Suppressing viscous fingering in structured porous media

Finger-like protrusions that form along fluid−fluid displacement fronts in porous media are often excited by hydrodynamic instability
when low-viscosity fluids displace high-viscosity resident fluids. Such interfacial instabilities are undesirable in many natural and engineered displacement processes.We report a phenomenon whereby gradual and monotonic variation of pore sizes along the
front path suppresses viscous fingering during immiscible displacement, that seemingly contradicts conventional expectation of enhanced instability with pore size variability. Experiments and pore-scale numerical simulations were combined with an analytical
model for the characteristics of displacement front morphology as a function of the pore size gradient. Our results suggest that the
gradual reduction of pore sizes act to restrain viscous fingering for a predictable range of flow conditions (as anticipated by gradient
percolation theory). The study provide insights into ways for suppressing unwanted interfacial instabilities in porous media, and provides design principles for new engineered porous media such as exchange columns, fabric, paper, and membranes with respect to their desired immiscible displacement behavior.