Microfluidic channels with integrated pillars are fabricated on SU8 and PDMS substrates to understand the capillary flow. Microscope in conjunction with high-speed camera is used to capture the meniscus front movement through these channels for ethanol and isopropyl alcohol, respectively. In parallel, numerical simulations are conducted, using volume of fluid method, to predict the capillary flow through the microchannels with different pillar diameter to height ratio, ranging from 2.19 to 8.75 and pillar diameter to pitch ratio, ranging from 1.44 to 2.6. The pillar size (diameter, pitch and height) and the physical properties of the fluid (surface tension and viscosity) are found to have significant influence on the capillary phenomena in the microchannel. The meniscus displacement is non-uniform due to the presence of pillars and the non-uniformity in meniscus displacement is observed to increase with decrease in pitch to diameter ratio. The surface area to volume ratio is observed to play major roles in the velocity of the capillary meniscus of the devices. The filling speed is observed to change more dramatically under different pillar heights upto 120 μm and the change is slow with further increase in the pillar height. The details pertaining to the fluid distribution (meniscus front shapes) are obtained from the numerical results as well as from experiments. Numerical predictions for meniscus front shapes agree well with the experimental observations for both SU8 and PDMS microchannels. It is observed that the filling time obtained experimentally matches very well with the simulated filling time. The presence of pillars creates uniform meniscus front in the microchannel for both ethanol and isopropyl alcohol. Generalized plots in terms of dimensionless variables are also presented to predict the performance parameters for the design of these microfluidic devices. The flow is observed to have a very low Capillary number, which signifies the relative importance of surface tension to viscous effects in the present study.

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Experimental and numerical investigation of capillary flow in SU8 and PDMS microchannels with integrated pillars

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This paper investigates the effects of nanoparticles on surface tension and equilibrium contact angle of TiO 2 - DI water nanofluids. Experimental measurements of surface tension by using the pendant droplet method show that the surface tension of the TiO 2 - DI water nanofluids depends weakly on nanoparticle concentration; however, at higher nanoparticle concentrations the surface tension is lower. Various mechanisms are reported to explain this behavior. Experimental measurements of contact angles of the TiO 2 - DI water nanofluids droplets on borosilicate glass slides exhibit strong nanoparticle dependence, and the general trend is increment of the contact angles with nanoparticle concentration. The effect from the so-called disjoining pressure due to the presence of nanoparticles within the thin nanofluid film wedge at the vicinity of the three-phase contact line is examined. However, the phenomenon is attributed to the pinning of contact line and local changes in solid-liquid interfacial tension due to the depositing of nanoparticles on adsorption sites on solid surface.

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Three kinds of silica nanoparticles with different surface functional groups (amino groups, epoxide groups, and alkyl chain groups) were prepared and used as fillers in the amine-cured epoxy resin systems to investigate the relationship between the interfacial interaction and the rheological and thermal properties of silica nanoparticles reinforced epoxy nanocomposites. It has been found that attractive interfacial interaction significantly contributed to reduce the viscosity and dramatically enhance the viscosity stability as well as reinforce the thermal stability of epoxy nanocomposites. While repulsive interfacial interaction had considerable negative effects on both of the viscosity and viscosity stability. Glass transition temperature of epoxy nanocomposites is closely related to the effects of the interfacial interaction on the curing process of bulk epoxy. This study may also shed lights on the choice of an optimum surface functional group for other inorganic fillers to improve the rheological properties and meanwhile keep good comprehensive thermal properties for epoxy nanocomposites.

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