Department of Physics, Indian Institute of Science, Bangalore 560012, India.
Proc Natl Acad Sci U S A. 2011 May 31;108(22):8996-9001. doi: 10.1073/pnas.1018685108. Epub 2011 May 16.
A monotonic decrease in viscosity with increasing shear stress is a known rheological response to shear flow in complex fluids in general and for flocculated suspensions in particular. Here we demonstrate a discontinuous shear-thickening transition on varying shear stress where the viscosity jumps sharply by four to six orders of magnitude in flocculated suspensions of multiwalled carbon nanotubes (MWNT) at very low weight fractions (approximately 0.5%). Rheooptical observations reveal the shear-thickened state as a percolated structure of MWNT flocs spanning the system size. We present a dynamic phase diagram of the non-Brownian MWNT dispersions revealing a starting jammed state followed by shear-thinning and shear-thickened states. The present study further suggests that the shear-thickened state obtained as a function of shear stress is likely to be a generic feature of fractal clusters under flow, albeit under confinement. An understanding of the shear-thickening phenomena in confined geometries is pertinent for flow-controlled fabrication techniques in enhancing the mechanical strength and transport properties of thin films and wires of nanostructured composites as well as in lubrication issues.
一般来说，复杂流体对剪切流动的流变学响应是剪切应力随剪切应力单调下降，而絮凝聚合物则是一个特殊的例子。在这里，我们在改变剪切应力时展示了一个不连续的剪切增稠转变，其中多壁碳纳米管 (MWNT) 的絮凝聚合物的粘度在非常低的重量分数（约 0.5%）下急剧跳跃四到六个数量级。流变光学观察表明，剪切增稠状态是 MWNT 絮凝聚合物的渗流结构，跨越整个系统尺寸。我们提出了非布朗 MWNT 分散体的动态相图，揭示了起始的堵塞状态，随后是剪切变稀和剪切增稠状态。本研究进一步表明，作为剪切应力函数获得的剪切增稠状态可能是流动下分形团簇的一个普遍特征，尽管是在受限的情况下。在受限几何形状中对剪切增稠现象的理解对于增强薄膜和纳米结构复合材料的细丝的机械强度和输运性能的流动控制制造技术以及润滑问题都是相关的。
