Alladi Ramakrishnan Hall

Rheology of dense emulsions - a shear start-up and steady state study

Vishwas Vasisht

Georgetown University, USA

Under shear deformation, dense emulsions compressed above the so-called jamming transition, how properties of a marginal solid and display fluid-like behaviour when a sufficiently large shear stress, also termed as yield stress, is applied. In the vicinity of yield stress one observes prominent dynamical heterogeneities and a competition between long restructuring time and shear induced rejuvenation, which lead to the shear banding phenomenon. In this work we present an extensive study of dense emulsions both during the start-up shear as well as in the steady state. By combining confocal-rheology experiments on compressed emulsions and numerical 3D simulations, we show the primary microscopic mechanisms for flow, that, just as the macroscopic flow, are strongly influenced by the rate of the imposed deformation. When shearing fast, small coordinated clusters of droplets move collectively as in a conga line, while at low rates the flow emerges from bursts of droplet rearrangements, correlated over large domains. The shear start-up behaviour of dense emulsions, is studied in 3D simulations of repulsive soft spheres in athermal condition. Our study show that the shear banding feature in a purely repulsive system, which even though is transient, but is long lived - longer than the structural relaxation time set by inverse shear rate. The shear banding originates at the stress overshoot and the time required for the system to reach homogeneous flow - fluidisation time, has a strong dependence on the shear rate as well as on the age of the sample.