The gliding comfort and performance of personal care and wellness products is strongly influenced by the sliding friction behaviour of human skin. In the open literature, most of the results on skin friction are related to the performance of cosmetic products or to the slip and grip properties of surfaces. Experiments were usually carried out on the forearm or the fingertips. The influence of the surface roughness and the material of engineering surfaces have received little attention so far, especially not in sliding contact with the skin of the cheek, or under different climate conditions. A custom-built rotating ring device was used to study the influence of the probe surface roughness (R a = 0.1-10 lm), the probe material (metals, plastics), the climate conditions (21-29°C, 37-92% RH) and skin hydration on the frictional behaviour of the skin on the cheek and the forearm. The amplitude of the surface roughness has a dominant influence on the friction behaviour: the smoother the surface, the higher the friction. Differences can be as large as a factor 5-10, especially in the range R a \ 1 lm. The probe material itself has no significant influence; except for PFTE which reduces the friction by approximately 25% compared to the other materials. In a humid climate, the skin becomes hydrated and the friction is twice as high as in a dry climate. The effect of skin hydration is smaller on the cheek than on the forearm, probably due to the presence of beard stubbles. A simple friction model for human skin is presented, based on adhesion friction, contact mechanics of rough surfaces and the interfacial shear stress of thin organic films. The model explains the effects of the probe surface roughness and skin compliance. Quantitative application of the model indicates that the biomechanical indentation and shearing behaviour of the stratum corneum is influenced by the same physical process, i.e. the intercellular bonding strength of the corneocytes.
We describe an apparatus designed to quantitatively measure friction dynamics at the mesoscopic scale. This lateral force apparatus, LFA, uses double parallel leaf springs in leaf-spring units as force transducers and two focus error detection optical heads, optical heads, to measure deflections. The design of the leaf-spring units is new. Normal spring constants are in the range of 20–4000 N/m, and lateral spring constants are 7–1000 N/m. The optical heads combine a 10 nm sensitivity with a useful range of about 100 μm. The proven range of normal forces is 400 nN–150 mN. The leaf-spring units transduce friction and normal forces independently. Absolute values of normal and friction forces are calibrated. Typical errors are less than 10%. The calibration is partly in situ, for the sensitivity of the optical heads, and partly ex situ for the normal and lateral spring constants of the leaf-spring units. There is minimal coupling between the deflection measurements in the lateral and normal directions. This coupling is also calibrated in situ. It is typically 1% and can be as low as 0.25%. This means that the displacements of the tip can be measured accurately in the sliding direction and normal to the surface. Together, these characteristics make the LFA, well suited for quantitative study of friction dynamics at mesoscopic scales. Furthermore the design of the leaf-spring unit allows exchange of tips which may be fabricated (e.g., etched) from wire material (d≈0.4 mm) and can have customized shapes, e.g., polished flat squares. The ability of the LFA to study friction dynamics is briefly illustrated by results of stick-slip measurements on soft polymer surfaces.
The sliding friction of hard, micron-sized single asperities sliding on soft polyester films was studied. Transitions from steady sliding to so-called ''stick-slip'' or nonstationary motion occur for decreasing driving speed, decreasing driving spring stiffness, increasing normal load, decreasing tip radius, and decreasing crosslink density. Normal displacements of the tip during sliding were studied in some detail. It is argued these play an important role in the dynamics of the system, being the dominant factor in determining the contact area between asperity and substrate. A rather simple model is proposed that is related to rate-and-state descriptions of stick-slip phenomena. In this particular description the normal displacement plays a part analogous to that of the state parameter. In a limited comparison of experiment and numerical results we find qualitative agreement on all measured trends.
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