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.acoustic | squeeze film | biotribology | roughness | haptics H olding a glass of wine, searching for keys in one's pockets, and assessing the quality of fabric are everyday tasks that involve precise and unambiguous perception of the friction between the skin and the environment. The somatosensory and motor control systems integrate multiple neural signals to determine the state of adhesion of the surface in contact with the skin, thus enabling perception (1-3) and in the context of grasp, ensuring that slippage is under control (4-6). Considering the central role of fingertip-surface friction in both manipulation and tactile perception, it is not surprising that many technologies attempt to control this effect to produce artificial and programmable tactile sensations (7-9). The use of transverse ultrasonic vibrations to reduce tactile friction (10) has proven to be a strong candidate for surface haptic displays that might be integrated with the ubiquitous touchscreen interface (11-13). A typical architecture consists of a glass plate-which may be placed in front of a graphical display-with piezoelectric actuators glued along one edge and used to excite a 0 × n flexural resonance. The resonant frequency may be ∼30 kHz and the peak to peak vibration amplitude may be up to 5 μm at the antinodes. A finger placed on the plate experiences markedly reduced friction as the vibration amplitude is increased as shown in Movie S1.A full understanding of the physical principle behind friction reduction has proven elusive. Two leading hypotheses have been put forward. The first hypothesis stems from an application of Reynolds' lubrication theory to the thin film of air between the fingertip and vibrating plate. The vibrations lead to time-averaged compression of the air, thereby creating an overpressure that levitates the skin. The second hypothesis postulates that the skin does not stay in close contact with the surfa...

Abstract-The tactual scanning of five naturalistic textures was recorded with an apparatus capable of measuring the tangential interaction force with a high degree of temporal and spatial resolution. The resulting signal showed that the transformation from the geometry of a surface to the force of traction, and hence to the skin deformation experienced by a finger is a highly nonlinear process. Participants were asked to identify simulated textures reproduced by stimulating their fingers with rapid, imposed lateral skin displacements as a function of net position. They performed the identification task with a high degree of success, yet not perfectly. The fact that the experimental conditions eliminated many aspects of the interaction, including low-frequency finger deformation, distributed information, as well as normal skin movements, shows that the nervous system is able to rely on only two cues: amplitude and spectral information. The examination of the "spatial spectrograms" of the imposed lateral skin displacement revealed that texture could be represented spatially despite being sensed through time and that these spectrograms were distinctively organized into what could be called "spatial formants". This finding led us to speculate that the mechanical properties of the finger enables spatial information to be used for perceptual purposes in humans without any distributed sensing, a principle that could be applied to robots.

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