Pressure-driven flow in a microchannel induces a streaming current due to the presence of an electrical double layer in the interface between the electrolyte solution and channel wall. As the streaming current is of the order of a nano-amphere and is additive, we propose here a method to develop an electrokinetic battery consisting of an array of microchannels that converts the hydrostatic pressure of a liquid into electrical work. We have given oscillating analytical solutions by means of an electrical circuit analysis to model the multi-microchannel battery. Using superposition of the appropriate Fourier series, the derived analytical solutions are useful to predict the current when there is more general time-dependent flow through a microchannel array. To illustrate the idea, we have studied steady-state pressure-driven flow in micropore porous glass filter and compared the results with those predicted from our model. From a 30 cm hydrostatic pressure drop, an external current of 1–2 µA was obtained by means of water passing through the micropore porous glass filter. A larger current can be obtained by simply using a solution with higher salt concentration. This results in a new and potentially useful method of energy conversion by means of an array of microchannels.

Poly(methyl methacrylate) (PMMA) has been modified via a dc pulsed oxygen plasma for different treatment times. The modified surfaces were characterized by X-ray photoelectron spectroscopy (XPS), optical profilometer, zeta potential, and advancing contact angle measurements. The measured advancing contact angles of water decreased considerably as a function of discharge. Several oxygen-based functionalities (carbonyl, carboxyl, carbonate, etc.) were detected by XPS, while zeta potential measurements confirmed an increase in negative charge for the treated PMMA surface. Evaluating the correlation between the concentration of polar chemical species and zeta potential, we found that increase in surface hydrophilicity results from the coeffect due to incorporation of oxygen functional groups and creation of charge states. The electrical double layer (EDL) effect was also considered in contact angle interpretation by introducing an additional surface tension term into Young's equation. We also found that EDL contribution to the solid-liquid interfacial tension is negligible and can be safely ignored for the systems considered here.

We show, by natural occurring phenomena of charge separation near the solid-liquid interface in microchannels, that electricity can be generated by forcing water through a ceramic rod with no moving part and emission. A single hand push on a syringe is our source of power which easily generates a streaming potential of over 20 V and a streaming current of 30 microA. By means of streaming potentials, two capacitors were charged and discharged alternatively to light-up two Light-Emitting-Diodes in every ten seconds. From our specific choice of liquid/solid pair, an efficiency of 0.8% was obtained. A mobile-ion-drain method is also demonstrated to increase the streaming potential.

Miniaturization of chemical analysis using microfabrication is an emerging technology. The use of polymeric materials as opposed to conventional glass substrate is also a promising alternative. As most polymeric materials are hydrophobic relative to glass, we describe here the implication for the loading process of electroosmotic flow (EOF) when a three-phase (solid-liquid-vapor) contact line exists. The presence of these interfaces can result in a large Laplace pressure that resists EOF and hence hinders its flow performance. This effect depends on the phenomenological contact angle at the solid-liquid interface. In our model for EOF, we considered simultaneously the presence of an electric double layer, liquid slips via a weaker solid-liquid interaction and Laplace pressure across a liquid-vapor interface.

It is typically assumed that flow field has very little or no effect on the electric double layer distribution. Here, we study this effect by means of a dilute electrolyte (deionized water) in a streaming potential mode for a 40-μm parallel-plate microchannel. By measuring the electrical potential downstream along the channel surface, we have shown that the potential distribution along a streamwise direction can be nonlinear. By means of a current continuity equation, we also presented numerical simulation results of potential distribution for pressure-driven flow with electrokinetic effects. The simulated results agree well with those from experiment. Results indicate that the conduction current in the streamwise direction is only a fraction of the streaming current. As a result, the ζ-potential values determined from streaming potential measurements are generally smaller than those from streaming current measurements. It is expected that this discrepancy would be smaller for a more concentrated electrolyte.

In many lubrication processes, lubricating oils containing polymer additives are subject to high shear rate through very small clearance channels. While the influence of shear rate on the performance of these lubricants has been well examined, very little is known about the effects of channel size. In this study a specially designed microchannel viscometer has been used to experimentally characterize the influence of channel height on the effective viscosity of oil lubricants with two different polymer additives (a radial hydrogenated styrene-isoprene copolymer and an A-B-A block ethylene-propylene copolymer) commonly used as viscosity index (VI) modifiers. The mass concentration of the polymer solutions ranged from 0.5% to 1.5% in this study. The viscosity was measured over a range of shear rates in steel slit microchannels with heights of 4.5, 7 and 11.5 lm, respectively. For all solutions a significant viscosity dependence on channel size was observed. In the higher shear rate range the smaller channels exhibited a lower viscosity while in the lower shear rate range all solutions exhibited a significant increase in viscosity. Generally, this observed increase in viscosity is more dramatic in the smaller channels. Possible causes of these behaviors were discussed in this paper. Ó

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Electrokinetic (EK) pumping efficiency has recently been a hot topic due largely to its applications in microfluidics for liquid transport. As the Onsager reciprocity relation suggests that the exact opposite of EK pumping is EK generation, experimental study on the efficiency of the latter is seldom reported. This paper examines electrokinetic generation efficiency in the presence of finite external loads through the use of different ceramic filters. The electrical output power was measured experimentally as a function of different external loadings. When the experiments were performed continuously on subsequent days, the EK generation efficiency for all the tested filters can be made to increase and approach a maximum limit. The experimental data also indicated that, when pore size of the filter decreases, the curve for potential versus current becomes more non-linear and the maximum efficiency does not occur when the current is half the zero-potential current.