Eliminating injection and memory effects in bubble rise experiments within yield stress fluids

Bubbles rising steadily in Carbopol often have an inverted teardrop shape with pointed tail, in contrast to steady bubble rise computations using simple yield stress fluid models. The cause of the pointed tail is suspected to be viscoelasticity, but another possibility is that the tail arises during the bubble formation and the shape is retained via some form of fluid memory effect (elastic, thixotropic or yield stress). Here we present results of a study in which we eliminate all possibility of memory effects using a multi-layered experimental design, but still find the pointed tail in steady bubble propagation. Thus, the tail shape can only result from a combination of the rheology and steady fluid deformation around the rising bubble. Further experiments show that the same steady shapes are found for quite different initial bubble shapes.