Stream lamination and rapid mixing in a microfluidic jet for X-ray spectroscopy studies

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

Liquid Heterostructures: Generation of Liquid-Liquid Interfaces in Free-Flowing Liquid Sheets

Chemical reactions and biological processes are often governed by the structure and transport dynamics of the interface between two liquid phases. Despite their importance, our microscopic understanding of liquid-liquid interfaces has been severely hindered by difficulty in accessing the interface through the bulk liquid. Here we demonstrate a method for generating large-area liquid-liquid interfaces within free-flowing liquid sheets, which we call liquid heterostructures. These sheets can be made thin enough to transmit photons from across the spectrum, which also minimizes the amount of bulk liquid relative to the interface and makes them ideal targets for a wide range of spectroscopies and scattering experiments. The sheets are produced with a microfluidic nozzle that impinges two converging jets of one liquid onto two sides of a third jet of another liquid. The hydrodynamic forces provided by the colliding jets both produce a multilayered laminar liquid sheet with the central jet is flattened in the middle. Infrared microscopy, white light reflectivity, and imaging ellipsometry measurements demonstrate that the buried layer has a tunable thickness and displays well-defined liquid-liquid interfaces, and that the inner layer can be thinner than 100 nm.Comment: 30 pages, 8 figures, 1 table. Supplement: 19 pages, 8 figure