ICAMS / Interdisciplinary Centre for Advanced Materials Simulation


Yield stress and blunted flow profile in a model glass

Date: 14.12.2009
Place: Midwinter meeting of the British Rheological Society, Edinburgh,UK

Fathollah Varnik, Max-Planck-Institut für Eisenforschung, Düsseldorf, Germany

The flow of a simple glass forming system(a 80:20 binary Lennard-Jones mixture) through a planar channel is studied via molecular dynamics simulations. The flow is driven by an external body force similar to gravity. Precious studies show that the model exhibits both a static and a dynamic yield stress in the glassy phase. These observations are corroborated by the present work, where we investigate how the presence of a yield stress may affect the system behaviour in a Poiseuille-type flow geometry.
In particular, we observe a blunted velocity profile across the cahnnel: A relatively wide region in the channel center flows with a constant velocity (zero shear rate) followed by a non linear change of the shear rate as the walls are approached. The observed velocity gradients are compared to those obtained from the knowledge of the shear stress across the channel and the flow-curves (stress versus shear rate), the latter being determined in our previous simulations of homogeneous shear flow.
Furthermore, using the value of the (dynamic) yield stress known from previous simulations, we estimate the threshold body force for a complete arrest of the flow. Indeed, a blockage is observed as the imposed force falls below this threshold value. Small but finite shear rates are observed at stresses above the dynamic but below the static yield stress. We discuss the possible role of the stick-slip like motion for this observation.

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