mitigation of fouling. To explain these effects, Mishchuk et al. [1] suggested that due to inertial forces electroosmotic mixing can remain during the pause. By solving fully coupled Nernst–Planck–Poisson–Navier–Stokes equations, we compute the distribution of velocity, concentration and electric fields in an ED channel. We simulate the situation where electroconvective vortices occur or not in the pause, and show that the total vortex attenuation takes tenths of a second. However, inertial forces are effective only first 0.01 s, then the vortices are fed by the chemical energy of non-uniform concentration field, the relaxation time of which is several seconds. The remnant vortices contribute to earlier onset of electroconvection. However, more important is the formation of new vortices after voltage re-application. For the first time, we show that nonuniformity of concentration field produces an effect similar to the action of electrically or geometrically non-uniform surface. It causes formation of spatially non-uniform electric body force, which hastens electroconvection.

Approximation for modelling electro-osmotic mixing in the boundary layer of membrane systems

A Computational Fluid Dynamics (CFD) model is used to compare the reliability and accuracy of the Helmholtz–Smoluchowski (HS) approximation against a one-dimensional charge density distribution (CD) solution of an electro-osmotic perturbation in a 2D membrane channel for reverse osmosis (RO). Electro-osmotic flow within the boundary layer is induced via the placement of a pair of electrodes parallel to the membrane surface and perpendicular to the bulk flow. Although this electrode configuration results in a two-dimensional electric field, the HS approximation considers only the electric field in the direction tangential to the membrane and not the normal component. The effect of the normal component of the electric field (Ey) on the flow field in the CD solution is compared against the solution using the HS approximation and it is found that the effect of Ey is minimal. Greater agreement between the CD solution and the HS approximation is found at higher bulk solute mass fractions, such as typically found on the membrane surface in RO systems. The relationships between cross-flow velocity, perturbation velocity and mixing are explored.

Numerical Investigation of Electroosmotic Mixing in Microchannels with Heterogeneous Zeta Potential

Numerical Investigation of Electroosmotic Mixing in Microchannels with Heterogeneous Zeta Potential

Numerical analysis of mixing enhancement for micro-electroosmotic flow

Micro-electroosmotic flow is usually slow with negligible inertial effects and diffusion-based mixing can be problematic. To gain an improved understanding of electroosmotic mixing in microchannels, a numerical study has been carried out for channels patterned with wall blocks, and channels patterned with heterogeneous surfaces. The lattice Boltzmann method has been employed to obtain the external electric field, electric potential distribution in the electrolyte, the flow field, and the species concentration distribution within the same framework. The simulation results show that wall blocks and heterogeneous surfaces can significantly disturb the streamlines by fluid folding and stretching leading to apparently substantial improvements in mixing. However, the results show that the introduction of such features can substantially reduce the mass flow rate and thus effectively prolongs the available mixing time when the flow passes through the channel. This is a non-negligible factor on the effectiveness of the observed improvements in mixing efficiency. Compared with the heterogeneous surface distribution, the wall block cases can achieve more effective enhancement in the same mixing time. In addition, the field synergy theory is extended to analyze the mixing enhancement in electroosmotic flow. The distribution of the local synergy angle in the channel aids to evaluate the effectiveness of enhancement method.

AC electro-osmotic mixing induced by non-contact external electrodes

We demonstrate efficient mixing in a micro-fluidic reservoir smaller than 10 microL using ac electro-osmosis driven by field-induced polarization. Our mixing device, of that electrodes are outside of the mixing unit, consists of three circular reservoirs (3mm in diameter) connected by a 1 mm x 1 mm channel. Unlike dc electro-osmosis, whose polarization is from charged substrate functional groups, this new mechanism uses the external field to capacitively charge the surface and the surface capacitance becomes the key factor in the electrokinetic mobility. The charging and mixing are enhanced at tailor-designed channel corners by exploiting the high normal fields at geometric singularities. The induced surface dielectric polarization and the resulting electric counter-ion double layer produce an effective Zeta potential in excess of 1 V, over one order of magnitude larger than the channel Zeta potential. The resulting ac electro-osmotic slip velocity scales quadratically with respect to the applied field, in contrast to the linear scaling of dc electro-osmosis and at 1cm/s and larger, exceeds the classical dc values by two orders of magnitude. The polarization is non-uniform at the corners due to field leakage to the dielectric substrate and the inhomogeneous slip velocity produces intense mixing vortices that effectively homogenize solutes in 30s in a 3mm reservoir, in contrast to hour-long mixing by pure diffusion.

ELECTROOSMOTIC MIXING INDUCED BY NON-UNIFORM ZETA POTENTIAL AND APPLICATION FOR DNA MICROARRAY IN MICROFLUIDIC CHANNEL

In this study, we describe a modification of the conventional microarray format. 20-mer oligonucleotide probes and singly labeled 20-mer targets, representative of the T-cell acute lymphocytic leukemia 1 (TAL1) gene, has been used to elucidate the performance of this hybridization approach. DNA microarray is integrated with microfluidic channel on a poly(methyl methacrylate) (PMMA) to generate non-uniform zeta potential inside the channel. A microtrench is designed and fabricated on a PMMA chip using a widely available CO2 laser scriber The electroosmotic mixing effect induced by the non-uniform zeta potential is utilized to enhance the DNA-DNA hybridization. The flow field in microfluidic channel is measured by particle image velocimetry (PIV). The enhanced signal to noise (S/B) ratio and reduced hybridization time is observed when the electroosmotic mixing is applied in the DNA-DNA hybridization.

Electroosmotic mixing in microchannels

Mixing is an essential, yet challenging, process step for many Lab on a Chip (LOC) applications. This paper presents a method of mixing for microfluidic devices that relies upon electroosmotic flow. In physical tests and in computer simulations, we periodically vary the electric field with time to mix two aqueous solutions. Good mixing is shown to occur when the electroosmotic flow at the two inlets pulse out of phase, the Strouhal number is on the order of 1, and the pulse volumes are on the order of the intersection volume.