Bidirectional AC electroosmotic pumping by hydrodynamic channeling using 2D and 3D photoresist-derived carbon microelectrodes

Abstract

In microfluidics, fluid movement is keystone for the correct operation of different system stages, including convective mixing, electrochemical sensing, and affinity bonding in immunoassays. Even though the most commonly developed microfluidic pumps are unidirectional, over the last decade, bidirectional approches have been developed to considerably improve the overall performance of microfluidic systems. Furthermore, the exploration of new approaches for bidirectional pumping allows the creation on fully integrated, stand-alone systems for applications including drug delivery, clinical analyses, cell culture, among many others.

AC electroomosis is a suitable electrokinetic approach for fluid driving in microchannels. Opposite to commonly-used microfluidic pumping approaches, in AC electroosmosis no moving elements are required, and only a set of electrodes is used, thus fabrication of devices is remarkably simple and cost-effective as a standalone pump, or for integration with other microfluidic components. In this work we propose a hydrodynamic mechanism to reverse the flow in AC electroosmotic micropumps in order to attain bidirectional pumping. The flow reversal mechanism involves the channeling of the vortices generated by electroosmotic manners, by taking advantage of the microchannel geometrical properties; particularly, the microchannel height, which is closely related to the stimulating electrodes’ width and, in consequence, to the enclosed vortices’ size.

We use the Carbon-MEMS fabrication process to develop two electrode architectures: asymmetric planar and high-aspect-ratio microposts. The carbonaceous material obtained from this process is characterized to validate the adecuate properties for electrokinetic applications, and the micropumps are then assembled by bonding a PDMS microchannel. The flow development by AC electroosmotic means is profoundly studied using a bidimensional finite element model comprehending the Poisson-Nernst-Plank-Navier-Stokes equation set to closely emulate the vortex formation on the electrodes surface. To explore the effect of electrode asymmetry ratio on the fluid velocity, we compare three asymmetry ratios for both, coplanar, and highaspect-ratio architectures.

Experimental results are presented and a fluid velocity analysis is carried out for forward and reverse flows. An explanation of how flow reversal is achieved by hydrodynamic channeling is detailed, and experimental test analysis provides the conditions under flow reversal is produced, such as amplitude and frequency of the applied AC signal, and asymmetry ratio of electrodes. Furthermore, the effect of microposts on fluid velocity and flow reversal is thoroughly discussed. Hydrodynamic channeling using AC electroomosis is a new approach for bidirectional pumping, thus areas of opportinity are presented for future developments in this field, such as optimization of electrode asymmetry ratio, micropost spacing and microchannel height.