(421ab) Mechanisms of Bipolar Charging in Insulators in a Hopper-Chute Assembly
Triboelectrification ,or contact electrification is a process is which two materials exchange charged species (ions or electrons) leaving oppositely charged bodies after a binary contact. Usually, the electron exchange is thought to take place based on difference of work function of the contacting materials. Bipolar charging is a special case procuring two opposite polarities are procured on different sized particles of the same material (usually, smaller particles getting negatively charged and bigger particles getting positively charged). In an attempt to explain this unusual behavior, various groups have presented hypothesis based on the differences in the adsorbed impurities and surface roughness of different sized particles which can change the apparent work function of the contacting material(s).The purpose of the study is to investigate the same, using various analytical tools ranging from X-ray photoelectron spectroscopy (XPS), Time of Flight- Secondary Ion Mass Spectroscopy (ToF-SIMS), Energy Dispersive Using X-Ray analysis (EDEX), Interferometer-Profilometer study, Scanning Electron Microscopy (SEM), Thermogravimetric Analysis (TGA) and Dynamic Vapor Sorption (DVS) to thoroughly map the surfaces of various materials. Theoretical predictions of work function modulation due adsorbed impurities (moisture) based on Density Functional Theory (DFT) calculations were performed on these materials. Hopper-chute flow experiments were conducted to study the possibility of bipolar charging due to third body interaction.
A major difference in the amount of water sorption (smaller particles sorbing ca. 10 times more water as compared to bigger particles) was observed. No significant differences in the surface or bulk impurities (XPS, ToF-SIMS, EDEX) or in surface roughness (Interferometer-profilometer and SEM) was found. Accordingly DFT calculations were performed on molecular slabs with and without water to study the effect of adsorbed water on the work-function values. Water sorption was found to lower the work function values of the smaller particles due to formation of dense electron cloud on the surface from the oxygen’s lone pair of electrons as follows. Work-function of; MCC: 6.15 eV, MCC with adsorbed water: 5.94 eV, Lactose: 6.29 eV and Lactose with adsorbed water: 6.06 eV. Water sorption on smaller particles leads to lowering of work function values, which suggests that electron transfer is not the sole or most dominant mechanism behind bipolar charging and there is a significant involvement of other mechanisms such as ion transfer in the process. Hence bipolar charging might be a result of preferential adsorption of negatively charged OH- ions on smaller particles due to higher surface area and greater water sorption on the same.