Fingertip real contact area scales quadratically with input voltage in electrostatic actuation

Touchscreens have become the dominant interface in consumer electronics, yet interactions with them remain primarily visual. Incorporating haptic feedback that simulates touch sensations could make these interactions more natural and intuitive. Electrostatic actuation, which modulates friction by attracting the finger toward a capacitive surface using an alternating voltage, offers a promising approach. The resulting increase in friction is often attributed to the rise in real contact area; however, direct experimental evidence linking voltage input parameters to real contact area and contact forces remains limited. Here, we use frustrated total internal reflection to directly image the real contact area while simultaneously measuring contact forces during controlled finger sliding under electrostatic actuation. We systematically vary voltage amplitude (75-150 V) and excitation frequency (30-230 Hz) and quantify the changes in contact area and forces as functions of these parameters. Our results reveal a quadratic dependence of real contact area, electro-static attraction, and tangential force on voltage amplitude, with comparatively small effects of excitation frequency. These findings clarify the respective roles of voltage amplitude and frequency in the electrostatic modulation of finger contact mechanics, providing design guidelines for haptic display design.