Electro-Osmotic Flow in Reservoir-Connected Flat Microchannels With Non-Uniform Zeta Potential

Abstract

The effects of non-uniform zeta potentials on electro-osmotic flows in flat microchannels have been investigated with particular attention to reservoir effects. The governing equations, which consist of a Laplace equation for the distribution of external electric potential, a Poisson equation for the distribution of electric double layer potential, the Nernst-Planck equation for the distribution of charge density, and modified Navier-Stokes equations for the flow field are solved numerically for an incompressible steady flow of a Newtonian fluid using the finite-volume method. For the validation of the numerical scheme, the key features of an ideal electro-osmotic flow with uniform zeta potential have been compared with analytical solutions for the ionic concentration, electric potential, pressure, and velocity fields. When reservoirs are included in the analysis, an adverse pressure gradient is induced in the channel due to entrance and exit effects even when the reservoirs are at the same pressure. Non-uniform zeta potentials lead to complex flow fields, which are examined in detail.

An analytical approach to the effect of finite‐sized end reservoirs on electroosmotic transport through narrow confinements

“…The coefficient k in Eq. (11a) is referred to as the doublelayer thickness parameter [9]. The average number of positive or negative ions in the buffer n 0 is given by, n 0 5 1000CN A , where C is the molar concentration of ions, N A is Avogadro's number.…”

“…Equations (5), (6) and (8)(9)(10) are simultaneously solved in a two-dimensional framework by employing a fully time implicit finite volume technique, leading to a steady-state solution of the velocity field. A coupling between the pressure and velocity is accomplished by using the SIMPLE algorithm [19].…”

“…Such abrupt change in crosssectional area is likely to cause significant viscous dissipation which in turn leads to the irreversible pressure losses at both inlet and exit sections of the channel. Since both the reservoirs are kept at same pressure, electroosmotic flow has to adjust to those pressure boundary conditions by inducing an adverse pressure gradient along the stream-wise direction [8][9][10]. It is important to recognize here that the electroosmotic driving mechanism actually needs to overcome this adverse pressure field in order to maintain unidirectional fluidic transport.…”

Influence of varying electroosmotic flow on the effective diffusion in electric field gradient separations

“…The majority of EOF studies in the literature have addressed conditions where a constant applied electric field has been imposed along the capillary length. Several recent studies have explored the characteristics of EOF in channels exhibiting a streamwise varying zeta potential [1,[26][27][28][29][30][31]. These studies have focused on the effects of step changes in z, linear and periodic variations in z, and a randomly varying z along the capillary axis.…”

“…Such applications include micropower generation, microscale heat exchangers, chemical and biological separation processes, and other biomedical diagnostic techniques. Electroosmotic pumping can often provide a sound alternative to classical pressure-forced motion at low flow rates in microscale applications due to better flow control and the absence of moving parts [1]. Furthermore, in microscale separations processes utilizing channels of diameter in the nominal range of 20-100 mm, the velocity distribution is uniform in the absence of pressure gradients [2].…”