Hydrodynamic dispersion in a combined magnetohydrodynamic- electroosmotic-driven flow through a microchannel with slowly varying wall zeta potentials (original) (raw)
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Proceedings of the 3rd World Congress on Mechanical, Chemical, and Material Engineering, 2017
In a parallel-flat plate microchannel, with nonuniform zeta potential of the wall, we analyse the dispersion of a passive solute under the simultaneous influence of electroosmotic (EOF), and magnetohydrodynamic (MHD) forces. The hydrodynamic of the flow was solved using the lubrication approximation theory (LAT) and we assume a Newtonian fluid. The solution of the electrical potential is based on the Debye-Hückel approximation for a weak potential of a symmetric (z : z) electrolyte solution. It is shown that the interaction between the non-uniform wall zeta potential induces a pressure gradient so as to satisfy the continuity of flow, generating a no plug like velocity profiles that contribute directly to dispersion. It is also shown that with the adding of the MHD the velocity flow increase two times its value, and the dispersion may increase more than four times as compared against the case of a purely electroosmotic forces.
Micromachines
This paper investigates the electroosmotic micromixing of non-Newtonian fluid in a microchannel with wall-mounted obstacles and surface potential heterogeneity on the obstacle surface. In the numerical simulation, the full model consisting of the Navier–Stokes equations and the Poisson–Nernst–Plank equations are solved for the electroosmotic fluid field, ion transport, and electric field, and the power law model is used to characterize the rheological behavior of the aqueous solution. The mixing performance is investigated under different parameters, such as electric double layer thickness, flow behavior index, obstacle surface zeta potential, obstacle dimension. Due to the zeta potential heterogeneity at the obstacle surface, vortical flow is formed near the obstacle surface, which can significantly improve the mixing efficiency. The results show that, the mixing efficiency can be improved by increasing the obstacle surface zeta potential, the flow behavior index, the obstacle heig...
Electro-osmotic flow of viscoelastic fluids in microchannels under asymmetric zeta potentials
Journal of Engineering Mathematics, 2011
The flow of viscoelastic fluids between parallel plates under the combined influence of electro-osmotic and pressure gradient forcings with asymmetric boundary conditions, by considering different zeta potentials at the walls, is investigated. The fluids are z-z symmetric electrolytes. The analytic solutions of the electrical potential, velocity distributions and streaming potential are based on the Debye-Hückel approximation for weak potential. The viscoelastic fluids used are modelled by the simplified Phan-Thien-Tanner constitutive equation, with linear kernel for the stress coefficient function, and the Finitely Extensible Nonlinear Elastic dumbbells model with a Peterlin approximation for the average spring force. The combined effects of fluid rheology, electrical doublelayer thickness, ratio of the wall zeta potentials and ratio between the applied streamwise gradients of electrostatic potential and pressure on the fluid velocity and stress distributions are discussed.
Numerical Investigation of Electroosmotic Mixing in Microchannels with Heterogeneous Zeta Potential
Advanced Science, …, 2011
The present study reports numerical analysis of micromixing for mixed electroosmotic/pressure driven flow of Newtonian fluid in microchannels. Two dimensional Laplace, Poisson-Boltzmann, momentum, and species concentration equations are solved numerically using finite volume method and SIMPLE algorithm. The equations are solved for rectangular microchannels with heterogeneous zeta potential distribution along the channel walls. Flow streamlines are presented for microchannels with and without electroosmotic effect. The simulation results indicate an enhancement in species mixing by introducing the heterogeneous electroosmotic effect. In addition, effects of zeta potential, bulk electrolyte concentration, external applied electric field, Reynolds number, and channel height are investigated on characteristics of species mixing. Results show that species mixing is improved by increasing the zeta potential, bulk electrolyte concentration, external applied electric field, and the channel height; whilst mixing efficiency is decreased by increasing Reynolds number.
Analytical Chemistry, 2004
The hydrodynamic dispersion of a nonadsorbed and nonelectrolyte solute is considered for the case of a flow driven through a straight microchannel by pressure and electric potential differences. The analysis is conducted using a thin double layer approximation developed in the previous paper (Zholkovskij, E. K.; Masliyah, J. H.; Czarnecki, J. Anal. Chem. 2003, 75, 901-909). On the basis of this approach, an expression is derived to address the dispersion coefficient for arbitrary electrokinetic potential, electrolyte type, and cross-section geometry. In the derived expression, the influence of cross-section geometry manifests itself through the channel hydrodynamic radius and through three dimensionless geometrical factors. The procedure for obtaining the geometrical factors is presented for an arbitrary cross-section geometry. The geometrical factors are evaluated for several examples of cross section: (i) unbounded parallel planes; (ii) circle; (iii) annulus; (iv) ellipse; (v) rectangle. The dependency of the dispersion coefficient on different parameters is discussed. It is shown that the dependencies are substantially affected by the cross-section geometry, electrolyte type, and electrokinetic potential.
