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.

对Hele-Shaw单元中屈服应力流体的径向位移流中可能出现的不稳定性进行了研究。从理论上讲，当屈服应力流体被牛顿流体驱替时，Saffman-Taylor界面失稳的粘塑性形式就会出现。但是，如果屈服应力流体置换了牛顿流体，则该界面有望保持稳定。然后使用Carbopol的水悬浮液进行各种实验。根据理论上的预测，当Carbopol被空气或不混溶的油驱替时，会观察到Saffman-Taylor的不稳定性，而当驱油反过来时，就不会观察到不稳定。但是，当在置换实验中使用水时，出现了其他不稳定性，这些不稳定性表现为在受拉界面的截面上Carbopol局部断裂的形式。裂缝以稳定和不稳定的Saffman-Taylor构型出现，导致Hele-Shaw单元内的图案丰富。我们认为，这种模式的形成不能用最近提出的剪切稀疏的延伸流的不稳定性来解释，而不管细胞板上的有效滑动程度如何。相反，我们将断裂归因于与水接触时悬浮液断裂能的降低。我们认为，这种模式的形成不能用最近提出的剪切稀疏的延伸流的不稳定性来解释，而不管细胞板上的有效滑动程度如何。相反，我们将断裂归因于与水接触时悬浮液断裂能的降低。我们认为，这种模式的形成不能用最近提出的剪切稀疏的延伸流的不稳定性来解释，而不管细胞板上的有效滑动程度如何。相反，我们将断裂归因于与水接触时悬浮液断裂能的降低。