Frictional fluid instabilities shaped by viscous forces

Multiphase flows involving granular materials are complex and prone to pattern formation caused by competing mechanical and hydrodynamic interactions. Here we study the interplay between granular bulldozing and the stabilising effect of viscous pressure gradients in the invading fluid. Injection of aqueous solutions into layers of dry, hydrophobic grains represent a viscously stable scenario where we observe a transition from growth of a single frictional finger to simultaneous growth of multiple fingers as viscous forces are increased. The pattern is made more compact by the internal viscous pressure gradient, ultimately resulting in a fully stabilised front of frictional fingers advancing as a radial spoke pattern. Fingering patterns form spontaneously when a non-wetting viscous liquid displaces a dry granular mixture in a confined flow cell. The authors show how these patterns are controlled by the balance between viscous, capillary, and frictional forces.

Automated summary

Multiphase flows involving granular materials often exhibit complex pattern formation due to simultaneous mechanical and hydrodynamic interactions. In this study, the authors investigate the interplay between granular bulldozing and the stabilizing effect of viscous pressure gradients in fluid invasion. They observe a transition from single to multiple fingers growth as viscous forces increase in a scenario where aqueous solutions are injected into dry, hydrophobic grains. The patterns are further compacted by internal viscous pressure gradients, leading to a stabilized front of frictional fingers advancing as a radial spoke pattern. Fingering patterns form spontaneously when a non-wetting viscous liquid displaces a dry granular mixture in a confined flow cell, and this study reveals how the resulting patterns are regulated by the balance between viscous, capillary, and frictional forces.