We investigate the conditions leading to large drag force fluctuations in granular materials. The study is based on a set of experimental drag tests, which involve pulling a plate vertically through a cohesionless granular material. In agreement with previous observations, drag force exhibits significant and sudden drops-up to 60%—when the plate is pulled out at low velocities. We further find that this instability vanishes at higher pullout velocities and near the surface. We empirically characterise the frequency and amplitude of these fluctuations and find that these properties are not consistent with a classical stick-slip dynamics. We therefore propose an alternative physical mechanism that can explain these force fluctuations.

中文翻译：

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颗粒材料中取决于速率的阻力不稳定性

我们调查了导致颗粒材料中大的阻力波动的条件。该研究基于一组实验阻力测试，其中包括垂直拉动板通过无粘性的颗粒材料。与以前的观察结果一致，当以低速拉出板时，阻力会突然显着下降至60％。我们进一步发现，这种不稳定性在较高的拔出速度和靠近地面的地方消失了。我们凭经验地描述了这些波动的频率和幅度，发现这些特性与经典的粘滑动力学不符。因此，我们提出了可以解释这些力波动的另一种物理机制。