Slip and banding in soft glassy materials with rough boundaries
Gerhard Jung (Institut für Physik)

Soft materials like dense emulsions or foams display universal features in their low frequency shear rheology that indicate an underlying slow, glassy dynamics. Exemplary experimental observations are their nonlinear flow behavior, and a power-law decay of the storage and loss moduli with the shear frequency. Even more interesting is the behavior of soft materials in creep experiments: The creep curves are characterized by an initial elastic response of the soft particles to an external stress, followed by a pronounced yielding transition.

In this talk I will present several important insights into the flow behavior of soft glasses. The system under investigation is a wet foam, modeled with the immersed boundary technique, confined in a narrow channel with rough boundary conditions. This model is not only able to reproduce the universal features mentioned above, but it can also realistically capture single yielding events inside the material. I will show that the slip at the boundary strongly depends on the roughness of the walls. Strikingly, the transition between slip and no-slip boundary conditions is very sharp and occurs if the size of the bumps in the walls corresponds to about 15-20% of the particle size. Additionally, I will discuss the formation of shear bands at the yielding transition and how the flow profile evolves from the initial elastic response to the steady state flow.