Electrostatic precursors to granular slip events

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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.