Electrostatic Actuation Induces Competing Adhesion and Vibration Regimes at Fingertip Contact

Electrostatic actuation enables programmable tactile feedback by modulating finger-surface friction via oscillating electric fields. Despite its potential, widespread adoption is hindered by an incomplete understanding of the underlying physical mechanisms, particularly the dynamics of finger-surface contact. To address this problem, this study presents the first time-resolved measurements of real contact area modulation under electrostatic actuation, obtained concurrently with contact forces. Experiments with ten participants sliding their fingers on an electrostatic display revealed an inverted U-shaped dependence of mean contact area and tangential force on actuation frequency, with a pronounced peak near 116 Hz—consistent with the frequency-dependent response of the fingertip-display system captured by mass-spring-damper and contact models. Two regimes emerged: a vibration regime below 320 Hz, where the voltage increased the contact area more than the tangential force, thereby reducing interfacial shear stress relative to the baseline; and an adhesion regime at higher frequencies, where skin viscoelasticity attenuated oscillations and restored or increased shear stress. For moist fingers, vibration effects were reduced, weakening the modulation of both tangential force and contact area. These findings reveal how adhesion and vibration jointly govern finger-surface interactions, guiding the design of next-generation electrostatic haptic interfaces.