A wide range of environmental, industrial, and energy processes rely on transport in disordered 3D porous media. In many of these settings, transport is limited by strong flow heterogeneities and the steady, laminar flow imposed by geometric confinement (Re«1). Polymer additives have potential as a key engineering tool for modifying these flows to improve transport. However, the flow behavior of these rheologically-complex fluids remains poorly understood in these disordered settings—in large part due to imaging limitations.

We address this gap by fabricating transparent 3D porous media and directly imaging the flow in situ. We find that polymer stretching can give rise to an elastic instability that generates turbulent-like fluctuations under conditions prohibitive for traditional turbulence. Combining microscopic flow measurements with theoretical modeling, we demonstrate that viscous dissipation associated with this instability is responsible for an anomalous increase in macroscopic flow resistance, resolving an over-50-year-old puzzle. We then show how these chaotic fluctuations can be harnessed to enhance pore-scale mixing of solutes by 3–6×, and the rate of chemical reactions by an order of magnitude. This work demonstrates how couplings between complex geometries and complex fluid rheology can give rise to intriguing flow behaviors—providing new avenues to understand, control, and engineer transport in confined spaces.