Accelerating mixing and reaction kinetics in porous media using an elastic instability

A wide range of environmental, industrial, and energy processes rely on reactive transport in disordered 3D porous media, but laminar flow under strong geometric confinement (Re«1) imposes a fundamental limit on reagent transport. Here, we report a novel technique to mimic turbulent-enhanced reactivity using by the addition of dilute high molecular weight polymers, which induce an elastic flow instability. Micro-scale imaging within a transparent porous medium reveals dynamic chaotic fluctuations that stretch and fold solute gradients exponentially in time—analogous to turbulent Batchelor mixing, despite the low Re. We observe a dramatic reduction in the required mixing length and improvement in the dispersion of concentration gradient, suggesting a cooperation between the elastic instability and the laminar chaotic advection inherent to the disordered 3D porous media. We show these two mixing mechanisms can be modeled with additive independent mixing rates, representing a dramatic conceptual simplification. We then extend these results to reactive mixing, accelerating a model reaction by 75% while simultaneously increasing throughput by 20×—circumventing the traditional trade-off between throughput and reactor length. Our results thus provide the first demonstration, to our knowledge, that elastic flow instabilities can provide turbulent-like enhancements in chemical reaction rates, which can operate cooperatively with laminar chaotic advection in industrially-relevant geometries.