How do soft particle glasses yield and flow near solid surfaces?

Abstract

We use fluorescence microscopy and particle tracking velocimetry to image the motion of concentrated emulsions and microgel suspensions near solid surfaces. The local deformation involves a combination of slip and bulk flow, which are found to be controlled by surface forces. With smooth surfaces, two slip mechanisms are identified depending on whether particle–wall interactions are repulsive or weakly attractive. In the former case, the materials yield uniformly and the local rheology can be mapped on the macroscopic flow curve. In the latter case, yielding is non-uniform which reveals a continuous distribution of states from the immediate vicinity of the smooth surface to the bulk of the material. The effect of the surface is long-ranged and decays exponentially with the distance, which can be described by a non-local fluidity model. Our results establish a link between surface forces, lubrication and yielding in soft glassy or jammed materials and open new routes to manipulate their flow through the surface chemistry of the confining boundaries.