We show the emergence of reaction hotspots induced by three-dimensional (3D) vortices with a simple A + B → C reaction. We conduct microfluidics experiments to visualize the spatial map of the reaction rate with the chemiluminescence reaction and cross-validate the results with direct numerical simulations. 3D vortices form at spiral saddle type stagnation points, and the 3D vortex flow topology is essential for initiating reaction hotspots. The effect of vortices on mixing and reaction becomes more vigorous for rough-walled channels, and our findings are valid over wide ranges of channel dimensions and Damköhler numbers. PACS numbers: 47.56.+r, 47.60.+i, 67.40.Hf, 67.55.Hc, 94.10.Lf Vortices commonly occur in various channel flow systems such as rock fractures [1][2][3][4], porous media [5][6][7][8], pipe flows [9, 10], micromixers [11], and blood vessels [12,13]. Specifically, vortices can have a distinctive flow topology [14][15][16], and the topology of a flow field is known to control mixing processes, which in turn control reaction dynamics [17][18][19]. Vortices at fluid flow intersections are particularly important because fluids with different properties can mix and react at flow intersections [20,21]. Notably, vortices may alter mixing dynamics and initiate local reaction hotspots where reaction rates are locally maximum. Nevertheless, to the best of our knowledge, there has been no study that elucidated the role of three-dimensional (3D) vortices on mixing and reaction at flow intersections.In this study, we combined laboratory microfluidic experiments and direct numerical simulations to establish a previously unrecognized link between the 3D flow topology of vortices and reaction hotspots. A novel chemiluminescence reaction was adopted to visualize the spatial map of reaction rates in channel intersections across a wide range of Reynolds numbers (Re). Further, flow and reactive transport simulations were experimentally cross-validated and used to demonstrate the role of 3D vortex topology on the emergence of reaction hotspots where reaction products are actively produced. To demonstrate the ubiquitous nature of vortex-induced reaction hotspots, we conducted experiments on rough-walled channels and also performed simulations over wide ranges of channel dimensions and Damköhler numbers (Da).Microfluidic experiment We conducted microfluidic experiments with chemiluminescence reaction [22] to visualize mixing and reaction at intersections. The mixing-induced reaction was performed by injecting two reactive solutions, labeled A and B, into two separate inlets on a polydimethylsiloxane (PDMS) microfluidic chip using a pulsation-free syringe pump (neMESYS 290N, Cetoni, Korbussen, Germany). The channels had a constant aperture of 100 µm, a depth of 70 µm, and a channel length of 2 cm. The two channels intersected orthogonally at the center (1 cm) of their lengths * Corresponding author: pkkang@umn.edu at which the solutions mixed, and the chemiluminescence bimolecular reaction (A + B → C) occurred thereaft...

Bioaggregates are condensed porous materials comprising microbes, organic and inorganic matters, and water. They are commonly found in natural and engineered porous media and often cause clogging. Despite their importance, the formation mechanism of bioaggregates in porous media systems is largely unknown. Through microfluidic experiments and direct numerical simulations of fluid flow, we show that the rapid bioaggregation is driven by the interplay of the viscoelastic nature of biomass and hydrodynamic conditions at pore throats. At an early stage, unique flow structures around a pore throat promote the biomass attachment at the throat. Then, the attached biomass fluidizes when the shear stress at the partially clogged pore throat reaches a critical value. After the fluidization, the biomass is displaced and accumulated in the expansion region of throats forming bioaggregates. We further find that such criticality in shear stress triggers morphological changes in bioaggregates from rounded- to streamer-like shapes. This knowledge was used to control the clogging of throats by tuning the flow conditions: When the shear stress at the throat exceeded the critical value, clogging was prevented. The bioaggregation process did not depend on the detailed pore-throat geometry, as we reproduced the same dynamics in various pore-throat geometries. This study demonstrates that pore-throat structures, which are ubiquitous in porous media systems, induce bioaggregation and can lead to abrupt disruptions in flow.