Simon A. Johnson

Finger pad friction and its role in grip and touch

Many aspects of both grip function and tactile perception depend on complex frictional interactions occurring in the contact zone of the finger pad, which is the subject of the current review. While it is well established that friction plays a crucial role in grip function, its exact contribution for discriminatory touch involving the sliding of a finger pad is more elusive. For texture discrimination, it is clear that vibrotaction plays an important role in the discriminatory mechanisms. Among other factors, friction impacts the nature of the vibrations generated by the relative movement of the fingertip skin against a probed object. Friction also has a major influence on the perceived tactile pleasantness of a surface. The contact mechanics of a finger pad is governed by the fingerprint ridges and the sweat that is exuded from pores located on these ridges. Counterintuitively, the coefficient of friction can increase by an order of magnitude in a period of tens of seconds when in contact with an impermeably smooth surface, such as glass. In contrast, the value will decrease for a porous surface, such as paper. The increase in friction is attributed to an occlusion mechanism and can be described by first-order kinetics. Surprisingly, the sensitivity of the coefficient of friction to the normal load and sliding velocity is comparatively of second order, yet these dependencies provide the main basis of theoretical models which, to-date, largely ignore the time evolution of the frictional dynamics. One well-known effect on taction is the possibility of inducing stick–slip if the friction decreases with increasing sliding velocity. Moreover, the initial slip of a finger pad occurs by the propagation of an annulus of failure from the perimeter of the contact zone and this phenomenon could be important in tactile perception and grip function.

Comparison of predicted and actual consequences of missense mutations

Significance Computational tools applied to any human genome sequence identify hundreds of genetic variants predicted to disrupt the function of individual proteins as the result of a single codon change. These tools have been trained on disease mutations and common polymorphisms but have yet to be tested against an unbiased spectrum of random mutations arising de novo. Here we perform such a test comparing the predicted and actual effects of de novo mutations in 23 genes with essential functions for normal immunity and all possible mutations in the TP53 tumor suppressor gene. These results highlight an important gap in our ability to relate genotype to phenotype in clinical genome sequencing: the inability to differentiate immediately clinically relevant mutations from nearly neutral mutations. Each person’s genome sequence has thousands of missense variants. Practical interpretation of their functional significance must rely on computational inferences in the absence of exhaustive experimental measurements. Here we analyzed the efficacy of these inferences in 33 de novo missense mutations revealed by sequencing in first-generation progeny of N-ethyl-N-nitrosourea–treated mice, involving 23 essential immune system genes. PolyPhen2, SIFT, MutationAssessor, Panther, CADD, and Condel were used to predict each mutation’s functional importance, whereas the actual effect was measured by breeding and testing homozygotes for the expected in vivo loss-of-function phenotype. Only 20% of mutations predicted to be deleterious by PolyPhen2 (and 15% by CADD) showed a discernible phenotype in individual homozygotes. Half of all possible missense mutations in the same 23 immune genes were predicted to be deleterious, and most of these appear to become subject to purifying selection because few persist between separate mouse substrains, rodents, or primates. Because defects in immune genes could be phenotypically masked in vivo by compensation and environment, we compared inferences by the same tools with the in vitro phenotype of all 2,314 possible missense variants in TP53; 42% of mutations predicted by PolyPhen2 to be deleterious (and 45% by CADD) had little measurable consequence for TP53-promoted transcription. We conclude that for de novo or low-frequency missense mutations found by genome sequencing, half those inferred as deleterious correspond to nearly neutral mutations that have little impact on the clinical phenotype of individual cases but will nevertheless become subject to purifying selection.

An experimental study of the nano-scratch behaviour of poly(methyl methacrylate)

An experimental study of poly(methyl methacrylate) (PMMA) is described with the aim of investigating the potential for the formation of wear particles by the interaction of multiple scratches under plastic ploughing conditions. The work was carried out using a nano-indenter fitted with a Berkovich probe. It was found that orthogonally intersecting scratches produced lumpy protuberances which could act as a precursor for wear particles. However, parallel scratches in close proximity resulted in a self-protective mechanism due to the load carrying capacity of the strain hardened pile-up from neighbouring scratches.

Effects of wetting hysteresis on pendular liquid bridges between rigid spheres

Abstract The effects of wetting hysteresis on the properties and behaviour of pendular liquid bridges between spherical particles have been investigated both experimentally and theoretically. A thermodynamic analysis shows the existence of metastable states that would account for the observed pinning of the three-phase contact line. The most important findings, with respect to the role of capillary interactions in agglomeration processes, are that (a) some of the kinetic energy of “wet” agglomerates may be dissipated during collisions by such hysteresis, (b) the bridge rupture distance is increased and (c) pinning will result in a maximum in the force–separation characteristics.

Role of Occlusion in Non-Coulombic Slip of the Finger Pad

Understanding how fingers slip on surfaces is essential for elucidating the mechanisms of haptic perception. This paper describes an investigation of the relationship between occlusion and the non-Coulombic slip of the finger pad, which results in the frictional force being a power law function of the normal load, with an index \( n \); Coulombic slip corresponds to \( n = 1 \). For smooth impermeable surfaces, occlusion of moisture excreted by the sweat glands may cause up to an order of magnitude increase in the coefficient of friction with a characteristic time of ~20 s. This arises because the moisture plasticises the asperities on the finger print ridges resulting in an increase in their compliance and hence an increase in the contact area. Under such steady state sliding conditions a finger pad behaves like a Hertzian contact decorated with the valleys between the finger print ridges, which only act to reduce the true but not the nominal contact area. In the limit, at long occlusion times (~50 s), it can be shown that the power law index tends to a value in the range \( {2 \mathord{\left/ {\vphantom {2 {3 \le n \le 1}}} \right. \kern-0pt} {3 \le n \le 1}} \). In contrast, measurements against a rough surface demonstrate that the friction is not affected by occlusion and that a finger pad exhibits Coulombic slip.

