Three-Dimensional Structure of Electroosmotic Flow over Heterogeneous Surfaces

Abstract

Electroosmotic flow is widely used as a primary method of species transport in microscale biological and chemical analysis systems commonly referred to as labs-on-a-chip. In these systems, surface electrokinetic heterogeneity can be introduced either intentionally through micromanufacturing technology, such as microcontact printing, or unintentionally through, for example, bioparticle adhesion. In either case it is desirable to examine the influence of these surface heterogeneities on the electroosmotic flow structure. In this paper a numerical model based on a simultaneous solution to the Nernst-Planck, Poisson, and Navier-Stokes equations is used to examine the electroosmotically driven flow through a microchannel exhibiting a periodically repeating patchwise heterogeneous surface pattern. The simulations have revealed a distinct three-dimensional flow structure that, depending on the degree of heterogeneity, varies from a weak circulation perpendicular to the applied electric field to a fully circulatory flow system. In general the induced flow structure is found to penetrate into the bulk flow no deeper than the length scale of the heterogeneous patches. In addition the electrophoritic influence of the applied electric field on the net charge density in the double layer is shown to cause a significant deviation from the traditional Poisson-Boltzmann distribution. The overall effect of this double-layer rearrangement on the flow structure, however, is found to be negligible.

Electrokinetic mixing in microfluidic systems

“…For example, various researchers have performed theoretical studies of electroosmotic flows in 2-D microchannels driven by time-dependent zeta potentials or a time-dependent external electric field, and have demonstrated that chaotic advection can be induced by time-wise periodic alternations of the zeta potential and can improve the mixing performance as a result (Qian and Bau 2002); these are to be described in the section of active mixing scheme. In addition, chaotic advection or transverse flows can also be induced in three-dimensional steady electroosmotic flows with time-independent zeta potentials through the use of specific surface charge patterning configurations (Ajdari 1996(Ajdari , 2001Erickson and Li 2003;Ng et al 2004;Yang and Chang 2004;Biddiss et al 2004;Chang and Yang 2006). Generally, heterogeneous surface charges or non-uniform time-independent zeta potentials are obtained through passive approaches such as coating the microchannel walls with different materials (Liu et al 2000;Stroock et al 2000;Fushinobu and Nakata 2005) or applying suitable surface-chemistry treatments (Hau et al 2003;Krishnamoorthy et al 2006).…”

Theory, fabrication and applications of microfluidic and nanofluidic biosensors

“…In such cases, when the flow is being driven both by a pressure gradient and an applied potential difference, the pressure gradient induces a flow of charge in the diffuse layer and generates a finite voltage called the streaming potential. The electric field in the fluid then is no longer one-dimensional [100]. In such cases, numerical solutions are often needed.…”

An experimental study on an electro-osmotic flow-based silicon heat spreader

“…However, both materials have poor thermal conductivities. Thus, the micro-fluidic devices made by PDMS and glass have difficulty in dissipating the heat which is generated internally when a high electric field is applied between the electrodes (Tang et al 2004;Erickson and Li 2003). Due to the reasons cited above, silicon has been chosen as the material to fabricate the EO heat spreader in the present study.…”