A glass surface feels more slippery to the touch when vibrated at ultrasonic frequencies. The phenomenon has long been known, but the underlying mechanisms have remained unclear. Michaël Wiertlewski and colleagues now suggest that the reduced friction is due to the skin bouncing on a 'squeeze film' that forms as the air between the fingertip and the vibrating glass plate is compressed. This mechanism decreases the contact area and thus the friction.
The picture of load sharing between compressed air and the skin emerged from a series of tribometric experiments in which the area of fingertip–surface contact was measured optically. Wiertlewski et al. relied on frustrated total internal reflection to image only skin within a few hundred nanometres of the glass surface. They could therefore reliably estimate the area of contact and monitor with high spatiotemporal resolution how that area changed when the surface was vibrated.
How these findings can be of practical value is not difficult to see: as we increasingly interact with our electronic devices using our fingertips, modulating the 'touching experience' using ultrasonic vibrations might form the basis for a new form of haptic feedback.