Biology subjects

Vardar, Y.

Publications and source records attributed to Vardar, Y..

4 recordsLinked to original sources

bioRxiv2026

Fingertip-Surface Interfacial Shear Stress Varies with Sliding Conditions and Electrostatic Actuation

Fingertip friction plays a central role in tactile exploration and object manipulation. During sliding, tangential force depends jointly on the real contact area and the interfacial shear stress, both of which can be influenced by sliding conditions. However, changes in fingertip friction are often interpreted primarily through changes in real contact area, whereas the accompanying changes in interfacial shear stress remain less well characterized. This gap is especially relevant for electrostatic surface haptic displays, which modulate fingertip friction by applying a voltage between the finger and the touch surface. Here, we experimentally quantify the mean interfacial shear stress of a sliding fingertip on an electrostatically actuated touchscreen using simultaneous measurements of tangential force and optically resolved real contact area. Ten participants performed sliding trials across three speeds and three normal forces with and without electrostatic actuation. Interfacial shear stress increased with speed and decreased with normal force; in both cases, these trends arose because real contact area varied more strongly than tangential force. Electrostatic actuation further reduced interfacial shear stress, as increasing voltage produced a larger increase in real contact area than in tangential force. These findings show that interfacial shear stress varies systematically with sliding conditions and electrostatic actuation, clarifying how changes in real contact area and interfacial shear stress combine to shape fingertip-surface friction.

biophysics↗

bioRxiv2026

Physics-Informed Estimation of Electrostatic Attraction During Fingertip Sliding Under Varying Speed and Normal Force

Electrostatic actuation is an emerging technology for generating tactile sensations on capacitive touchscreens through voltage-induced attractive forces between a fingertip and the surface. However, accurate control of electrostatic attraction during natural touchscreen interactions remains challenging because the applied normal force and sliding speed continuously vary, and their effects on the fingertip-screen contact and resulting actuation strength are not fully characterized. Here, we show how normal force and sliding speed systematically alter fingertip- screen contact area and electrical impedance, and use these measured changes to estimate electrostatic attraction during sliding. Contact area, interaction forces, and electrical impedance were measured simultaneously as participants slid their fingertips across an electrostatic surface under systematically varied normal forces and sliding speeds. These measurements revealed condition-dependent changes in fingertip contact, electrical interaction impedance, effective capacitance, derived effective gap thickness, and electrostatic attraction. We then incorporated these measured contact quantities into a physics-informed, data-driven model based on parallel-plate capacitor theory, in which effective capacitance, apparent contact area, and effective voltage determine the estimated electrostatic attraction. The resulting model links force- and speed-dependent changes in these quantities to electrostatic attraction while accounting for inter-participant variability through a participant-specific scaling factor. These findings provide experimentally grounded guidance for designing electrostatic surface-haptic feedback and future adaptive control strategies under realistic touch conditions.

biophysics↗

bioRxiv2026

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.

biophysics↗

bioRxiv2026

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

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.

biophysics↗