  Polymer solutions are frequently used in enhanced oil recovery and
groundwater remediation to improve the recovery of trapped non-aqueous fluids.
However, applications are limited by an incomplete understanding of the flow in
porous media. The tortuous pore structure imposes both shear and extension,
which elongates polymers; moreover, the flow is often at large Weissenberg
numbers Wi at which polymer elasticity in turn strongly alters the flow. This
dynamic elongation can even produce flow instabilities with strong spatial and
temporal fluctuations despite the low Reynolds number Re. Unfortunately,
macroscopic approaches are limited in their ability to characterize the
pore-scale flow. Thus, understanding how polymer conformations, flow dynamics,
and pore geometry together determine these non-trivial flow patterns and impact
macroscopic transport remains an outstanding challenge. Here, we describe how
microfluidic tools can shed light on the physics underlying the flow of polymer
solutions in porous media at high Wi and low Re. Specifically, microfluidic
studies elucidate how steady and unsteady flow behavior depends on pore
geometry and solution properties, and how polymer-induced effects impact
non-aqueous fluid recovery. This work provides new insights for polymer
dynamics, non-Newtonian fluid mechanics, and applications such as enhanced oil
recovery and groundwater remediation.
