arXiv subjects

Tom F. O'Hara

Publications and source records attributed to Tom F. O'Hara.

4 recordsLinked to original sources

arXiv2026

It Takes Two to Tribo: Stochastic Charge Evolution in Repeated Binary Collisions of Acoustically Levitated Particles

The mechanisms governing triboelectric charging between insulating particles of the same material remain an open question in nonequilibrium physics, with several competing models proposed to explain observed charging behaviour. Here, we investigate charge evolution in a minimal system consisting of two acoustically levitated particles undergoing repeated binary collisions. To quantify particle charge transfer, purpose-built Faraday cage picoammeters were developed and calibrated. The MultiLev acoustic levitation system was used alongside Ultraino simulations to generate transducer control signals, enabling controlled collision and separation of polystyrene (PS) particles. Although individual collision events exhibit stochastic charge transfer, described by skew-normal distributions, we demonstrate for the first time that the cumulative charge evolution of an individual acoustically levitated particle pair follows the saturation behaviour predicted by the condenser model of triboelectric charging, with fits achieving $R^2 > 0.99$. Under identical conditions, conductive graphite-coated PS particles also undergo charge transfer, but accumulate substantially less charge, consistent with the distinct charging behaviour expected for conductive materials.

cond-mat.soft↗

arXiv2025

Surface Material Dependence in Powder Triboelectric Charging

Triboelectric charging of granular materials against container walls is a critical yet poorly understood phenomenon affecting many industrial powder handling processes. Charge accumulation can cause material flow disruptions, adhesion issues, and pose serious safety risks, such as providing ignition sources for dust cloud explosions. This study investigates particle-wall charging behaviour for materials with varying size, shape, and electrical resistivity. Aluminium surfaces are used as a reference case, followed by analysis of labradorite, an analogue for volcanic ash, to examine charging interactions with various wall materials. Finally, the triboelectric response of industrially relevant materials used in flexible intermediate bulk containers, including both lined and unlined variants, is assessed. Results show that stainless steel surfaces generate the least charge, while the presence of insulating polyethene liners increases charging significantly. These findings contribute to a deeper understanding of triboelectric charging mechanisms in powder transfer operations and inform safer industrial practice.

cond-mat.soft↗

arXiv2025

The Scaling of Triboelectric Charging Powder Drops for Industrial Applications

Triboelectrification of granular materials is a poorly understood phenomenon that alters particle behaviour, impacting industrial processes such as bulk powder handling and conveying. At small scales ($< 1 g$) net charging of powders has been shown to vary linearly with the total particle surface area and hence mass for a given size distribution. This work investigates the scaling relation of granular triboelectric charging, with small, medium ($< 200 g$), and large-scale ($\sim 400 kg$) laboratory testing of industrially relevant materials using a custom powder dropping apparatus and Faraday cup measurements. Our results demonstrate that this scaling is broken before industrially relevant scales are reached. Charge (Q) scaling with mass (m) was fitted with a function of the form $Q \propto m^b$ and $b$ exponents ranging from $0.68\ \pm\ 0.01$ to $0.86\ \pm\ 0.02$ were determined. These exponents lie between those that would be expected from the surface area of the bulk powder ($b = 2 / 3$) and the total particle surface area ($b = 1$). This scaling relation is found to hold across the powders tested and at varying humidities.

cond-mat.soft↗

arXiv2024

Faraday Cup Measurements of Triboelectrically Charged Granular Material: A Modular Interpretation Methodology

The triboelectric charging of granular materials remains a poorly understood phenomenon with a wide range of scientific and industrial applications, from volcanic lightning to pharmaceutical manufacturing. The Faraday cup is the most commonly used apparatus for studying triboelectric charging, yet current methods of interpreting measurements are overly simplistic, often conflating charging due to particle-particle interactions with other charging mechanisms. In this study, we present a modular approach for interpreting Faraday cup measurements, which allows for more detailed exploration of triboelectric phenomena. The approach involves fitting approximated charge distribution shapes to experimental Faraday cup data, using measured size distributions alongside simplified models of charge distribution and particle dynamics. This modular framework is adaptable, allowing for fine-tuning at each step to suit specific application cases, making it broadly applicable to any insulating granular material. As a case study, we examine volcanic ash samples from Grímsvötn and Atitlán volcanoes, finding that the Grímsvötn ash exhibited a higher proportion of charge due to particle-particle interactions. Experimental validation with sieved volcanic ash fractions revealed that larger particle sizes showed stronger particle-particle charging. Additionally, non-particle-particle charging was found to scale with particle size as $\propto d_p^{-0.85 \pm 0.03}$, approximately following the particles' effective surface area.

cond-mat.soft↗