Profile blunting and flow blockage in a yield stress fluid: A molecular dynamics study

TL;DR

This study uses molecular dynamics simulations to explore how yield stress influences flow behavior in a glass-forming system, revealing flow blunting, flow blockage, and the role of stick-slip motion in a channel flow setup.

Contribution

It demonstrates how yield stress affects flow profiles and flow arrest in a Lennard-Jones glass system, linking microscopic properties to macroscopic flow behavior.

Findings

Flow exhibits a blunted velocity profile with a central flowing region.

Flow blockage occurs below a threshold body force related to the yield stress.

Finite shear rates are observed between the dynamic and static yield stresses, possibly due to stick-slip motion.

Abstract

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. Previous studies show that the model exhibits both a static [Varnik et al. J. Chem. Phys. 120, 2788 (2004)] and a dynamic [F. Varnik and O. Henrich Phys. Rev. B 73, 174209 (2006)] yield stress in the glassy phase. \blue{These observations are corroborated by the present work, where we investigate how the presence of a yield stress may affect the system behavior in a Poiseuille-type flow geometry.} In particular, we observe a blunted velocity profile across the channel: 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 \blue{stick-slip like motion} for this observation.