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...

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Friction modulation technology enables the creation of textural effects on flat haptic displays. However, an intuitive and manageably small design space for construction of such haptic textures remains an unfulfilled goal for user interface designers. In this paper, we explore perceptually relevant features of fine texture for use in texture construction and modification. Beginning with simple sinusoidal patterns of friction force that vary in frequency and amplitude, we define irregularity as a third building block of a texture pattern and show it to be a scalable feature distinct from the others using multidimensional scaling. Additionally, subjects' verbal descriptions of this 3-dimensional design space provide insight into their intuitive interpretation of the physical parameter changes.

Compared to relatively mature audio and video human-machine interfaces, providing accurate and immersive touch sensation remains a challenge owing to the substantial mechanical and neurophysical complexity of touch. Touch sensations during relative lateral motion between a skin-screen interface are largely dictated by interfacial friction, so controlling interfacial friction has the potential for realistic mimicry of surface texture, shape, and material composition. In this work, we show a large modulation of finger friction by locally changing surface temperature. Experiments showed that finger friction can be increased by ~50% with a surface temperature increase from 23° to 42°C, which was attributed to the temperature dependence of the viscoelasticity and the moisture level of human skin. Rendering virtual features, including zoning and bump(s), without thermal perception was further demonstrated with surface temperature modulation. This method of modulating finger friction has potential applications in gaming, virtual and augmented reality, and touchscreen human-machine interaction.

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