LAUSR

Abstract A study is presented of the instabilities that may arise in radial displacement flows of yield-stress fluid in a Hele-Shaw cell. Theoretically, the viscoplastic version of the Saffman–Taylor interfacial instability is predicted to arise when the yield-stress fluid is displaced by a Newtonian one. The interface is expected to remain stable, however, if the yield-stress fluid displaces the Newtonian one. A variety of experiments are then performed using an aqueous suspension of Carbopol. As predicted theoretically, the Saffman–Taylor instability is observed when the Carbopol is displaced by either air or an immiscible oil, and no instabilities are observed when the displacement is the other way around. However, when water is used in the displacement experiments, other instabilities appear that take the form of localized fractures of the Carbopol over the sections of the interface that are under tension. The fractures arise in both the stable and unstable Saffman–Taylor configurations, leading to a rich range of patterns within the Hele-Shaw cell. We argue that this pattern formation cannot be explained by a recently proposed instability of shear-thinning extensional flow, whatever the degree of effective slip over the plates of the cell. Instead, we attribute the fractures to a reduction in the fracture energy of the suspension when placed in contact with water.