A “tactile white” for the fingertip: structuring materials for low friction

• Applicant: Professor Dr. Roland Bennewitz
• Subject Area: Synthesis and Properties of Functional Materials
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 466754480

Friction between materials and the human fingertip is an essential part in our everyday life. It gives us the grip to handle objects and is key ingredient in tactile perception, providing us with the input for haptic recognition and affective touch. Flat surfaces offer good contact with the skin and a strong sticking, for example to lift a glass bottle. Rippled surfaces deform the skin and give better grip, for example to turn the lid and open the bottle. However, tactile functions of surface structures go beyond grip: They transport information, steer attention, and appeal to us. Already today most of the surfaces we touch are engineered. The demand for sustainable design and digital interfaces will increase the need to develop surfaces with haptic functions and attractions. Neither flat nor rippled surfaces are particularly appealing to touch, while surfaces with a fine texture offer low friction and the pleasant feel of silk. In this project, we explore the physics underlying the friction of small-scale random roughness and develop design rules for materials with a surface microstructure that provides lowest fingertip friction. In a classical two-term description, friction forces originate in adhesive shear at the skin-material interface and in deformation of the skin through roughness asperities. Low friction can be achieved when the interfacial contact area is reduced by increasing surface roughness while limiting deformation of the skin through protruding surface asperities. For the case of fingertip friction, contact mechanics depend not only on surface texture, but also on the papillary ridge structure at the fingertip. Therefore, an optimization of the surface texture also involves its spatial spectrum with respect to distances between papillary ridges. For our experiments we calculate surfaces with defined spectral properties and produce prototypes by advanced additive manufacturing. Tactile exploration experiments with many participants provide us with the data to validate models for fingertip friction as function of the spectral composition of roughness.We also explore the perception of friction by participants. We are particularly interested to reveal possible differences in the perception of interfacial and deformation friction when varying the roughness amplitude at certain length scales. The results will shed new light on the roles of roughness and friction in materials perception. The low friction and the irregular force fluctuations produced by our randomly rough surfaces are expected to result in a smooth, pleasant touch. The successful project will thus result in a better understanding of skin friction on natural and engineered surfaces. It will provide us with design rules for pleasant surface structures which may be described as a “tactile white” standard and thus offer a novel starting point for the creation of materials with tactile contrast and haptic functions.

• DFG Programme: Research Grants