Although triboelectrification (TE) is essential in many industrial and scientific fields, its charge transfer mechanisms are still not fully understood. In this paper, the charging-induced electric potential on the friction surface and the discharging-induced light emission from the contact region during sliding frictions between insulators have been observed simultaneously. The results show that, in the absence of discharging, the temporal variations of surface potential at all the contact points are almost the same, experiencing a rapid growth in the initial stage, followed by a slow growth, and eventually reaching a stable value. To explain such a dynamics of electron transfer, a theoretical expression for the temporal evolution of the surface potential during TE process is proposed by considering the electron transfer as the charging process of a capacitor formed by contacting surfaces, and is found consistent with the experimental measurements. The experiments further indicate that, when discharging occurs, it has no influence on the charging process of the initially negatively charged surface, but can greatly change the charging of the initially positively charged surface, on which the potential will increase initially, soon begin to decrease, and eventually reach a stable value. Such a significant difference in the potential variation when discharging occurs can be attributed to the huge difference in mass between electrons and positive ions produced in the discharging process. The present work may offer a new perspective for understanding the electron transfer dynamics in TE and may provide potential applications in numerous fields involving TE.

Triboluminescence has been investigated by many researchers due to its many applications in various fields related to industry and life. However, the triboluminescence mechanism is still unclear. The triboluminescence mechanisms are further investigated here by experimentally studying the triboluminescence properties of SiO₂ crystals with various crystal orientations. The experimental results show that the crystal orientation significantly affects the triboluminescence. Measurements of the triboluminescence spectra showed that the triboluminescence comes from the gas discharge caused by triboelectrification. The different triboluminescence intensities in different crystal orientations are then shown to be based on the differences in the work functions for different orientations.

The effects of impacting particles from a jet of liquid on the removal of a surface material (on the impacted workpiece) were investigated. Experimental observations show that the cross section of the material removed changed from 'W'-shaped to 'U'-shaped as the size of abrasive particles was increased. Comparisons between removed material profiles and particle collision distributions indicate that the particle-surface collisions are the main reason for the material removal. The deduced number of atoms removed by a single collision implies that a transition occurs in the removal mode. For nanoscale particles, the polished surface is likely to be removed in an atom-by-atom manner, possibly due to the chemisorption of the impacting particles on the impacted surface. Contrarily, for the case of microscale particles, bulk material removal produced by particle bombardment is more likely to occur. The present mechanism of material removal for particle-surface collisions is further corroborated experimentally.