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.

中文翻译：

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颗粒状物质像复杂的流体一样流动

诸如沙子，粉末和泡沫之类的粒状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会形成稳定的结构，但是在受到诸如敲击或剪切之类的外部影响的情况下，它们“放松”，从而自然变成流体。通常认为粒状体系的弛豫动力学与热玻璃成形体系的弛豫动力学相似。但是，到目前为止，尚不可能通过实验确定颗粒级的三维颗粒系统的动力学特性。缺乏实验数据，加上粒状颗粒的运动涉及摩擦这一事实（而热玻璃成型系统中颗粒的运动却没有摩擦），意味着缺乏对粒状材料弛豫动力学的准确描述。在这里，我们使用X射线断层扫描来确定经受振荡剪切的硬颗粒椭球体的微观尺度松弛动力学。我们发现，椭球位移的分布可以通过Gumbel定律很好地描述（类似于小位移的高斯分布，但是对于大位移，尾部较重），其形状参数与振幅的大小无关。剪切应变和时间。尽管存在这种普遍性，但单个椭球体的均方位移仍遵循幂定律作为时间的函数，其指数确实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（因此，椭圆体在给定时间点的运动取决于其先前的运动）。我们的观察结果表明，在颗粒水平上，粒状系统的动态行为在性质上不同于热玻璃成形系统，而是与复杂流体的动态行为更为相似。我们得出的结论是，即使驱动力较弱，粒状材料也可以松弛。