屈服应力流体中剖面钝化和流动堵塞的分子动力学研究
Profile blunting and flow blockage in a yield stress fluid: A molecular dynamics study
摘要：
通过分子动力学模拟研究了一个简单的玻璃形成系统（80：20二元Lennard-Jones混合物）通过平面通道的流动。流动由类似于重力的外部体力驱动。先前的研究表明，该模型表现出静态[Varnik et al. J. Chem. Phys. 120，2788（2004）]和动态[F. Varnik和O. Henrich Phys.Rev.B73，174209（2006）]在玻璃相中的屈服应力。\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.}特别是，我们观察到一个钝的速度分布在整个通道：一个相对较宽的区域，在通道中心流动的恒定速度（零剪切速率），然后由非线性变化的剪切速率接近墙壁。所观察到的速度梯度进行比较，从跨通道的剪切应力的知识和流量曲线（应力与剪切速率），后者被确定在我们以前的模拟均匀剪切流。此外，使用从以前的模拟已知的（动态）屈服应力的值，我们估计一个完整的逮捕流动的阈值体力。实际上，当施加的力下降到该阈值以下时，观察到阻塞。在高于动态屈服应力但低于静态屈服应力的应力下观察到小但有限的剪切速率。我们讨论了可能的作用的\blue{stick-slip like motion}的这一观察。
英文摘要：
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.
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