The rupture of microscopic contacts between metallic or insulating spheres and adhesive tape results in the separation of charges. An electric breakdown of air between the separating charged surfaces can be observed as emission of light in the ultraviolet regime. We have simultaneously recorded pulling force and light emission during the rupture process. The results indicate the formation of multiple filaments of the adhesive even in microscopic contacts. The rupture of each filament causes a light flash. We find proportionality between the amount of emitted light and the pulling force at the moment of rupture. The strength of this rupture force depends on the normal force applied in the contact formation and on the contact time.
We have investigated the amount of separated charge by calibrating the light intensity. The calibration has been achieved by measuring the light intensity of micro-gap discharges for known amounts of charge in the same geometry. We find charge density of 0.01-0.1 C/m2 for the breaking contacts between steel and silicon nitride balls and adhesive tape. This amount is magnitudes higher than typical values for the contact electrification between insulators. The typical contact diameter was 300 µm and typical rupture forces were of the order of 100 mN.