Permeability heterogeneity and bulk linear elasticity of displaced clay suspensions determine interfacial pattern morphologies in Hele–Shaw experiments

Abstract When a less viscous Newtonian fluid displaces an aging aqueous clay suspension in a confined space, a rich array of interfacial patterns emerges due to a predominantly viscous instability. Earlier studies on clay displacement identified macroscopic morphologies such as fingering and fracturing, but the local finger propagation modes did not receive much attention. For the present work, we controlled the mechanical properties of clay suspensions by incorporating additives and observed a range of local finger propagation modes when the suspensions were radially displaced by water in a Hele–Shaw cell. When the elasticity of clay was low, the interfacial dynamics exhibited features of nonlinear viscous fingering in heterogeneous media. By tuning the nature and content of additives that delay clay aging, we uncovered two novel finger propagation mechanisms: skewering and zig–zag growth, both characterised by tortuous trajectories and thick finger profiles. These patterns have hitherto never been observed in experiments with colloidal systems. For moderate clay elasticities, we report that shear-thinning-induced flow anisotropy leads to the formation of dendrites with dominant side branches. As clay elasticity increases due to the incorporation of salts, the energy required to create fractures becomes smaller than that for system-wide yielding. This scenario is characterised by the emergence of viscoelastic fractures. Our work demonstrates that incorporating additives in the suspension medium significantly modifies clay elasticity and permeability and is an effective strategy to manipulate the growth and morphologies of instabilities at clay-water interfaces.