Berlin 2008 – wissenschaftliches Programm

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DY: Fachverband Dynamik und Statistische Physik

DY 18: Soft matter

DY 18.1: Hauptvortrag

Mittwoch, 27. Februar 2008, 14:00–14:30, MA 001

Rate Dependence and Role of Disorder in Linearly Sheared Two-Dimensional Foams. — •Martin Van Hecke, Gijs Katgert, and Matthias Moebius — Kamerlingh Onnes Lab, PObox 9504 2300 RA Leiden, the Netherlands

We experimentally probe the rheology of two dimensional foams sandwiched between the fluid phase and a top-plate, and find that these flows depend crucially on both the applied strain rate and the degree of disorder of the foam: (1) Disordered, bidisperse foams exhibit rate dependent flow profiles, which become increasingly shear-banded for large shear rates. (2) Ordered, monodisperse foams exhibit rate independent flows.

These findings are captured in a model in which the averaged drag forces between bubble and top plate are balanced by the inter-bubble drag forces. We show that nonlinear scaling of these forces and rate dependent flows are intimately connected.

The fact that disorder plays such a crucial role for the foam flow, evidences that the translation from individual inter-bubble drag forces to the average inter-bubble drag forces is nontrivial. We find that for disordered foams the average inter-bubble drag forces scale differently from the individual inter-bubble drag forces. This is consistent with earlier suggestions that the averaged viscous drags are enhanced over what might be expected from the local interactions, due to disordered, non-affine, bubble motion. We discuss how anomalous scaling of bulk properties caused by non-affine motion at the micro scale appears to be a general feature of disordered systems close to jamming.