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Abstract:
Immiscible fluid displacement with average front velocities in the capillary-dominated
regime is studied in a transparent Hele-Shaw cell with cylindrical posts. Employing various
combinations of fluids and wall materials allows us to cover a range of advancing contact
angles 46◦ θa 180◦ of the invading fluid in our experiments. In parallel, we study
the displacement process in particle-based simulations that account for wall wettability.
Considering the same arrangement of posts in experiments and simulation, we find a
consistent crossover between stable interfacial displacement at θa 80◦ and capillary
fingering at high contact angles θa 120◦. The position of the crossover is quantified
through the evolution of the interface length and the final saturation of the displaced
fluid. A statistical analysis of the local displacement processes demonstrates that the shape
evolution of the fluid front is governed by local instabilities as proposed by Cieplak and
Robbins for a quasistatic interfacial displacement [Cieplak and Robbins, Phys. Rev. Lett. 60,
2042 (1988)]. The regime of stable front advances coincides with a corresponding region
of contact angles where cooperative interfacial instabilities prevail. Capillary fingering,
however, is observed only for large θa , where noncooperative instabilities dominate the invasion process.
