What is the fundamental mechanism behind charge generation in triboelectricity?

The fundamental mechanism behind charge generation in triboelectricity is contact electrification, where charge transfer occurs between two dissimilar materials when they come into contact and then separate. This charge transfer arises due to the difference in the materials' electronic structures, specifically their work functions or electron affinities. The work function is the minimum energy required to remove an electron from a solid to a point in the vacuum immediately outside the solid surface. When two materials with different work functions are brought into contact, electrons flow from the material with the lower work function to the material with the higher work function until their electrochemical potentials equalize. This electron transfer results in one material becoming positively charged (losing electrons) and the other becoming negatively charged (gaining electrons). Upon separation, these charges remain on the surfaces of the materials, creating an electrostatic potential difference. The magnitude and polarity of the generated charge depend on the materials' positions in the triboelectric series, which ranks materials according to their tendency to gain or lose electrons. For example, when Teflon (high electron affinity) contacts nylon (lower electron affinity), electrons transfer from nylon to Teflon, making Teflon negatively charged and nylon positively charged upon separation.