Faraday cup measurements of triboelectrically charged granular material: a modular interpretation methodology

The triboelectric charging of granular material is a long-standing and poorly understood phenomenon, with numerous scientific and industrial applications ranging from volcanic lightning to pharmaceutical production. The most widely utilised apparatus for the study of such charging is the Faraday cup, however, existing analysis of the resulting measurements is often simplistic and fails to distinguish charging due to particle-particle interactions from charging occurring through other mechanisms. Here, we outline a modular approach for interpreting these measurements, enabling triboelectric phenomena to be explored in greater detail. Our approach fits approximated charge distribution shapes to experimental Faraday cup traces. The fitting process uses measured size distributions in combination with simplified models of charge distribution and particle dynamics to predict the relative charging contributions. This modular approach allows scope for adaptation of each aspect to fine-tune the process to specific application cases, making the technique broadly generalisable to any insulating granular material. An example case of volcanic ash showed that samples from the Grímsvötn volcano charged with a greater proportion of particle-particle interactions than ash from Atitlán. Experimental validation is provided using sieved fractions of volcanic ash, where the broader size fractions were found to exhibit greater particle-particle charging. Non-particle-particle charging was also shown to scale with particle size as ∝ d-0.85±0.03p, roughly scaling with the particles' effective surface area.