Adhesion or Vibration? Frequency-Dependent Fingertip Contact on Electrostatic Displays

Electrostatic actuation enables programmable tactile feedback on touchscreens by modulating finger-surface friction through an oscillating electric field. Previous studies have attributed this modulation to adhesion, where increased real contact area enhances friction. However, adhesion alone cannot explain the frequency-dependent behavior observed under oscillation, indicating a role of vibration-driven fingertip dynamics. Here, finger-glass contact is directly visualized and quantified in 10 participants using frustrated total internal reflection, providing the first time-resolved measurements of real contact area modulation synchronized with normal and tangential forces. The real contact area and tangential force exhibited an inverted U-shaped dependence on actuation frequency, consistent with models of fingertip mass-spring-damper systems and contact mechanics. Below 320 Hz, a vibration regime increased the real contact area more rapidly than the tangential force, reducing interfacial shear stress. At higher frequencies, skin viscoelasticity attenuated oscillations and restored or increased interfacial shear stress, yielding an adhesion regime. Increased fingertip moisture reduced the modulation amplitude of both real contact area and tangential force. These findings reveal how adhesion and vibration jointly govern finger-surface interactions, guiding the design of next-generation electrostatic haptic interfaces.