Role of Fingerprint Mechanics and non-Coulombic Friction in Ultrasonic Devices

International audienceUltrasonic vibration of a plate can be used to modulate the friction of a finger pad sliding on a surface. This modulation can modify the user perception of the touched object and induce the perception of textured materials. In the current paper, an elastic model of finger print ridges is developed. A friction reduction phenomenon based on non-Coulombic friction is evaluated based on this model. Then, a comparison with experimental data is carried out to assess the validity of the proposed model and analysis

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

Friction Reduction through Ultrasonic Vibration Part 1: Modelling Intermittent Contact

International audienceUltrasonic vibration is employed to modify the friction of a finger pad in way that induces haptic sensations. A combination of intermittent contact and squeeze film levitation has been previously proposed as the most probable mechanism. In this paper, in order to understand the underlying principles that govern friction modulation by intermittent contact, numerical models based on finite element (FE) analysis and also a spring-Coulombic slider are developed. The physical input parameters for the FE model are optimised by measuring the contact phase shift between a finger pad and a vibrating plate. The spring-slider model assists in the interpretation of the FE model and leads to the identification of a dimensionless group that allows the calculated coefficient of friction to be approximately superimposed onto an exponential function of the dimensionless group. Thus, it is possible to rationalise the computed relative reduction in friction being (i) dependent on the vibrational amplitude, frequency, and the intrinsic coefficient of friction of the device, and the reciprocal of the exploration velocity, and (ii) independent of the applied normal force, and the shear and extensional elastic moduli of the finger skin provided that intermittent contact is sufficiently well developed. Experimental validation of the modelling using real and artificial fingertips will be reported in part 2 of this work, which supports the current modelling

Texture Rendering Strategies with a High Fidelity - Capacitive Visual-Haptic Friction Control Device

International audienceUltrasonic vibrations of a plate can modify the perception of the friction between a surface and a sliding finger. This principle, coupled with modern position sensing techniques, is able to reproduce textured materials. In this paper , an open loop control through model inversion of the friction force between the finger and the plate is presented. The device incorporating the control system is described, and two different reproduction strategies are formalized to address the reproduction of objects and textures. In the end, a psychophysical experiment evaluating the two control strategies is described

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

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

Fingerprint ridges allow primates to regulate grip

Fingerprints are unique to primates and koalas but what advantages do these features of our hands and feet provide us compared with the smooth pads of carnivorans, e.g., feline or ursine species? It has been argued that the epidermal ridges on finger pads decrease friction when in contact with smooth surfaces, promote interlocking with rough surfaces, channel excess water, prevent blistering, and enhance tactile sensitivity. Here, we found that they were at the origin of a moisture-regulating mechanism, which ensures an optimal hydration of the keratin layer of the skin for maximizing the friction and reducing the probability of catastrophic slip due to the hydrodynamic formation of a fluid layer. When in contact with impermeable surfaces, the occlusion of the sweat from the pores in the ridges promotes plasticization of the skin, dramatically increasing friction. Occlusion and external moisture could cause an excess of water that would defeat the natural hydration balance. However, we have demonstrated using femtosecond laser-based polarization-tunable terahertz wave spectroscopic imaging and infrared optical coherence tomography that the moisture regulation may be explained by a combination of a microfluidic capillary evaporation mechanism and a sweat pore blocking mechanism. This results in maintaining an optimal amount of moisture in the furrows that maximizes the friction irrespective of whether a finger pad is initially wet or dry. Thus, abundant low-flow sweat glands and epidermal furrows have provided primates with the evolutionary advantage in dry and wet conditions of manipulative and locomotive abilities not available to other animals.ope

Sensory mechanisms involved in obtaining frictional information for perception and grip force adjustment during object manipulation

Sensory signals informing about frictional properties of a surface are used both for perception to experience material properties and for motor control to be able to handle objects using adequate manipulative forces. There are fundamental differences between these two purposes and scenarios, how sensory information typically is obtained. This thesis aims to explore the mechanisms involved in the perception of frictional properties of the touched surfaces under conditions relevant for object manipulation. Firstly, I show that in the passive touch condition, when the surface is brought in contact with immobilised finger, humans are unable to use existing friction-related mechanical cues and perceptually associate them with frictional properties. However, a submillimeter range lateral movement significantly improved the subject's ability to evaluate the frictional properties of two otherwise identical surfaces. It is demonstrated that partial slips within the contact area and fingertip tissue deformation create very potent sensory stimuli, enabling tactile afferents to signal friction-dependent mechanical effects translating into slipperiness (friction) perception. Further, I demonstrate that natural movement kinematics facilitate the development of such small skin displacements within the contact area and may play a central role in enabling the perception of surface slipperiness and adjusting grip force to friction when manipulating objects. This demonstrates intimate interdependence between the motor and sensory systems. This work significantly extends our understanding of fundamental tactile sensory processes involved in friction signaling in the context of motor control and dexterous object manipulation tasks. This knowledge and discovered friction sensing principles may assist in designing haptic rendering devices and artificial tactile sensors as well as associated control algorithms to be used in robotic grippers and hand prostheses

Investigating the formation of functional and smart materials by nanospinning and other spinning techniques

Functional, smart fibres and fibres with different morphologies have been produced from different materials using different spinning methods. The effect of processing parameters on different nano fibre morphologies was studied by SEM. The spinning solution properties such as viscosity, surface tension, conductivity, UV-visible spectra were studied. The fibres were characterised by DSC, FTIR, XRD, strength test. Antibacterial, hygroscopic, humectant Manuka honey (MH) functional nanofibres have been produced successfully by single needle electrospinning (SNE) using polyethylene oxide (PEO) as matrix. Electrospinning parameters such as higher feed rate, higher proportion of MH, lower applied voltage, lower needle to collector distance produced merged, thicker, flat 15% (wt/wt) MHPEO nanofibres and vice versa. 15%MHPEO fibres of diameters from 0.198μm to 0.924μm were produced using different parameters. The 50% and 65% (wt) MHPEO mats showed antibacterial property. DSC result showed reduction in melting temperature as the MH proportion increased. FTIR results showed respective peaks for MH and PEO. MHPEO nanofibres can be used for medical end use such as wound healing. Ethyl cellulose (EC) nanofibres have been successfully electrospun using different combination of toluene and ethanol (0:100, 40:60, 50:50, 60:40,100:0) as solvent by SNE. Round and elongated bead on string to smooth bead-less 15% (wt/wt)EC fibres produced as proportion of toluene increased in the solvent mixture. Thin, bead-less fibres were obtained by 60:40 (toluene: ethanol) with average fibre diameters ranging from 0.483μm to 0.631μm. EC nanofibres have been also produced by high output bubble electrospinning (BE) method. EC fibres of diameters from 0.188μm to 0.41μm were produced by BE. Comparison between effect of electorspinning parameters on fibre revealed that the fibre morphologies followed different trends in SNE and BE. The beaded structure can be used for loading drugs in advanced medical textiles and smooth bead-less fibrous mat can be used for application such as filtration. In order to develop thermochromic (smart) nanofibres by meltelectrospinning, thermochromic polypropylene fibres have been developed by meltspinning. The pure polypropylene and thermochromic. DSC and FTIR results showed separate peaks for the thermochromic effect and for the polypropylene. SEM images verified the presence of thermochromic pigments. Thermochromic filaments can be used in garment fashion, or as sensors in yarn or fabric form