TRIBOLOGY OF TOUCH

With the rapid development of haptic devices, there is an increasing demand to understand finger pad topography under different conditions, especially for investigation of the human–machine interface in surface haptic devices. An accurate description of finger pad topography across scales is essential for the study of the interfaces and could be used to predict the real area of contact and friction force, both of which correlate closely with human tactile perception. However, there has been limited work reporting the heterogeneous topography of finger pads across scales. In this work, we propose a detailed heterogeneous finger topography model based on the surface roughness power spectrum. The analysis showed a significant difference between the topography on ridges and valleys of the fingerprint and that the real contact area estimation could be different by a factor of 3. In addition, a spatial–spectral analysis method is developed to effectively compare topography response to different condition changes. This paper provides insights into finger topography for advanced human–machine interaction interfaces.

With the commercialization of haptic devices, understanding behavior under various environmental conditions is crucial for product optimization and cost reduction. Specifically, for surface haptic devices, the dependence of the friction force and the electroadhesion effect on the environmental relative humidity and the finger hydration level can directly impact their design and performance. This article presents the influence of relative humidity on the finger-surface friction force and the electroadhesion performance. Mechanisms including changes to Young's modulus of skin, contact angle change and capillary force were analyzed separately with experimental and numerical methods. Through comparison of the calculated capillary force in this paper and the electroadhesion force calculated in published papers, it was found that electrowetting at high voltage could contribute up to 60% of the total friction force increase in electroadhesion. Therefore, in future design of surface haptic devices, the effect of electrowetting should be considered carefully.

Surface haptics is an important branch of the growing haptic industry. The finger-device interface is crucial for the design and optimization of surface haptic devices. Our research aims to understand the tribology and Micro/Nano scale mechanics to design novel surface textures that have minimal sensitivity to the environment and user variations. To study the fingertip-substrate interaction, we designed and developed a custom tribometer. The tribometer is designed to measure the friction force between the human fingertip and different surfaces (including electroadhesion enabled surfaces). A special normal force adjustment mechanism is developed to better control the normal force during the measurement of friction. We utilized an environmental chamber in the tribometer's design and development which enables us to measure and analyze friction force between the finger and different textured surfaces under different relative humidities.

Custom tribometer