Patterned plates mould the shape of yield-stress fluids in lifted Hele-Shaw cells

As fluids displace one another in lifted Hele-Shaw cells, complex fingering patterns can emerge. Our understanding of such fingering instabilities in complex materials, such as yield-stress fluids, is still in its fledgling stage. Here we experimentally consider the finger growth dynamics of a yield-stress fluid displaced by air in a lifted Hele-Shaw cell confined by various groove patterned plates. As air displaces the yield-stress fluid, the emergent fingering pattern depends on the lifting speed, initial plate separation and plate texture. Interestingly, we find that the plate’s texture influences finger growth, even outside the regime of traditional unstable viscous fingering, which will be discussed.