Contact mechanics of the human finger pad under compressive loads

The coefficient of friction of most solid objects is independent of the applied normal force because of surface roughness. This behaviour is observed for a finger pad except at long contact times (greater than 10 s) against smooth impermeable surfaces such as glass when the coefficient increases with decreasing normal force by about a factor of five for the load range investigated here. This is clearly an advantage for some precision manipulation and grip tasks. Such normal force dependence is characteristic of smooth curved elastic bodies. It has been argued that the occlusion of moisture in the form of sweat plasticises the surface topographical features and their increased compliance allows flattening under an applied normal force, so that the surfaces of the fingerprint ridges are effectively smooth. While the normal force dependence of the friction is consistent with the theory of elastic frictional contacts, the gross deformation behaviour is not and, for commonly reported values of the Youngs modulus of stratum corneum, the deformation of the ridges should be negligible compared with the gross deformation of the finger pad even when fully occluded. This paper describes the development of a contact mechanics model that resolves these inconsistencies and is validated against experimental data.

Chapter 28 Pendular capillary bridges

Publisher Summary It is well established that liquid junctions play a major role in granulation processes. Essentially they are responsible for the forces of attraction that bind together the primary particles in an agglomerate. Under static conditions only capillary forces act among the particles and their strength depends on such factors as the geometry, surface free energy and surface topography of the particles, the separation distance among the particles and also the size of the liquid bridge together with the surface tension of the liquid. In this chapter, the special case of pendular liquid bridges between nominally smooth and rigid spherical particles is considered. A particular focus of the current chapter is to describe closed-form approximations for calculating the forces and stability associated with capillary bridges. This avoids the difficulties of exact calculations that require numerical integration. A similar approach is described in order to assess the influence of wetting hysteresis and gravity. For bridges that are greater than some critical size, gravity causes a distortion of the bridge geometry and hence influences both the capillary forces and the bridge stability. For liquids that do not perfectly wet the particles, wetting hysteresis commonly occurs and some of the fundamental aspects of this problem are described. Discrete simulation schemes—for example, granular dynamics studies of wet agglomerate impact, are an important application of the closed-form approximations. They are used as computationally efficient algorithms for the interaction laws.

Frictional dynamics of finger pads are governed by four length-scales and two time-scales

The evolution of the contact area of a finger pad against a surface is critical during tactile interaction, whether for gripping or discriminating surfaces. The contact area made by a finger pad is commonly considered at two distinct length scales corresponding to the gross area, Agross, and to the smaller ridge area, Aridge, that excludes the interstitial spaces between the ridges. Here, these quantities were obtained from high-resolution imaging of contacts during loading and stress relaxation. While AgroSS rapidly reaches an ultimate value, the contact made by the ridges is initially formed from unconnected junctions with a total contact area, Ajunct, which continues to increase for several seconds during the holding period. Thus, the contact area grows in a two-step process where the number of junctions made by the ridges first increases, followed by a growth of their size and connectivity. Immediately after contact the stratum corneum is in a glassy state and the individual junctions form a multiple asperity contact. At longer contact times, the asperities soften owing to the occlusion of moisture excreted from the sweat pores in the ridges. Thus, the real area of contact, Areal, which drives the creation of friction, grows with time at a relatively slow rate. It is concluded that multi-asperity dynamic contact models should be preferred compared with static models in order to describe the physics of finger pad contact mechanics and friction.

Why 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. The process by which human fingers gives rise to stable contacts with smooth, hard objects is surprisingly slow. Using high-resolution imaging, we found that, when pressed against glass, the actual contact made by finger pad ridges evolved over time following a first-order kinetics relationship. This evolution was the result of a two-stage coalescence process of microscopic junctions made between the keratin of the stratum corneum of the skin and the glass surface. This process was driven by the secretion of moisture from the sweat glands, since increased hydration in stratum corneum causes it to become softer. Saturation was typically reached within 20 s of loading the contact, regardless of the initial moisture state of the finger and of the normal force applied. Hence, the gross contact area, frequently used as a benchmark quantity in grip and perceptual studies, is a poor reflection of the actual contact mechanics that take place between human fingers and smooth, impermeable surfaces. In contrast, the formation of a steady-state contact area is almost instantaneous if the counter surface is soft relative to keratin in a dry state. It is for this reason that elastomers are commonly used to coat grip surfaces.

Nanoindentation of Poly (Methyl Methacrylate)

For the indentation of an elastic-plastic homogeneous half-space with a pyramidal indenter, the load theoretically increases with the square of the total penetration depth. Experimental data are presented in the current paper that demonstrate the validity of this relationship for an organic polymer and a Berkovich indenter, provided that appropriate account is taken of the tip defect and viscoplasticity. It is also shown that a simple analytical solution exists for the ratio of the contact depth to the total penetration depth. These findings assist in identifying procedures for obtaining the rate-dependent mechanical properties of thin polymer coatings or polymeric materials with depth-dependent mechanical properties.