Surface softness

• Applicants: Professor Dr. Roland Bennewitz; Professor Dr. Knut Drewing
• Subject Area: General, Cognitive and Mathematical Psychology ; Synthesis and Properties of Functional Materials
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 566601345

Every day we perceive soft surfaces in our environment, be it when touching cloths, wrappings, pets, or human skin. We casually or purposefully brush over the surfaces with our fingers and keep an impression of their softness. This impression is deemed to be appealing and positive and, therefore, soft surfaces play a central role in product design and marketing. Nevertheless, it is largely unexplored which sensory mechanisms underlie this perception, and which physical properties provoke the impression. The physical properties of textures and surfaces comprise many components, such as smoothness and friction, the interaction of surface layers with the substrate material, and the deformability of microstructures. The role of these characteristics cannot be studied systematically with natural surfaces. Only a microstructural material design which engages into interaction with psychological experiments can lead to insights about the basic haptic experiences of our environment. Therefore, this interdisciplinary project brings together psychologists and materials scientists. The shared aim is to elucidate physical and psychological principles that serve to understand the perception and to improve the design of soft surfaces. At the INM Leibniz Institute for New Materials, materials with well-defined parameters are produced and their interaction with the probing fingertip is characterized. At the Department of General Psychology at Justus-Liebig University Giessen, psychophysical experiments are performed with these materials, which can answer the question of the perceptual mechanisms for surface softness. We investigate classes of materials whose properties are derived from everyday observations: Based on velvety or furry materials, we will investigate the softness of micro-fibrillar structures, whose density and bending stiffness we systematically vary. We mimic the softness of silky upholstery covers by applying thin surface layers of varying stiffness to deformable substrates. Similarly, we simulate soft skin by additionally mimicking the human forearm in shape, temperature and texture of the samples. Psychophysical experiments assess the perception of materials through direct judgments and comparisons between materials, and measure movements and forces when materials are explored. We will create models that predict the perception of surface softness from the physical parameters and their sensory effects. Our results shed light on the mechanisms that underlie the judgement of surfaces between hard and soft. We contribute to the understanding of well-being in our environment and lay a foundation for the design of polymer materials with surfaces that can make medical devices comfortable, invite the touch of social robots, or increase the acceptance of sustainable materials.

• DFG Programme: Research Grants