Interdiffusion in microfluidic interface rotation between two laminarly flowing liquids of different densities

Targeted and rapid mixing is a common task in microfluidic devices. Owing to their small dimensions, flows in micromixers are laminar, and mixing is primarily diffusion driven. Y-mixers, which have two inlet channels and one mixing channel, are particularly well studied. In these channels, interface-reorientation can occur: the interface tilts from a side-by-side flow to a top-to-bottom flow. This impacts mixing performance and may complicate product extraction. Studies have demonstrated that the density gradient between fluids and the interdiffusion of solutes affect the interface-reorientation. However, systematic experimental research considering both influences is lacking. Traditional experimental methods, such as confocal fluorescence microscopy, struggle to provide highly resolved and marker-free measurements of interdiffusion constants. These methods rely on added markers, and it remains uncertain whether these impact fluid properties. In this work, we employ confocal Raman microscopy as a quantitative, highly resolved, and marker-free method to simultaneously measure the interdiffusion constants and interface orientation in situ. Isopropanol:water, deuterated water:water, and ethanol:water in 3-dimensional (3D)-printed microfluidic Y-channels were measured. A general model for interface-reorientation, considering densities and interdiffusion, is derived using the Buckingham pi theorem. Diffusion positively affects the interface rotation rate. The diffusion constants below which the diffusive influence can be neglected are on the order of 1.0×10−10 to 15.2×10−10 m2/s. A fundamental comprehension of interface-reorientation is essential for the strategic engineering of microfluidic devices, the selection of optimal fluid systems, and the precise determination of external parameters, such as fluid flow velocity. Applications include membraneless fuel cells, chemical processing cells, in-channel surface functionalization, and in-channel micropatterning.