Mixing with herringbone-inspired microstructures: overcoming the diffusion limit in co-laminar microfluidic devices

Abstract

Enhancing mixing is of uttermost importance in many laminar microfluidic devices, aiming at overcoming the severe performance limitation of species transport by diffusion alone. Here we focus on the significant category of microscale co-laminar flows encountered in membraneless redox flow cells for power delivery. The grand challenge is to achieve simultaneously convective mixing within each individual reactant, to thin the reaction depletion boundary layers, while maintaining separation of the co-flowing reactants, despite the absence of a membrane. The concept presented here achieves this goal with the help of optimized herringbone flow promoting microstructures with an integrated separation zone. Our electrochemical experiments using a model redox couple show that symmetric flow promoter designs exhibit laminar to turbulent flow behavior, the latter at elevated flow rates. This change in flow regime is accompanied by a significant change in scaling of the Sherwood number with respect to the Reynolds number from Sh ~ Re(0.29) to Sh ~ Re(0.58). The stabilized continuous laminar flow zone along the centerline of the channel allows operation in a co-laminar flow regime up to Re ~325 as we demonstrate by micro laser-induced fluorescence (μLIF) measurements. Micro particle image velocimetry (μPIV) proves the maintenance of a stratified flow along the centerline, mitigating reactant cross-over effectively. The present work paves the way toward improved performance in membraneless microfluidic flow cells for electrochemical energy conversion.

Co-laminar flow cells for electrochemical energy conversion

“…The second study in this work characterized the crossover losses and reactant recirculation possibilities of both single and dual pass architectures [109]. Similarly, a study by Marschewski et al provided a thorough analysis of microfluidic mixing demonstrating the utility of a raised neutral region in between turbulence promoting herringbone structures to preserve co-laminar flow [110].…”

“…In general, this single-layer device platform has spawned many derivative studies [48], [50], [55], [76], [77], [80]- [83], [85], [113], [119]- [123] due to its simplicity of construction and the possibility for direct visualization of the co-laminar interface quality and reactant conversion through the clear PDMS channel substrate. This unique feature has recently been utilized to visually track and quantify the effect of herringbone mixers on crossover of reactants via micro particle image velocimetry [110]. It has also been shown that parallel and series multiplexing of these cells within the same PDMS layer is possible but suffers from low overall device power density due to the added area required for separate manifolds [37], [105], [107].…”

Influence of perturbance frequency on intensity of mixing of fluids in microchannel devices

“…The conventional passive methods of increasing the mixing velocity involve decreasing the thickness of the fluid layers by changing the microchannel size ( [1], [2]) and extending the contact area with adjacent liquids. To increase the mixing velocity in the microchannel, an excitation of circulations in the flow and a curvature of the flow lines can be used.…”

On-chip recalcification of citrated whole blood using a microfluidic herringbone mixer

“…For microfluidic chambers, it is ideal to conduct this mixing on-chip in order to reduce blood volumes. However, in addition to the well-documented challenges of mixing in low Reynolds number flows, [23][24][25] mixing with whole blood is further hindered by the low diffusivity of suspended blood cells. In this report, we describe a method for continuous on-chip recalcification of sodium citrate anticoagulated whole blood, hereafter referred to as citrated blood, for microfluidic flow assays.…”

“…Mixing in microfluidic devices poses several well-documented challenges 24,25,27 due to low Reynolds number flows and high P eclet numbers. For a solution or suspension to be well mixed, there must be no concentration gradients.…”