Electroosmotic effect on flows in a serpentine microchannel with varying zeta potential
2007
Electroosmosis is widely observed in different micro-scale flows. Flow modification due to electroosmosis is important for applications related to fuel cells such as micro direct methanol fuel cells and micro-scale devices. Mechanism for delivering methanol into the micro-fuel cell is very important process affecting its power density and often they need to be stand alone systems with no external pumps or other ancillary devices to eliminate the parasitic power loss accounting from fuel feed. In this study, numerical simulation has been carried out for the electroosmotic effect on pressure-driven flows in a serpentine microchannel whose side walls are subjected to variable zeta potentials. It is observed that for a non-uniform zeta potential, the secondary vortex pair and their strength of motion generated by electroosmosis is modified asymmetrically, corresponding to the degree of the variation in and the magnitude of the zeta potential. It is also found that the flow profiles are quite different for variable zeta potentials as compared to a constant zeta potential applied on the channel walls. The flow phenomena at the bend of the serpentine channel has also been investigated, and flow control is possible by regulating the applied zeta potential on the channel walls. It is observed that vortices shift in a clockwise direction as the applied zeta potential is increased on the walls of the serpentine channel. Formation of additional vortices has also been observed for very low values of applied zeta potential.
Micromachines, 2018
Electroosmotic flow (EOF) is one of the most important techniques in a microfluidic system. Many microfluidic devices are made from a combination of different materials, and thus asymmetric electrochemical boundary conditions should be applied for the reasonable analysis of the EOF. In this study, the EOF of power-law fluids in a slit microchannel with different zeta potentials at the top and bottom walls are studied analytically. The flow is assumed to be steady, fully developed, and unidirectional with no applied pressure. The continuity equation, the Cauchy momentum equation, and the linearized Poisson-Boltzmann equation are solved for the velocity field. The exact solutions of the velocity distribution are obtained in terms of the Appell’s first hypergeometric functions. The velocity distributions are investigated and discussed as a function of the fluid behavior index, Debye length, and the difference in the zeta potential between the top and bottom.
Micromachines, 2017
In this paper, a systematic study of a fully developed electroosmotic flow of power-law fluids in a rectangular microchannel bounded by walls with different zeta potentials is described. Because the upper and lower layers of most microchannels are made of different materials, it is necessary to study the flow characteristics for cases in which the microchannels have different zeta potentials at each wall. The electrical potential and momentum equations were solved numerically using a finite element analysis. The velocity profiles and flow rates were studied parametrically by varying the fluid behavior index, channel aspect ratio, and electrochemical properties of the liquid and the bounding walls. The calculated volumetric flow rates in a rectangular microchannel were compared with those between two infinite parallel plates.
Numerical Modelling of Electrokinetic Flow in Microchannels: Streaming Potential and Electroosmosis
2020
Investigating the flow-behavior in microfluidic systems has become of interest due to the need for precise control of the mass and momentum transport in microfluidic devices. In multiphase flows, precise control of the flow behavior is much more challenging as it depends on multiple parameters. The following thesis focuses on two aspects of microfluidics discussed in two chapters: the flow reversal phenomenon in streaming potential flows and the magnetic fields generated by electroosmotic and streaming potential flows. In the first chapter, the proposed microfluidic system consists of an aqueous solution between a moving plate and a stationary wall, where the moving plate represents a charged oil-water interface. A numerical model was developed to predict the streaming potential flow created due to the shear-driven motion of the charged upper wall along with its associated electric double layer (EDL) effect. Additionally, analytical expressions were derived by solving the nonlinear Poisson-Boltzmann equation along with the simplified Navier-Stokes equation in order to describe the effect of the EDL on the sheardriven flow of the aqueous electrolyte solution. Results show that the interfacial charge of the moving interface greatly impacts the velocity profile of the flow and can reverse its overall direction. The numerical results were validated by the analytical expressions, where both models predicted that flow can reverse its overall direction when the surface potential of the oil-water interface exceeds 120mV. For the second chapter, models were constructed for the transient electrokinetics, for both the electroosmotic flow and for the shear driven streaming potential flow, in a charged nanocapillary channel. Additionally, the transient effects of ionic currents and the magnetic field generated both inside and outside the microchannel were evaluated, and the results compared with known iii analytical solutions for verification purposes. In order to correctly simulate the above models, the following partial differential equations are solved together for the electrolyte continuum to capture the physics of the problem: a) the Navier-Stokes equation for the fluid flow b) Poisson-Nernst-Planck equations for the electric potential distribution and ion transport and c) Ampere-Maxwell's law for the associated magnetic field. The obtained results showed that the magnetic field detected outside of the nanochannels can be used as a secondary electromagnetic signal for biomolecules as a part of a sequencing technique.