Growth and Arrest of Reactive Mixing Fronts from Spherical Point-Source Injections

Many reactive processes in the environment involve the formation of chemical fronts as reactant A is injected from a point source into a three-dimensional medium containing reactant B, and a bimolecular A + B → C reaction takes place. While bimolecular reaction fronts have been extensively studied in planar and cylindrical geometries, their dynamics in spherical configurations remain experimentally unexplored, despite their relevance to environmental applications. Using a bimolecular redox reaction in a hydrogel-based medium, we show experimentally that point-like injection in three-dimensional porous media can lead to a self-organized stationary sphere in which the reactive zone remains at a fixed radius. We measure the spatial distribution of the product of these reactive mixing fronts, the front position, and the amount of product formed with time. The stationary front radius scales linearly with the flow rate, and inversely with the diffusion coefficient ratio and logarithm of the initial concentration ratio. We use this experimental data set to test recent theoretical predictions regarding reactive front growth and arrest for point injections of reactive elements in three-dimensional (3D) domains. Furthermore, we analyze numerically the influence of reactant diffusivity differences on front position, width, maximum reaction rate, and total product formation. These findings highlight the existence of new dynamical regimes in reactive transport, emphasizing the importance of considering full three-dimensional modeling of reactive dynamics in a range of environmental applications that involve point injections of reactive solutes in the subsurface.