Density saturation of densely contact-electrified negative charges on a thin silicon oxide sample due to the Coulomb repulsive force

To compare the mechanism of the contact electrification on a thin silicon oxide sample in air with that in vacuum, the contact voltage V-C and contact time t(0) dependences of the peak value and the full width at half maximum (FWHM) of the electrostatic force distributions induced by the deposited negative charges just after the contact electrification were measured. In air, the peak value of the electrostatic force was found to saturate with increase in both V-C and t(0). Also, the FWHM was found to increase monotonically with increase in both V-C and t(0). These results suggest that density saturation of the deposited negative charges occurs and that the saturated charge density is independent of both V-C and t(0). Further, the deposited negative charges diffuse owing to the Coulomb repulsive force between the deposited negative charges. On the other hand, in vacuum, the peak value of the electrostatic force was found to saturate with increase in t(0), whereas it increases monotonically with increase in V-C. Also, the FWHM of the electrostatic force was found to be nearly independent of both V-C and t(0). Thus deposited negative charges do not diffuse even during contact electrification and the incessant deposition seems to be inhibited by the Coulomb repulsive force at the charge density where the Coulomb repulsive force due to the deposited negative charges equilibrates with the force for the contact electrification due to V-C.