Electrokinetic flow in a free surface-guided microchannel

Abstract

The purpose of this study is to investigate electro-osmotic flow in a free surface-guided microchannel. Although multiphase microfluidics has attracted interests over the past few years, electro-osmotic flow involving free surfaces has yet to be studied in great detail. Several proposed theoretical models describing this type of electro-osmotic flow need to be verified by experiments. In this work, a surface-guided microchannel was fabricated using an innovative fabrication process. Because the liquid stream was confined by surface properties, solid sidewalls did not exist in this microchannel. Instead, the sidewalls were water-air or water-oil interfaces. Using this microchannel, two systems were investigated: water-air system and water-oil system. The experimental results were compared against three proposed models in order to gain more understandings on this type of electro-osmotic flow. Experimental results show that the liquid velocity near the liquid-fluid interface resembles a pluglike profile for both water-air and water-oil systems. Computer simulation results show that with the consideration of the electrical double layer and the surface charges, the electric forces inside the electrical double layer are counteracted by the surface forces at the liquid-fluid interface, also resulting in a pluglike velocity profile in the microchannel. Therefore, the model that considers both the electrical double layer and the surface charges at the liquid-fluid interface best describe the physical phenomenon observed in experiments.