Solute Dispersion by Electroosmotic Flow in Nonuniform Microfluidic Channels

Introduction Dispersion is a phenomenon that determines the resolution of several types of liquid phase chromatography, and is an important component of overall transport through electrokinetic flow systems. The severe dispersion of solute as it travels through a U-turn, for example, has attracted recent attention for this reason [1]. In “ideal” electroosmotic flow through uniform, straight channels (in the limit of thin double layers), solute dispersion is simply due to axial diffusion. However, in straight microfabricated channels, channel nonuniformities may arise due to surface roughness (e.g., due to etching or powderblasting fabrication methods [2]), or imperfections (e.g., etch pits, waviness due to poor resist adhesion, or imperfect bonding of channel lids). Other nonuniformities may be intentional, e.g., engineered post arrays for capillary electrochromatography. In this work, we measure dispersion in several types of nonuniform channels, and report their geometric Taylor-Aris dispersion coefficients αo. The dispersion coefficient relates the effective diffusion coefficient Deff = D12(1 + αoPe), to the binary diffusion coefficient, D12 as a function of the Peclet number [3,4].