Beyond Laminar Mixing: the Advantages of Combining Mixing Approaches in Microfluidic Chips

The effective mixing of two or more fluids is a key target for virtually all microfluidic-assisted processes. However, making this action rapid is generally difficult because of the inherently laminar character of the flow in small channels (low Reynolds / high Peclet numbers). The most common approaches (passive micromixers) are the induction of chaotic features (e.g. staggered herringbone chips) and the enhancement of liquid-liquid interfaces (e.g. split-and-recombine chips).This study focuses on a micromixer design that combines a split-and-recombine approach (effective already at low Reynolds numbers) with slanted-groove connectors (boosting chaotic dynamics at high Re). Both experiments (epifluorescence and confocal microscopy using low-diffusion fluorescent probes) on 3D printed chips, and Computational Fluid Dynamics (CFD) simulations (particle tracing) show that this combination is particularly effective at intermediate to high Re. For example, already after a first mixing unit, at Re = 10 the mixing efficiency is 45-60% (depending on the chip junction) vs. ≈30% for a staggered herringbone chip with comparable length, and further increases at higher Re. At low Re, where the chip predominantly acts as a split-and-recombine mixer, we have found that increasing the amount of liquid-liquid interface at the junction point is a particularly useful approach; this increases the mixing efficiency from e.g. 35-40% with a simple Y junction, to 50-55% with 2D flow-focusing.In terms of chip design, our results therefore show that the combination of flow-focused split-and-recombine (low-Reynolds) and chaotic flow (high-Reynolds) is a particularly effective and additive combination to enhance mixing behaviour.