Turbulent flows have been used for millennia to mix solutes; a familiar
example is stirring cream into coffee. However, many energy, environmental, and
industrial processes rely on the mixing of solutes in porous media where
confinement suppresses inertial turbulence. As a result, mixing is drastically
hindered, requiring fluid to permeate long distances for appreciable mixing and
introducing additional steps to drive mixing that can be expensive and
environmentally harmful. Here, we demonstrate that this limitation can be
overcome just by adding dilute amounts of flexible polymers to the fluid.
Flow-driven stretching of the polymers generates an elastic instability (EI),
driving turbulent-like chaotic flow fluctuations, despite the pore-scale
confinement that prohibits typical inertial turbulence. Using in situ imaging,
we show that these fluctuations stretch and fold the fluid within the pores
along thin layers (``lamellae'') characterized by sharp solute concentration
gradients, driving mixing by diffusion in the pores. This process results in a
3× reduction in the required mixing length, a 6× increase in
solute transverse dispersivity, and can be harnessed to increase the rate at
which chemical compounds react by 3× -- enhancements that we rationalize
using turbulence-inspired modeling of the underlying transport processes. Our
work thereby establishes a simple, robust, versatile, and predictive new way to
mix solutes in porous media, with potential applications ranging from
large-scale chemical production to environmental remediation