All mixed up: Harnessing elastic instabilities for improved mixing and reaction kinetics in porous media

Many industrial and environmental processes involve the slow flow of fluids through porous media at low Reynolds numbers. Under these conditions, diffusion and laminar flow strongly limit solute mixing. Here, we describe how this fundamental limitation can be overcome by adding a dilute amount of flexible, high molecular weight polymers to the fluid. In particular, in situ medium reveals that the polymer generates an elastic instability with strong chaotic flow fluctuations that stretch and fold solute gradients in a manner analogous to turbulent Batchelor mixing—despite the low Reynolds numbers in the flow. This phenomenon results in a ~80% reduction in the length required to mix solutes and an ~8-fold increase in the rate of a test chemical reaction. Altogether, our results provide a quantitative link between elastic instabilities, the chaotic flows they generate in a porous medium, and enhanced solute mixing and chemical reactivity, with important implications for a range of applications that require enhanced mixing in confinement.