Pore structure creates vortex-induced reaction hot spots in low Reynolds number porous media flows: A microfluidic and numerical investigation

Abstract:

While flow in porous media systems, such as groundwater and rock fracture flow, is usually laminar (Re < 500), it has been increasingly recognized that inertial flow structures can appear in these systems at low (Re < 1) to moderate Re numbers (Re ~ 100), resulting in complex 3D flow structures [1]. These features often manifest as vortex-like structures that can have a significant impact on mixing and reaction in porous media. Previous work with a chemiluminescent reaction in a cross intersection showed that 3D vortical flow structures form and create reaction hot spots for Re number as low as 100 [2]. A recent study also showed that vortical flow structures can control transport processes even in low Re number (Re << 1) porous media flows, but their impact on mixing and reaction have not been explored [3]. The overarching goal of this work, therefore, is to identify the physical, hydrodynamic, and geochemical conditions where complex flow structures create reaction hot spots.

Here, we combine 3D pore-scale numerical simulations and microfluidic experiments on a bimolecular chemiluminescent reaction to study the formation of reaction hotspots in low to moderate Re number flow. The figure shows 3D Flow, transport, and reaction simulation results for a bimolecular reaction and flow around two pillars at Re = 40 (referenced to one pillar diameter) shows reaction hotspots). We use a microfluidic channel as a porous media analog where two reactants are injected into separate channels that converge to a central channel containing a sequence of pillars. The appearance and characteristics of reaction hot spots in this system are controlled by the geometry of the channel features (i.e. relative pillar size, gap between pillars, etc.), Re, and the time scale of chemical reaction. The results of this work show that the critical Re to initiate vortical flow structures and reaction hotspots is sensitive to channel geometry. Further, these results imply that typical porous media geometries and hydrodynamic conditions will readily create vortical structures that induce reaction hot spots, which will influence many geochemical reactions, especially mineral precipitation and dissolution.

[1] D. L. Koch and R. J. Hill, Annu. Rev. Fluid Mech. 33, 619 (2001).

[2] S. H. Lee and P. K. Kang, Phys. Rev. Lett. 124, 144501 (2020).

[3] E. Crevacore, T. Tosco, R. Sethi, G. Boccardo, and D. L. Marchisio, Phys. Rev. E 94, 053118 (2016).