Triboelectric charging of insulating materials - understanding, manipulation and application to lunar in-situ resource utilisation

Abstract

The lunar surface is covered in a deep layer of generally very fine mineral regolith, presenting both opportunities and challenges for exploration and the utilisation of in-situ resources. The reduction of oxygen from lunar regolith will greatly reduce the cost of space exploration by providing fuel, metals and oxygen for life support. On the other hand, the very fine grain size of the regolith combined with the lunar vacuum lead to the build-up of electrical charge on dust grains and their subsequent adhesion to surfaces. Triboelectric charging is the transfer of charge between two surfaces upon contact. It can lead to unwanted charge build-up and subsequent dust adhesion, or it can be used to manipulate regolith for transport or the separation of oxygen bearing minerals. Triboelectric charging is notoriously difficult to predict due to its dependence on a number of environmental and material properties. This thesis presents a theoretical and experimental study of the saturation of triboelectric charge on insulating particles. A new method of simultaneous triboelectric charging and charge measurement is developed, providing a quick, accurate method for triboelectric charge characterisation. Electrical breakdown of air is identified as the primary saturation mechanism for triboelectric charge on polymer particles, explaining the inverse relationship between particle size and saturation surface charge density. Triboelectric charging is unaffected by humidity below a transition humidity level. This transition humidity level is found to be proportional to particle size. The transition humidity level is not determined by the effect of humidity on the dielectric strength of air and is most likely explained by the formation of electrical double layers on material surfaces, and the dependence of this formation on surface hydrophobicity and curvature. The cumulative electric field of multiple particles charged together can cause electrical breakdown at a smaller charge per particle, explaining the inverse relationship between saturation charge per particle and the number of particles charged together. Triboelectric saturation charge increases significantly in a vacuum where electrical breakdown is absent, however, it is possible to discharge particles almost completely and prevent triboelectric charge build-up by maintaining them in an intermediate low pressure environment. The results in this thesis predict triboelectric charging of insulating particles in both atmospheric and vacuum conditions, and both dry and humid air. They can aid in the design and optimisation of electrostatic dust mitigation methods, and mineral separation technologies for lunar and terrestrial mining processes.