Time: 4:30 p.m.
Place: Seminar room 0.08
Claus Heussinger, Universität Göttingen, Göttingen, Germany
The jamming paradigm aims at providing a unified view for the elastic and rheological properties of materials as different as foams, emulsions, suspensions or granular media. Structurally, these systems can all be viewed as dense assemblies of particles, and the particle volume fraction φ plays the role of the coupling constant that tunes the distance to the jamming transition.
In this contribution, we will discuss how certain features in the particle interactions affect the rheological properties of the material. First, we will discuss the effect of frictional forces, and show how the jamming phase diagram has to be modified as compared to the frictionless scenario. Essential findings are a discontinuous and hysteretic jamming transition, as well as a shear thickening regime. Secondly, we will investigate the role of weak attractive interactions between the particles. The attraction strength provides a second control parameter, next to particle volume fraction, that allows to efficiently tune the yield stress. For weakly attractive systems below the jamming density we find an unusual scaling regime where the yield stress scales super-linearly with attraction strength u, σy~u3/2. We show how this scaling is related to the connectivity z of the evolving particle network, which displays self-organization towards the minimal isostatic value ziso=2d (d spatial dimensions). In contrast to the isostatic "point" of repulsive systems, this happens over a broad interval of both volume fractions and attraction strengths. Going beyond the characterization of the yield stress we map out the full flow curve of the material. Associated with the super-linear scaling we find non-monotonic flow curves indicative of a shear banding instability. We illustrate how these features may allow to extract a long restructuring time-scale, which has been argued to be at the origin of the instability.