Enhanced Microfluidic Separations Using a Combination of Electrically- and Pressure-Driven Flows

Separation is an important and integral part of most chemical analysis methods. Miniaturization of separation systems could improve their performance. With the development of microfluidic and micro-fabrication methods, miniaturization of separation methods has become possible. By miniaturizing separation methods onto microchips, separations can be realized faster, with less sample consumption, and higher energy efficiency. Unfortunately however, the utility of microfluidic separation systems is often restricted to the analysis of relatively simple samples due to the availability of limited footprint area. The objective of this dissertation work is to improve the separation performance of microfluidic systems using a combination of electrically- and pressure-driven flows. Resolutions of three microfluidic separation methods have been enhanced based on this strategy. In microfluidic capillary zone electrophoresis (CZE), a steady/periodic pressure-driven backflow was generated to counteract the electroosmotic flow in the forward direction which increases the residence time of the sample molecules in the electric field. This backflow was generated in our current work by fabricating a shallow segment downstream of the separation duct and applying an electric field across it. Another on-chip pumping unit which relies on sealing one end of a microchannel and applying an electric field across the channel has been shown to generate a strong and reproducible pressure in nanoscale ducts. A charge based separation was then developed in nanochannels by transporting the analyte bands using pure pressure-driven flow. A steady/periodic electrically-driven backflow was employed in this device to counteract the pressure-driven flow in the forward direction which not only enhanced the separation field but also increased the residence time of the sample molecules in it. In microfluidic hydrodynamic chromatography (HDC), a novel pre-concentration technique for focusing particle clusters was first developed while migrating them through a micochannel with a uniform electric field. An on-chip HDC separation of particle-based samples using a pressure-driven flow was improved by inducing an electrically-driven flow in the forward direction.