Viscoelastic flow instabilities in porous media

Many energy, environmental, industrial, and microfluidic processes rely on the viscous flow of polymer solutions through porous media. Unexpectedly, the macroscopic flow resistance often abruptly increases above a threshold flow rate in a porous medium—but not in bulk solution. The reason why has been a puzzle for over half a century. In this talk, I will describe how by directly visualizing the flow in a transparent 3D porous medium, we have experimentally demonstrated that this anomalous increase is due to the onset of an elastic instability in which the flow exhibits chaotic spatiotemporal fluctuations reminiscent of inertial turbulence, despite the vanishingly small Reynolds number. Our measurements enabled us to quantitatively establish that the energy dissipated by unstable pore-scale fluctuations generates the anomalous increase in the overall flow resistance. Because the macroscopic resistance is one of the most fundamental descriptors of fluid flow, our results both help deepen understanding of complex fluid flows and provide guidelines to inform a broad range of applications. As a further demonstration of this point, we demonstrated that this flow instability can be harnessed to homogenize the uneven partitioning of flow that arises in structurally-heterogeneous porous media—providing a new approach to homogenizing fluid and passive scalar transport in heterogeneous porous media at low Reynolds numbers. Ultimately, by linking viscoelastic flow instabilities at the pore scale to transport at the macroscale, this work yields generally applicable guidelines for predicting and controlling polymer solution flows.