Tribological interactions of the finger pad and tactile displays

This thesis summarise the results of an investigation of the tribological interactions of the human finger pad with different surfaces and tactile displays. In the wide range of analyses of the mechanical properties of the finger pad, an attempt has been made to explain the nature of the interactions based on critical material parameters and experimental data. The experimental data are presented together with detailed modelling of the contact mechanics of the finger pad compressed against a smooth flat surface. Based on the model and the experimental data, it was possible to account of the loading behaviour of a finger pad and derive the Young’s modulus of the fingerprint ridges. The frictional measurements of a finger pad against smooth flat surfaces are consistent with an occlusion mechanism that is governed by first order kinetics. In contrast, measurements against a rough surface demonstrated that the friction is unaffected by occlusion since Coulombic slip was exhibited. The thesis includes an investigation of critical parameters such as the contact area. It has been shown that four characteristic length scales, rather than just two as previously assumed, are required to describe the contact mechanics of the finger pad. In addition, there are two characteristic times respectively associated with the growth rates of junctions formed by the finger pad ridges and of the real area of contact. These length and time scales are important in understanding how the Archardian-Hertzian transition drives both the large increase of friction and the reduction of the areal load index during persisting finger contacts with impermeable surfaces. Established and novel models were evaluated with statistically meaningful experiments for phenomena such as lateral displacement, electrostatic forces and squeeze-film that have advanced applications

Characterizing and imaging gross and real finger contacts under dynamic loading

We describe an instrument intended to study finger contacts under tangential dynamic loading. This type of loading is relevant to the natural conditions when touch is used to discriminate and identify the properties of the surfaces of objects — it is also crucial during object manipulation. The system comprises a high performance tribometer able to accurately record in vivo the components of the interfacial forces when a finger interacts with arbitrary surfaces which is combined with a high-speed, high-definition imaging apparatus. Broadband skin excitation reproducing the dynamic contact loads previously identified can be effected while imaging the contact through a transparent window, thus closely approximating the condition when the skin interacts with a non-transparent surface during sliding. As a preliminary example of the type of phenomenon that can be identified with this apparatus, we show that traction in the range from 10 to 1000 Hz tends to decrease faster with excitation frequency for dry fingers than for moist fingers

Why do pens have rubbery grips?

Significance Why does gripping a pen, tool, or handle feel more secure when it is coated with a rubbery material? The keratin of the skin outer layer is stiff and rough at a small scale. When encountering a smooth, stiff, and impermeable surface, such as polished metal or glass, the actual contact area is initially small as is the friction. Because the keratin softens when it is hydrated by the moisture secreted from the sweat pores, it requires many seconds for the contact area to increase to the value reached almost instantaneously with a soft material, such as a rubber. This mechanism might be used by our tactile sense to identify materials and has implications for the design of tactile displays.</jats:p

The molecular architecture of engulfment during Bacillus subtilis sporulation.

The study of bacterial cell biology is limited by difficulties in visualizing cellular structures at high spatial resolution within their native milieu. Here, we visualize Bacillus subtilis sporulation using cryo-electron tomography coupled with cryo-focused ion beam milling, allowing the reconstruction of native-state cellular sections at molecular resolution. During sporulation, an asymmetrically-positioned septum generates a larger mother cell and a smaller forespore. Subsequently, the mother cell engulfs the forespore. We show that the septal peptidoglycan is not completely degraded at the onset of engulfment. Instead, the septum is uniformly and only slightly thinned as it curves towards the mother cell. Then, the mother cell membrane migrates around the forespore in tiny finger-like projections, whose formation requires the mother cell SpoIIDMP protein complex. We propose that a limited number of SpoIIDMP complexes tether to and degrade the peptidoglycan ahead of the engulfing membrane, generating an irregular membrane front

Modeling and simulation in tribology across scales: An overview

This review summarizes recent advances in the area of tribology based on the outcome of a Lorentz Center workshop surveying various physical, chemical and mechanical phenomena across scales. Among the main themes discussed were those of rough surface representations, the breakdown of continuum theories at the nano- and micro-scales, as well as multiscale and multiphysics aspects for analytical and computational models relevant to applications spanning a variety of sectors, from automotive to biotribology and nanotechnology. Significant effort is still required to account for complementary nonlinear effects of plasticity, adhesion, friction, wear, lubrication and surface chemistry in tribological models. For each topic, we propose some research directions

Macroscopic Modeling of Fingerpad Friction Under Electroadhesion: Possibilities and Limitations

Electrovibration is one of the key technologies in surface haptics. By inducing controlled electrostatic forces, the friction within a sliding contact between the human finger and a capacitive screen is modulated, which in turn gives effective tactile feedback to the user. Such powerful haptic displays can be built into mobile phones, tablets, navigation devices, games consoles and many other devices of consumer electronics. However, due to the layered structure and complex material of human skin, the underlying contact mechanical processes have not yet been fully understood. This work provides new continuum-based approaches to macroscopic modeling of the electro-adhesive frictional contact. A solution of pure normal contact between a human finger and a rigid, smooth plane under electroadhesion is derived by applying Shull's compliance method in the extended regime of large deformations. Based on these results and assuming pressure-controlled friction, a model for the sliding electro-adhesive contact is developed, which adequately predicts the friction force and coefficient of friction over the whole range of relevant voltages and applied normal forces. The experimentally observed area reduction caused by the tangential force is incorporated in a more empirical than profound contact mechanical way. This effect is studied with the help of a two-dimensional finite element model of the fingertip, assuming non-linear elastic material for the skin tissue. Although the simulations are restricted to non-adhesive tangential contacts, they show a significant reduction of the contact area, which is caused by large deformations of the non-linear elastic material around the distal phalanx. This result indicates that adhesion is only of secondary importance for the area reduction.TU Berlin, Open-Access-Mittel – 202

Functional surface microstructures inspired by nature – From adhesion and wetting principles to sustainable new devices

In the course of evolution nature has arrived at startling materials solutions to ensure survival. Investigations into biological surfaces, ranging from plants, insects and geckos to aquatic animals, have inspired the design of intricate surface patterns to create useful functionalities. This paper reviews the fundamental interaction mechanisms of such micropatterns with liquids, solids, and soft matter such as skin for control of wetting, self-cleaning, anti-fouling, adhesion, skin adherence, and sensing. Compared to conventional chemical strategies, the paradigm of micropatterning enables solutions with superior resource efficiency and sustainability. Associated applications range from water management and robotics to future health monitoring devices. We finally provide an overview of the relevant patterning methods as an appendix

Functional surface microstructures inspired by nature : From adhesion and wetting principles to sustainable new devices

In the course of evolution nature has arrived at startling materials solutions to ensure survival. Investigations into biological surfaces, ranging from plants, insects and geckos to aquatic animals, have inspired the design of intricate surface patterns to create useful functionalities. This paper reviews the fundamental interaction mechanisms of such micropatterns with liquids, solids, and soft matter such as skin for control of wetting, self-cleaning, anti-fouling, adhesion, skin adherence, and sensing. Compared to conventional chemical strategies, the paradigm of micropatterning enables solutions with superior resource efficiency and sustainability. Associated applications range from water management and robotics to future health monitoring devices. We finally provide an overview of the relevant patterning methods as an appendix