Partial squeeze film levitation modulates fingertip friction

Significance Touchscreens have redefined human–computer interfaces. Although flexibility in the design of interfaces has dramatically increased, users are still confronted with a flat, featureless glass plate that cannot provide any tactile cues. Exciting this plate with ultrasonic waves reduces the friction experienced by a user’s finger, enabling tactile feedback directly on the surface. For vibration amplitudes of ±3 μm, we measured a reduction of up to 95% in the friction force experienced by a sliding finger. Using special illumination techniques and microsecond imaging, we show that this reduction of friction is because of the skin bouncing on a layer of air trapped between the plate and the surface of the finger. When touched, a glass plate excited with ultrasonic transverse waves feels notably more slippery than it does at rest. To study this phenomenon, we use frustrated total internal reflection to image the asperities of the skin that are in intimate contact with a glass plate. We observed that the load at the interface is shared between the elastic compression of the asperities of the skin and a squeeze film of air. Stroboscopic investigation reveals that the time evolution of the interfacial gap is partially out of phase with the plate vibration. Taken together, these results suggest that the skin bounces against the vibrating plate but that the bounces are cushioned by a squeeze film of air that does not have time to escape the interfacial separation. This behavior results in dynamic levitation, in which the average number of asperities in intimate contact is reduced, thereby reducing friction. This improved understanding of the physics of friction reduction provides key guidelines for designing interfaces that can dynamically modulate friction with soft materials and biological tissues, such as human fingertips.