Viscous textures: Velocity dependence in fingertip-surface scanning interaction

We explore the impact of fingertip velocity and material properties on the lateral force interaction between a fingertip and a texture. Three sinusoidal gratings of varying compliance were scanned by a finger at a variety of speeds while lateral force and fingertip position were measured. Two robust trends were noted: one, for more compliant textures, the DC component of lateral force was larger, and it increased with scanning speed (i.e., it had a viscous component); two, for all textures, but especially the more compliant ones, the 1 /f background noise component of lateral force decreased with increased scanning speed. Focusing on the first of these trends, we used a TPad haptic device to implement virtual gratings with multiple levels of viscosity and DC friction, and we performed a multidimensional scaling analysis as well as comparisons to two of the physical gratings. The results demonstrate that both DC friction level and viscosity have significant perceptual consequences, but suggest that subjects may not be able to distinguish readily between friction and viscosity, at least at the levels implemented here.