| Electrokinetic based microflows are studied both experimentally and theoretically. Soft lithography technique is used to form microchannels on poly-di-methyl siloxane (PDMS), while a glass slide or thin PDMS layer is used to cover the microchannels. For microflow quantification, an in-house micro scale particle image velocimetry (muPIV) system is developed from an existing PIV system.; The first part of the experimental work focuses on quantifying electrokinetic microflows in trapezoidal microchannels. In this investigation, pressure, electroosmotic, and mixed electroosmotic-pressure driven cases are considered. Experimental results obtained from muPIV are compared with three-dimensional numerical models; the results show an excellent agreement between experimental work and simulation. Trapezoidal microchannels provide tapered-cosine velocity profiles if there is pressure gradient in the streamwise direction. The experimental results verify that velocity distribution in mixed flow can be decomposed into pressure and electroosmotic driven components.; In the second part of the experimental work, a field-effect transistor, which locally modifies the surface charge condition, is developed to control flow in microfluidic chips. By applying a gate voltage to one side of the wall, zeta potential on the controlled surface is altered which results in a secondary electroosmotic flow in the lateral direction. Flow control is observed both quantitatively and qualitatively at relatively low voltage (in the order of 101 [V]) having leakage current through the interface between PDMS and glass layers. A leakage capacitance theory is introduced to estimate the zeta potential at the straight channel wall, and the predicted zeta potential agrees with the experimental results.; Finally, a theoretical study is presented to investigate the microflow behavior in mixed non-uniform electroosmotic flow and pressure driven flow due to step change in zeta potential. A biharmonic equation in terms of stream functions is solved with double-sided Laplace transformation, and the explicit analytical expressions for the entire velocity field and pressure distribution in a two-dimensional straight microchannel are found. Possibilities of recirculation and separation of the flow, which leads to hydrodynamic dispersion, band broadening, and deformation of the band shape, are examined. |
