Vázquez-Quesada Adolfo, Tanner Roger I, Ellero Marco
Zienkiewicz Centre for Computational Engineering, Swansea University, Swansea SA1 8EN, United Kingdom.
School of Aerospace, Mechanical and Mechatronic Engineering, University of Sydney, Sydney, New South Wales 2006, Australia.
Phys Rev Lett. 2016 Sep 2;117(10):108001. doi: 10.1103/PhysRevLett.117.108001. Epub 2016 Aug 31.
Shear thinning-a reduction in suspension viscosity with increasing shear rates-is understood to arise in colloidal systems from a decrease in the relative contribution of entropic forces. The shear-thinning phenomenon has also been often reported in experiments with noncolloidal systems at high volume fractions. However its origin is an open theoretical question and the behavior is difficult to reproduce in numerical simulations where shear thickening is typically observed instead. In this letter we propose a non-Newtonian model of interparticle lubrication forces to explain shear thinning in noncolloidal suspensions. We show that hidden shear-thinning effects of the suspending medium, which occur at shear rates orders of magnitude larger than the range investigated experimentally, lead to significant shear thinning of the overall suspension at much smaller shear rates. At high particle volume fractions the local shear rates experienced by the fluid situated in the narrow gaps between particles are much larger than the averaged shear rate of the whole suspension. This allows the suspending medium to probe its high-shear non-Newtonian regime and it means that the matrix fluid rheology must be considered over a wide range of shear rates.
剪切变稀——即悬浮液粘度随剪切速率增加而降低——据信在胶体系统中是由于熵力的相对贡献减少而产生的。在高体积分数的非胶体系统实验中也经常报道剪切变稀现象。然而，其起源是一个开放的理论问题，并且在数值模拟中很难再现这种行为，在数值模拟中通常观察到的是剪切增稠。在这封信中，我们提出了一种颗粒间润滑力的非牛顿模型来解释非胶体悬浮液中的剪切变稀现象。我们表明，悬浮介质的隐藏剪切变稀效应发生在比实验研究范围大几个数量级的剪切速率下，会在小得多的剪切速率下导致整个悬浮液显著的剪切变稀。在高颗粒体积分数下，位于颗粒间狭窄间隙中的流体所经历的局部剪切速率远大于整个悬浮液的平均剪切速率。这使得悬浮介质能够进入其高剪切非牛顿区域，这意味着必须在很宽的剪切速率范围内考虑基体流体的流变学。
