Roughness-controlled Tribocharging Governs Friction in Dry Glass Contacts

Friction is commonly reduced by polishing surfaces, based on the idea that roughness enhances mechanical interlocking and thus friction. Here we show that, for dry glass-glass contacts, increasing nanoscale roughness can instead reduce friction because it suppresses triboelectric adhesion. Using rheometer-based friction measurements in dry nitrogen, super-resolution imaging of the real contact area, soft x-ray discharge, and Faraday-cup electrometry, we demonstrate that sliding generates substantial tribocharges whose electrostatic attraction contributes significantly to friction. As the root-mean-square surface slope h'_rms of the glass ball is increased from 0.01 to 0.09, the real contact area and retained tribocharge both decrease strongly, while the average contact pressure increases by a factor of three; nevertheless, the friction coefficient drops by about 30%. Discharging the interface with soft x-rays largely removes the roughness dependence of friction. Our results show that nanoscale roughness controls tribocharging and electroadhesion in dielectric contacts, inverting the classical relation between roughness and friction and identifying triboelectric effects as a key design parameter for friction control.