This paper presents an analytical expression of the pressure barrier in a capillary-burst valve for flow regulation in centrifugal microfluidics. The analysis considers variations of the interfacial energies at the meniscus of three-dimensional (3D) configuration in a rectangular microchannel with a sudden expansion in cross-section. We derive a simple expression that predicts the critical burst pressure or rotational speed to overcome the capillary valve. Experiments were carried out for capillary valves that were integrated with microchannels on a rotating disk having various cross-sectional dimensions (300 and 400 mu m in width and 80-600 mu m in depth) and wedge angles (30 degrees-100 degrees) of sudden expansion. The flow visualization of the meniscus development across the capillary valve supports the assumptions made for the present analysis. The measurements of burst rotational speeds for the capillary valves are in good agreement with the predictions by the simple expression except that those with a larger channel width and wider wedge angles are nearly 10% lower than the predicted values

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The mixing phenomena for two fluid streams in pressure-driven rectangular microchannels are analyzed and directly compared with the measurements of mixing intensity for a wide range of aspect ratio (width/depth = 1-20). In the analysis, the three-dimensional transport equation for species mixing was solved using the spectral method in a dimensionless fashion covering a large regime of the normalized downstream distance. The analysis reveals the details of non-uniform mixing process, which originates from the top and bottom walls of the channel and stretches out toward the center of the channel, and its transition to uniformity. Employing different length scales for the non-uniform and uniform mixing regimes, the growth of mixing intensity can be expressed in a simple relationship for various aspect ratios in the large range. The mixing experiments were carried out on silicon- and poly(methyl methacrylate) (PMMA)-based T-type micromixers utilizing fluids of pH indicator (in silicon channel) and fluorescent dye (in PMMA channel) to evaluate the mixing intensity based on flow visualization images. Using conventional microscopes, the experiments demonstrate the mixing intensity as a power law of the stream velocity for all the microfluidic channels tested. The variations of measured mixing intensity with the normalized downstream distance are found in favorable agreement with the numerical simulations. The comparison between the experiments and simulations tells the capabilities and limitations on the use of conventional microscopes to measure the mixing performance