Inverse Saffman-Taylor Experiments with Particles Lead to Capillarity Driven Fingering Instabilities

Using air to displace a viscous fluid contained in a Hele-Shaw cell can create a fingering pattern at the interface between the fluids if the capillary number exceeds a critical value. This Saffman-Taylor instability is revisited for the inverse case of a viscous fluid displacing air when partially wettable hydrophilic particles are lying on the walls. Though the inverse case is otherwise stable, the presence of the particles results in a fingering instability at low capillary number. This capillary-driven instability is driven by the integration of particles into the interface which results from the minimization of the interfacial energy. Both axisymmetric and rectangular geometries are considered in order to quantify this phenomenon.