Van der Waals-Zeeman Institute, University of Amsterdam, Valckenierstraat 65, 1018XE Amsterdam, The Netherlands.
Phys Rev Lett. 2010 Nov 26;105(22):225502. doi: 10.1103/PhysRevLett.105.225502. Epub 2010 Nov 23.
Yield stress fluids have proven difficult to characterize, and a reproducible determination of the yield stress is difficult. We study two types of yield stress fluids (YSF) in a single system: simple and thixotropic ones. This allows us to show that simple YSF are simply a special case of thixotropic ones, and to pinpoint the difference between static and dynamic yield stresses, one of the major problems in the field. The thixotropic systems show a strong time dependence of the viscosity due to the existence of an internal percolated structure that confers the yield stress to the material. Using loaded emulsions to control the thixotropy, we show that the transition to flow at the yield stress is discontinuous for thixotropic materials, and continuous for ideal ones. The discontinuity leads to a critical shear rate below which no steady flows can be observed, accounting for the ubiquitous shear banding observed in these materials.
屈服应力流体的特性难以确定，且屈服应力的重现性测定也很困难。我们在单一体系中研究了两种类型的屈服应力流体（YSF）：简单屈服应力流体和触变屈服应力流体。这使我们能够证明简单屈服应力流体只是触变屈服应力流体的一个特殊情况，并指出静态和动态屈服应力之间的区别，这是该领域的主要问题之一。触变体系由于存在赋予材料屈服应力的内部渗透结构，其粘度具有很强的时间依赖性。使用负载乳液来控制触变性，我们表明，对于触变材料，屈服应力下的流动转变是不连续的，而对于理想材料则是连续的。这种不连续性导致临界剪切率以下无法观察到稳定流动，这解释了在这些材料中普遍观察到的剪切带现象。
