Granular materials flow like complex fluids
Granular materials flow like complex fluids
复制标题
颗粒材料像复杂流体一样流动
DOI：
10.1038/nature24062
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发表时间：
2017-11-16
期刊：
影响因子：
64.8
通讯作者：
Wang, Yujie
中科院分区：
文献类型：
--
作者：
Kou, Binquan;Cao, Yixin;Wang, Yujie
沙子、粉末和泡沫等颗粒材料在日常生活以及工业和岩土工程应用中无处不在。这些无序的系统在不受干扰时形成稳定的结构，但在存在外部影响（例如敲击或剪切）的情况下，它们会“松弛”，本质上变成流体。通常假设颗粒系统的弛豫动力学与热玻璃形成系统的弛豫动力学相似。然而，到目前为止，还不可能在粒子水平上通过实验确定三维颗粒系统的动态特性。实验数据的缺乏，加上粒状颗粒的运动涉及摩擦（而热玻璃形成系统中颗粒的运动则不涉及摩擦）这一事实，意味着缺乏对粒状材料松弛动力学的准确描述。在这里，我们使用 X 射线断层扫描来确定受到振荡剪切的硬粒状椭球体的微观弛豫动力学。我们发现椭球体位移的分布可以通过冈贝尔定律（类似于小位移的高斯分布，但对于较大位移具有较重的尾部）来很好地描述，其形状参数与剪切应变的幅度和时间无关。尽管存在这种普遍性，但单个椭球体的均方位移遵循幂律作为时间的函数，其指数取决于应变幅度和时间。我们认为这些结果与涉及摩擦和记忆效应的微观松弛机制有关（即椭球体在给定时间点的运动取决于其先前的运动）。我们的观察表明，在颗粒水平上，颗粒系统的动态行为与热玻璃形成系统的动态行为有本质上的不同，而更类似于复杂流体的动态行为。我们得出的结论是，即使驱动应变很弱，颗粒材料也可以松弛。
Granular materials such as sand, powders and foams are ubiquitous in daily life and in industrial and geotechnical applications,,,. These disordered systems form stable structures when unperturbed, but in the presence of external influences such as tapping or shear they ‘relax’, becoming fluid in nature. It is often assumed that the relaxation dynamics of granular systems is similar to that of thermal glass-forming systems,. However, so far it has not been possible to determine experimentally the dynamic properties of three-dimensional granular systems at the particle level. This lack of experimental data, combined with the fact that the motion of granular particles involves friction (whereas the motion of particles in thermal glass-forming systems does not), means that an accurate description of the relaxation dynamics of granular materials is lacking. Here we use X-ray tomography to determine the microscale relaxation dynamics of hard granular ellipsoids subject to an oscillatory shear. We find that the distribution of the displacements of the ellipsoids is well described by a Gumbel law (which is similar to a Gaussian distribution for small displacements but has a heavier tail for larger displacements), with a shape parameter that is independent of the amplitude of the shear strain and of the time. Despite this universality, the mean squared displacement of an individual ellipsoid follows a power law as a function of time, with an exponent that does depend on the strain amplitude and time. We argue that these results are related to microscale relaxation mechanisms that involve friction and memory effects (whereby the motion of an ellipsoid at a given point in time depends on its previous motion). Our observations demonstrate that, at the particle level, the dynamic behaviour of granular systems is qualitatively different from that of thermal glass-forming systems, and is instead more similar to that of complex fluids. We conclude that granular materials can relax even when the driving strain is weak.