This thesis presents the results of study on two topics, the electrokinetic effect in artificially fabricated porous membranes and the dissipation dynamics at the fluid-solid interface. The first topic involves the theoretical and experimental study of electroosmotic pump (EOP) and its reverse effect, the generation of streaming potential through a pressure gradient across a porous sample. We designed and fabricated samples with various fluid channel diameters. In particular, we use the Onsager relation to check the consistency of the two effects and to obtain a value for the surface conductivity of our samples. The results show that the efficiency of the EOP decreases with the increasing channel diameter, from 2.5 μm to 4.5 μm. This is opposite to the trend observed for samples with mu...[ Read more ]

This thesis presents the results of study on two topics, the electrokinetic effect in artificially fabricated porous membranes and the dissipation dynamics at the fluid-solid interface. The first topic involves the theoretical and experimental study of electroosmotic pump (EOP) and its reverse effect, the generation of streaming potential through a pressure gradient across a porous sample. We designed and fabricated samples with various fluid channel diameters. In particular, we use the Onsager relation to check the consistency of the two effects and to obtain a value for the surface conductivity of our samples. The results show that the efficiency of the EOP decreases with the increasing channel diameter, from 2.5 μm to 4.5 μm. This is opposite to the trend observed for samples with much smaller channel diameters fabricated on anodized aluminum oxide films. Thus there is an optimal diameter for achieving maximum efficiency in EOP, in agreement with the theoretical prediction that the best efficiency is at a Debye length that is on the order of 1/5 the diameter of the microchannel. We have also developed a digital approach to flow rate control in EOP, using pulsed voltage with varying duty cycles. This approach has shown much more stability and tunability at both high and low flow rates, thus making EOP better suited for various applications in micro total analysis systems (μTAS).

The second part of the thesis involves simulation support to the development of a new interfacial microrheology technique using atomic force microscope (AFM) as a force sensor. The probe used for microrheology contains a long vertical glass fiber with one end glued onto a rectangular shaped cantilever beam and the other end immersed through a water-air interface.

The motion of the modified cantilever can be accurately described by the Langevin equation for a damped harmonic oscillator, from which we obtain the friction coefficient ξ of the glass fiber in contact with the water. It is found that ξ contains two contributions. One is generated by the bulk fluid, which increases with the immersion length of the glass fiber. The other contribution comes from the contact line between the water-air interface and the glass fiber, which is obtained by a linear extrapolation of the measured ξ at the limit of zero immersion length. The experiment thus demonstrates an application of AFM in the studies of interfacial microrheology and contact line dynamics. The numerical results are in excellent agreement with experimental data.