Stream lamination and rapid mixing in a microfluidic jet for X-ray spectroscopy studies (original) (raw)
Microfluidic mixers offer new possibilities for the study of fast reaction kinetics down to the microsecond time scale, and methods such as soft X-ray absorption spectroscopy are powerful analysis techniques. These systems impose challenging constraints on mixing time scales, sample volume, detection region size and component materials. The current work presents a novel micromixer and jet device which aims to address these limitations. The system uses a so-called 'theta' mixer consisting of two sintered and fused glass capillaries. Sample and carrier fluids are injected separately into the inlets of the adjacent capillaries. At the downstream end, the two streams exit two micron-scale adjoining nozzles and form a single free-standing jet. The flow-rate difference between the two streams results in the rapid acceleration and lamination of the sample stream. This creates a small transverse dimension and induces diffusive mixing of the sample and carrier stream solutions within a time scale of 0.9 microseconds. The reaction occurs at or very near a free surface so that reactants and products are more directly accessible to interrogation using soft X-ray. We use a simple diffusion model and quantitative measurements of fluorescence quenching (of fluorescein with potassium iodide) to characterize the mixing dynamics across flow-rate ratios. Impact Statement This study presents the design, demonstration and quantification of a novel mixer designed to address constraints associated with reaction rate studies using soft X-ray spectroscopy. Low-flow-rate, rapid micromixers typically use laminar flow focusing where a sample stream is confined within a carrier stream and, often, within a microfluidic device. This limits the possible spectroscopic methods to hard X-ray spectroscopy, including significant absorption by the carrier stream and microfluidic device, and reduced energy resolution. In this study, the sample is laminated at the surface of a free jet to allow direct optical access to the mixing zone. We demonstrate and quantify a mixing time scale of 0.9 µs. The mixing and reaction occur within approximately 0.1 µm from the surface of the jet. This micromixer thus enables the analysis of reactions with fast kinetics using techniques with demanding experimental constraints such as the 3d transition metal Ledge X-ray absorption spectroscopy (XAS).
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