Electrostatic precursors to granular slip events

非技术摘要数以千计的粉末和颗粒用于聚合物、催化剂、药品和建筑材料的加工。不幸的是，这些材料在流动和堵塞之间表现出不可预测的转变。在大范围内，这些过渡表现为灾难性的滑坡事件，仅在美国每年就导致25至50人死于山体滑坡。在小范围内，滑移事件与材料不规则性密切相关，这无疑是人造陶瓷拒收的主要原因。最近的工作首次证明，电信号可以在粉床中滑动事件的任何向外迹象出现前几秒钟被持续和重复性地测量。在这个项目中，研究小组将调查这些电子前兆的原因，确定前兆依赖于什么材料性质，并为确定是否以及何时可以对滑动事件进行提前预测奠定基础。这肯定会加深对材料科学中一种全新现象的理解，并可能对地震和滑坡的预测以及对严格管制的材料(如陶瓷和药品)的质量控制产生影响。技术摘要粉末床中产生电压的机理尚不清楚。因此，这项研究的目标是在仔细的有针对性的实验中分析这一机制。具体地说，这些实验试图回答的问题是(1)在颗粒水平上发生了什么，导致电荷转移导致电压产生；(2)电信号在空间和时间上是如何变化的；以及(3)电压信号依赖于什么材料特性？将进行的工作将首先建立一个实验，其中局部电荷和应力分布可以在空间和时间上映射，第二，定量评估电压信号如何依赖于材料特性，如硬度、尺寸和粘结性。测量将使用排列在空间阵列中的静电电压表进行，并辅之以可视化技术。这些测量方法依赖于现有的和经过验证的技术，以独特的方式执行，以关注感兴趣的机制。

Non-Technical AbstractPowders and grains are used by the thousands of tons in the processing of polymers, catalysts, pharmaceuticals and building materials. Unfortunately, these materials exhibit unpredictable transitions between flow and jamming. On the large scale, these transitions manifest themselves in catastrophic slip events, causing the deaths of between 25 and 50 people annually in landslides in the US alone. On the small scale, slip events are intimately associated with material irregularities, which are far and away the dominant cause of rejections of manufactured ceramics. Recent work has demonstrated for the first time that electrical signals can be measured consistently and reproducibly as much as several seconds before any outward sign of a slip event in a powder bed. In this project, the research team will investigate the cause of these electrical precursors, establish what material properties the precursors depend on, and establish the groundwork to determine whether and when advance predictions of slip events can be made. This will certainly lead to improved understanding of an entirely new phenomenon in material science, and may conceivably have implications for earthquake and landslide prediction as well as for improved quality control for tightly regulated materials such as ceramics and pharmaceuticals.Technical AbstractThe mechanism underlying electrical voltage production in powder beds is not understood. The goal of this research is therefore to analyze this mechanism in carefully targeted experiments. In particular, questions that these experiments seek to answer are (1) what, mechanistically, occurs at the granular level to cause a transfer of charge leading to voltage production; (2) how do the electrical signals vary in space and in time; and (3) what material properties do the voltage signals depend on? The work that will be performed will be first to construct an experiment in which local charge and stress distributions can be mapped in space and in time, and second, to quantitatively evaluate how the voltage signals depend on material properties such as hardness, size and cohesivity. Measurements will be performed using electrostatic voltmeters arranged in a spatial array, complemented by visualization techniques. These measurement methods rely on existing and proven technologies, performed in unique ways designed to focus on the mechanism of interest.