School of Physics and Astronomy, University of Edinburgh, King's Buildings, Peter Guthrie Tait Road, Edinburgh EH9 3FD, UK.
School of Engineering, University of Edinburgh, King's Buildings, Peter Guthrie Tait Road, Edinburgh EH9 3JL, UK.
Soft Matter. 2020 Jan 7;16(1):229-237. doi: 10.1039/c9sm00041k. Epub 2019 Nov 28.
There is a growing consensus that shear thickening of concentrated dispersions is driven by the formation of stress-induced frictional contacts. The Wyart-Cates (WC) model of this phenomenon, in which the microphysics of the contacts enters solely via the fraction f of contacts that are frictional, can successfully fit flow curves for suspensions of weakly polydisperse spheres. However, its validity for "real-life", polydisperse suspensions has yet to be seriously tested. By performing systematic simulations on bidisperse mixtures of spheres, we show that the WC model applies only in the monodisperse limit and fails when substantial bidispersity is introduced. We trace the failure of the model to its inability to distinguish large-large, large-small and small-small frictional contacts. By fitting our data using a polydisperse analogue of f that depends separately on the fraction of each of these contact types, we show that the WC picture of shear thickening is incomplete. Systematic experiments on model shear-thickening suspensions corroborate our findings, but highlight important challenges in rigorously testing the WC model with real systems. Our results prompt new questions about the microphysics of thickening for both monodisperse and polydisperse systems.
人们越来越一致地认为，浓缩分散体的剪切增稠是由应力诱导的摩擦接触的形成驱动的。Wyart-Cates（WC）模型对此现象进行了描述，其中接触的微观物理学仅通过摩擦接触的分数 f 进入，该模型可以成功拟合弱多分散球体悬浮液的流动曲线。然而，其对于“现实生活”中多分散悬浮液的有效性尚未得到认真测试。通过对双分散球体混合物进行系统模拟，我们表明 WC 模型仅适用于单分散极限，并且当引入大量的多分散性时会失效。我们将模型的失败归因于其无法区分大-大、大-小和小-小的摩擦接触。通过使用依赖于每种接触类型分数的多分散类似物 f 来拟合我们的数据，我们表明剪切增稠的 WC 图像是不完整的。对模型剪切增稠悬浮液的系统实验证实了我们的发现，但强调了用真实系统严格测试 WC 模型所面临的重要挑战。我们的结果促使人们对单分散和多分散系统的增稠微观物理学提出了新的问题。
