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

材料和指尖之间的摩擦是我们日常生活中必不可少的一部分。它赋予我们握持物体的能力，是触觉感知的关键成分，为我们提供触觉识别和情感触摸的输入。平坦的表面可以很好地接触皮肤，并且具有很强的粘性，例如可以举起玻璃瓶。波纹状的表面会使皮肤变形，从而提供更好的抓地力，例如转动盖子和打开瓶子。然而，表面结构的触觉功能超越了抓地力：它们传递信息，引导注意力，并吸引我们。今天，我们接触的大多数表面都是经过设计的。对可持续设计和数字界面的需求将增加开发具有触觉功能和吸引力的表面的需求。无论是平坦的还是波纹的表面都不是特别吸引人的触摸，而纹理细腻的表面提供了低摩擦和丝绸的舒适感觉。在这个项目中，我们探索了小尺度随机粗糙度摩擦的物理基础，并为具有提供最低指尖摩擦的表面微观结构的材料制定了设计规则。在经典的两项描述中，摩擦力起源于皮肤-材料界面的粘接剪切和皮肤通过粗糙度的变形。当通过增加表面粗糙度来减少界面接触面积，同时通过突出表面凸起来限制皮肤的变形时，可以实现低摩擦。在指尖摩擦情况下，接触力学不仅取决于表面纹理，还取决于指尖乳头状脊结构。因此，表面纹理的优化还涉及其相对于乳头嵴之间距离的空间谱。在我们的实验中，我们计算了具有确定光谱特性的表面，并通过先进的增材制造生产了原型。众多参与者的触觉探索实验为我们验证指尖摩擦作为粗糙度光谱组成函数的模型提供了数据。我们还探讨了参与者对摩擦的感知。我们特别感兴趣的是揭示在一定长度尺度上改变粗糙度振幅时界面和变形摩擦的感知可能存在的差异。研究结果将揭示粗糙度和摩擦在材料感知中的作用。我们随机粗糙的表面产生的低摩擦和不规则的力波动预计会产生光滑，愉快的触摸。因此，成功的项目将更好地理解自然和工程表面上的皮肤摩擦。它将为我们提供令人愉悦的表面结构的设计规则，这可以被描述为“触觉白色”标准，从而为具有触觉对比和触觉功能的材料的创造提供一个新的起点。

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.