The effect of an electrical double layer (EDL) on microchannel flow has been studied widely, and a constant bulk electric conductivity is often used in calculations of flow rate or pressure drop. In our experimental study of pressure-driven micropipette flows, the pipette diameter is on the same order of magnitude as the Debye length. The overlapping EDL resulted in a much higher electric conductivity, lower streaming potential, and lower electroviscous effect. To elucidate the effect of overlapping EDL, this paper developed a simple model for water flow without salts or dissolved gases (such as CO(2)) inside a two-dimensional microchannel. The governing equations for the flow, the Poisson, and Nernst equations for the electric potential and ion concentrations and the charge continuity equation were solved. The effects of overlapping EDL on the electric conductivity, velocity distribution, and overall pressure drop in the microchannel were quantified. The results showed that the average electric conductivity of electrolyte inside the channel increased significantly as the EDL overlaps. With the modified mean electric conductivity, the pressure drop for the pressure-driven flow was smaller than that without the influence of the EDL on conductivity. The results of this study provide a physical explanation for the observed decrease in electroviscous effect for microchannels when the EDL layers from opposing walls overlap.
Micropipette injection has wide applications in genetic, physiological, pharmacological and micro-chemical research at pico-liter or sub-pico liter level. Micropipettes are generally tapered glass tubes with the inner exit diameter of 0.2 to a few microns. The quantitative relationship describing the injection volume and the operational parameters and pipette geometry in the microinjection process, however, has never established. This paper experimentally studied the injection flow rate as a function of injection pressure as well as the pipette geometry and fluid properties for the hydrophilic glass surface. It was found that the experimental pressure drop for the pressure-driven flow was always less than that was predicted by the classical theory with no slip boundary conditions. A model with slip boundary condition was developed for the axisymmetric conical flow and the result agreed well with computational simulation with slip boundary and the experimental data. The analysis indicated that the slip length was about 0.12 μ for water flow through micropipette of exit diameter 0.94 ~ 4.48 μm, half cone angle 3.3 ~ 5.2°, surface roughness 2.1 ~3.8 nm with shear rate ranging from 106 to 107 s-1.
The critical heat flux of fast transient boiling impacts the safety assessment of nuclear reactor designs. This paper conducted a series of experiments to explore the critical heat flux of saturated water boiling at different heating rates on a platinum wire. The wire is placed in a glass box surrounded by deionized water at saturation temperature. The diameter of wire is 126 μm, and a constant voltage power supply is used. Data is collected at 250k sample/s. A high-speed camera with 50,000 fps is used to record boiling video. Eight tests are presented with 2 steady boiling experiments and 6 transient boiling experiments. In steady boiling experiments, both the wire temperature and heat flux to the water increase until they reach a steady point. In the transient boiling experiments, the wire temperature keeps increasing. But the heat flux reaches a peak first, a critical heat flux; then it goes down to a certain value before the power is shut off. The higher the wire heating rate, the larger the critical heat flux peak will be. The mechanism of the critical heat flux peak relates to bubble formation, propagation, and coalescence, and is discussed based on the high-speed camera images. This initial research in transient boiling paved the way for further experiments with subcooling at prototypical temperature and pressure conditions in a nuclear pressured water reactor.
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