The effect of particle size and relative humidity on triboelectric charge saturation

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.

Triboelectric charge saturation on multiple particles in air and vacuum

“…PTFE spheres of 6.35 mm diameter were tribocharged in batches of 1 to 64 particles using the apparatus shown in Figure 1. The charging behaviour of single PTFE particles is well understood 38 , acquiring a consistent magnitude of negative charge after triboelectric charging against grounded stainless steel. All experiments in air were conducted at 30% relative humidity and 30 °C, which has been shown to produce reproducible experimental results for PTFE 38 .…”

“…The charging behaviour of single PTFE particles is well understood 38 , acquiring a consistent magnitude of negative charge after triboelectric charging against grounded stainless steel. All experiments in air were conducted at 30% relative humidity and 30 °C, which has been shown to produce reproducible experimental results for PTFE 38 . Figure 4 shows the experimental results for saturation charge on PTFE particles charged in batches, along with a domefirst simulation.…”

“…Experiments on the charge transferred per contact show that particles charge towards a saturation level in a manner similar to a capacitor, and that the rate of charging can be dependent on factors such as impact velocity and angle [33][34][35][36] . Particle saturation charge has been shown to be proportional to particle size 37 , but that the surface charge density reduces with increasing particle size 38 .…”

“…In the surface potential altering model, the electric field created by additional particles will induce an opposite charge in a conducting charging surface. This charge induction may alter the work function of the charging surface, in turn affecting the final saturation charge attainable by particles that contact it 38,43,44 . In the breakdown model, the electric field created by a particle or group of particles may become strong enough to cause the dielectric breakdown of the surrounding medium.…”

Robust Displacement Sensing by Direct‐Current Triboelectric Nanogenerator Via Intelligent Waveform Recognition

“…However, there is a concern that the robustness of the amplitude‐based TENG sensor is not as good as that of the frequency‐based ones because the voltage or current amplitude of the TENG‐based sensor is proportional to the surface charge density of its friction material. The humidity, [ 25 , 26 ] temperature, [ 27 , 28 ] abrasion, [ 29 ] and other factors all have non‐negligible influences on the surface charge density, and these non‐ideal factors cannot be completely avoided in practical applications. For example, in a rainy or hot day, the surface charge density is lower than normal due to increased humidity or temperature.…”