Lee Young Ki, Hyun Kyu, Ahn Kyung Hyun
School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, Seoul 08826, Korea.
School of Chemical and Biomolecular Engineering, Pusan National University, Busan 46241, Korea.
Soft Matter. 2020 Nov 11;16(43):9864-9875. doi: 10.1039/d0sm01204a.
We carry out a numerical study to investigate the dynamics of non-Brownian hard-sphere suspensions near the liquid and crystal coexistence region in small to large amplitude oscillatory shear flow. The first normal stress difference (N1) and related rheological functions are carefully analyzed, focusing on the strain stiffening phenomenon, which occurs in the large strain amplitude region. Under oscillatory shear, we observe several unique behaviors of N1. A negative nonzero mean value of N1 (N1,0) is observed for the applied strain amplitudes. The change of the sign, from negative to positive, at the maximum value of N1 (N1,max) is observed at a specific point, which is not consistent with the critical strain amplitude (γ0,c) at which the modulus begins to deviate from linear viscoelasticity. The behavior of N1 in the oscillatory shear flow is different from that of N1 in steady shear flow, that is, the characteristics of N1 in strain stiffening and shear thickening are quite distinguished from each other. We also perform structural analysis to confirm the relationship between the rheological properties and microstructure of the suspension. A strong correlation is observed between the global bond order parameter (Ψ6) and the distortions in both nonlinear shear and normal stresses. The most noticeable characteristic is captured through the maximum of the global bond order parameter (Ψ6,max). The strain amplitude at the slope change of Ψ6,max corresponds to the point where a unique behavior of N1 is observed, i.e. the change of the sign in N1,max, but a strong correlation is not captured at γ0,c. This demonstrates that the normal stress responds to particle ordering more sensitively than other rheological functions based on shear stress like dynamic moduli. As far as we are concerned, the behavior of N1 has rarely been fully explored and related with the strain stiffening of non-Brownian suspensions so far. Therefore, this study has significance as the first report to strictly analyze strain stiffening along with the first normal stress difference N1.
我们进行了一项数值研究，以研究在小振幅到大幅振荡剪切流中，非布朗硬球悬浮液在液体和晶体共存区域附近的动力学。仔细分析了第一法向应力差（N1）和相关的流变函数，重点关注在大应变振幅区域出现的应变强化现象。在振荡剪切作用下，我们观察到了N1的几种独特行为。对于所施加的应变振幅，观察到N1的负非零平均值（N1,0）。在特定点观察到N1的最大值（N1,max）处的符号从负变为正，这与模量开始偏离线性粘弹性时的临界应变振幅（γ0,c）不一致。振荡剪切流中N1的行为与稳态剪切流中N1的行为不同，也就是说，N1在应变强化和剪切增稠方面的特性彼此有很大区别。我们还进行了结构分析，以确认悬浮液的流变性质与微观结构之间的关系。观察到全局键序参数（Ψ6）与非线性剪切应力和法向应力的畸变之间存在很强的相关性。最显著的特征通过全局键序参数的最大值（Ψ6,max）得以体现。Ψ6,max斜率变化处的应变振幅对应于观察到N1独特行为的点，即N1,max中的符号变化，但在γ0,c处未观察到很强的相关性。这表明法向应力比基于剪切应力的其他流变函数（如动态模量）对颗粒排列的响应更敏感。就我们所知，到目前为止，N1的行为很少被充分探索并与非布朗悬浮液的应变强化相关联。因此，本研究作为首次严格分析应变强化以及第一法向应力差N1的报告具有重要意义。
