Combined effect of moisture and electrostatic charges on granular materials: an experimental approach

It is well known empirically by industrialists that the relative air humidity and the presence of electrostatic charges influence drastically granular material properties. The relative air humidity induces the formation of capillary bridges and modify the grain surface conductivity. The presence of capillary bridges produces cohesive forces. On the other hand, the apparition of electrostatic charges due to the triboelectric effect at the contacts between the grains and at the contacts between the grains and the container produces electrostatic forces. Therefore, in many cases, the cohesiveness is the result of the interplay between capillary and electrostatic forces. Unfortunately, the triboelectric effect is still poorly understood, particularly inside a granular material. Moreover, reproducible electrostatic measurements are difficult to perform. In this presentation, we will discuss a set of experimental results showing the sensitivity of tribocharging to relative air humidity with an optimum around 37%RH. Moreover, we show the consequences of this optimum on the flow and packing fraction dynamics. The fundamental aspects but also the consequences on a short selection of industrial processes will be discussed. From a fundamental point of view, we will link our experimental results with the recent advances in the field. In particular with the model based on patchy grains (donor/acceptor sites for electrostatic charges) and the models based on water ions exchange.