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To measure a microflow rate and to accelerate the reaction between proteins by an unbalanced impingement of feedback flow, we have proposed and verified the design of a self- flapping microfluidic oscillator. Three specific features - the large aspect ratio of the micronozzle, the structure of the sudden- expansion inlet, and the asymmetric feedback channels - are developed to induce stable oscillation. The large aspect ratio of the micronozzle diminishes the influence of viscous force, and the inlet structure triggers flow instability. The conjunction of both factors promotes the occurrence of the Coanda effect, and initiates oscillation. The asymmetric feedback channels produce an unbalanced impingement of the inlet flow, thus reinforcing the initial oscillation to become stably periodic. Beyond the function of a microflowmeter, the oscillatory characteristics are applicable to accelerate the biochemical reaction between two fluorescent proteins, B-phycoerythrin and an allophycocyanin alpha subunit. With fluorescence induced with a laser, we detected the proteins at a specific wavelength to define the region of interaction caused by the oscillatory motions, which clearly enhances the rate of reaction of these fluids. To focus on the reaction phenomenon of twin fluids, we demonstrated biotin-streptavidin binding that was detected via a fluorescence-resonance-energy-transfer (FRET) pair of fluorescent proteins. The FRET signal demonstrated conclusively that the biochemical reaction was promoted through the oscillatory function.