Surface effects of microchannel wall on microfluidics

Microfluidics is widely used in MEMS (microelectromechanical system) devices. Because of the large ratio of surface area to liquid volume of microfluidics, surface effects significantly influence the fluid behavior. In this thesis, two surface effects: electrical double layer (EDL) and hydrophobicity are studied. We have performed fundamental modelling of time-dependent electrokinetic flow in rectangular, circular and parallel-plate microchannels. A new model, combining electrokinetic phenomena with slippage of microfluidics, is created and the corresponding analytical solutions of time-dependent electrokinetic slip flow in rectangular, circular and parallel-plate microchannels are obtained. According to our study, electrokinetic phenomena and hydrophobicity should be considered simultaneously. Electroosmotic flow with a moving front is a typical case governed by EDL phenomena and surface hydrophobicity. In this thesis, we first propose an analytical model to describe this phenomena. Based on these models, we further study the microfluidic flow in a heterogeneous microchannel with nonuniform surface potential and hydrophobicity. We also investigate the effects of different EDL models on microchannel flow. Comparison of these models suggests that the Possion-Boltzmann equation is still a valid model to describe the charge distribution in EDL of microfluidics.; According to the above theoretical study, potential applications of microfluidics are proposed. A new method to determine zeta potential (or surface potential) and slip length is proposed, which overcomes the defect of the current zeta potential measurement method on hydrophobic surfaces. We also extend our research to membrane science, where a method is proposed to determine unknown parameters and structure of multilayer membranes by means of a high frequency alternating electric field. By means of streaming current or streaming potential, we designed two novel electrokinetic batteries, allowing us to light up two LEDs. We also found that slip coating can improve efficiency of microfluidic MEMS devices. A promising slip coating for MEMS devices, Self-Assembled Monolayers (SAMs), is proposed. In experiments, we observed slippage of flow on Au/S(CH 2)17CH3 Self-Assembled Monolayers (SAMs). The slip lengths of SAMs were experimentally determined.