Influence of geometric ordering on viscoelastic flow instabilities in 3D porous media

Many applications involve flow of viscoelastic polymer solutions in geometrically complex 3D porous media. Polymers accumulate elastic stresses as they navigate the pore space, leading to a flow instability characterized by spatiotemporally chaotic flow fluctuations. Our previous studies in disordered 3D media suggested that this instability onset is highly sensitive to medium geometry; however, how exactly geometry influences the flow instability remains unclear. We address this gap by directly imaging flow in microfabricated 3D porous media with precisely controlled geometries consisting of body-centered cuboid or simple-cubic arrays of spheres. Unexpectedly, in both cases, the flow instability is generated upstream of the contact regions between spheres rather than at sphere surfaces—suggesting that the consolidation of solid grains, inherent in naturally-occurring media, may play a pivotal role in establishing the flow instability in field settings. Further, the characteristics of the flow instability strongly depend on the unit cell geometry, and we quantify how the pore-scale flow features control the macroscopic flow resistance across the entire medium. Our work thus provides a key step towards elucidating how porous medium geometry shapes viscoelastic flow behavior.