Abstract
Triboelectric charging is present in numerous technologies and everyday processes, providing both problems and opportunities. Despite this, there is no generalised model for the amount of charge that will build up on surfaces in contact. Here, we develop a new model for the saturation charge on triboelectrically charged spherical insulators, accounting for both equalisation of surface potentials and electrical breakdown of the surrounding medium. Experiments are conducted under controlled temperature and humidity using two independent methods, measuring the saturation charge on polymer spheres contacting grounded stainless steel. The results verify our equalisation of surface potentials model which describes how saturation charge density increases for smaller particle sizes. Key triboelectric properties are calculated: The estimated saturation charge on a flat surface and the equalisation potential between different materials, which can be used to predict charge saturation and quantify a triboelectric series. The transition radius below which electrical breakdown will cause saturation of charge is also calculated theoretically. Limitations to the model are demonstrated experimentally. As particle size reduces, a point is reached at which the electrostatic adhesion of particles to the grounded charging surface prevents further charge build-up. Furthermore, it is found that the saturation charge for smaller particles in humid conditions is greatly reduced. These calculations, and the demonstrated procedure, can serve as a tool for the design of technologies and processes influenced by triboelectric charge build-up, including triboelectric nanogenerators and electrostatic mineral separators.
Highlights
Contacts between any two surfaces can lead to the transfer of charge, a process called triboelectric charging
The effect of particle size on surface charge density was investigated using PTFE, low-density polyethylene (LDPE) and polyvinyl chloride (PVC) spheres across a range of diameters
At 10% relative humidity and 30 °C the particles were charged to saturation by shaking in a stainless steel container using the non-ionising method from the previous section
Summary
Contacts between any two surfaces can lead to the transfer of charge, a process called triboelectric charging. The total saturation surface potential will be given by φsat = φS + φF due to the additive nature of electric fields This leads to the following new expression for the saturation surface charge density on an insulating particle as a function of its radius: ε0 φsat (5). Differences in the number of leftover charge carriers and whether they recombine or neutralise the particle mean that the actual measured saturation surface charge density should be stochastic in nature and located in a band below σmax Equations (1), (5) and (7) combine to produce a model for the saturation surface charge density of spherical insulating particles when they are contacted against a grounded conductor as a function of their radius and material properties. The chamber was fitted with an airlock that was connected to the dry nitrogen purging system, allowing the addition and removal of equipment without disturbing the atmosphere
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