Diffusion-based separation using bidirectional electroosmotic flow

In contrast to properties such as electrophoretic mobility, size, and chemical affinity, which are the basis of most standard separation techniques (e.g. electrophoresis, chromatography, dialysis), diffusivity, despite being a fundamental property of any molecule, has not been extensively used for separation purposes. Moreover, diffusion is often an undesired phenomenon as it is the source of dispersion, known as band broadening in chromatography and capillary-based methods. In this work, we report on an entirely new approach for separation of particles and molecules based on their diffusivity. We pattern the bottom of an unobstructed fluidic chamber with stripes of alternating surface charges, and upon the application of an external electric field parallel to the stripes, a bidirectional electroosmotic flow pattern is formed. The surface charge of each stripe is obtained by controlling the potential of an electrode array embedded in the floor of the microfluidic chamber and the potential in the chamber. High-diffusivity particles introduced into the bidirectional flow rapidly diffuse across stripes and experience an average zero velocity, whereas low-diffusivity particles remain on single stripes and therefore advect through the microfluidic chamber. We demonstrate the applicability of this method for separation of low-diffusivity beads from high-diffusivity dye, and a separation of labeled antibodies from free labeling dye. Together with experiments, we present a theoretical analysis of the system, discussing the dependence of the separation on the Péclet number, and provide engineering guidelines for its design and operation. This method could be particularly useful for small sample volumes that are often not compatible with standard separation techniques, and we believe this could enrich the toolset available for analysis and sample preparation in lab-on-chip applications.