Tribometrology of Skin

The quantitative assessment of both skin health and skin care products is suggested based on skin tribological properties. Simultaneous multi-sensor measurements of both coefficient of friction and contact electrical impedance allow for fast and quantitative evaluation of skin conditions such as dryness and moisturization, and early diagnosis of skin diseases or of the deterioration in skin functions at a stage that may not be easily discernable visibly. It may be instrumental in developing and testing skin cosmetics and medicine.     

Friction and Adhesion Hysteresis between Surfactant Monolayers in Water

We briefly review the model that correlates friction between two surfaces in adhesive contact with the loading-unloading adhesion hysteresis between them. We then examine in light of this model the observed low friction between two mica surfaces coated with a double-chained quaternary ammonium surfactant in intimate adhesive contact in water. This enables us to propose a mechanism for surfactant boundary lubrication in water that is rather different from the classic boundary lubrication in air: in this mechanism, adhesion takes place at the interface between the opposing surfactant hydrocarbon tails, whereas frictional sliding takes place at the interface between the hydrated surfactant headgroups and mica. The implications of our findings to biolubrication processes are discussed.     

The Lubricating Properties of Human Whole Saliva

We demonstrate the efficient boundary lubricating properties of human whole saliva (HWS) in a soft hydrophobic rubbing contact, consisting of a poly(dimethylsiloxane) (PDMS) ball and a PDMS disk. The influence of applied load, entrainment speed and surface roughness was investigated for mechanically stimulated HWS. Lubrication by HWS results in a boundary friction coefficient of μ ≈ 0.02, two orders of magnitude lower than that obtained for water. Dried saliva on the other hand results in μ ≈ 2–3, illustrating the importance of hydration for efficient salivary lubrication. Increasing the surface roughness increases the friction coefficient for HWS, while it decreases that for water. The boundary lubricating properties of HWS are less sensitive to saliva treatment than are its bulk viscoelastic properties. Centrifugation and ageing of HWS almost completely removes the shear thinning and elastic nature observed for fresh HWS. In contrast, the boundary friction coefficients are hardly affected, which indicates that the high-M _w (supra-)molecular structures in saliva, which are expected to be responsible for its rheology, are not responsible for its boundary lubricating properties. The saliva-coated PDMS surfaces form an ideal model system for ex-vivo investigations into oral lubrication and how the lubricating properties of saliva are influenced by other components like food, beverages, oral care products and pharmaceuticals.     

Structural characterization and tribological evaluation of quince seed mucilage

The mucilage, originating from the seeds of quince fruit was characterized as a potential bio-inspired water-based lubricant. The mucilage consists mainly of fine cellulose nanofibrils and charged hemicelluloses whose structure and properties were characterized here by atomic force microscopy (AFM) and tribological Pin-On-Disc (POD) experiments. The hemicellulose-decorated nanocellulose fibrils were 3.0±0.7 nm in thickness, had a very large aspect ratio and also had a tendency to self-align when dried on mica surface. Macroscale tribological tests showed that the mucilage was able to reduce the coefficient of friction of polyethylene/stainless steel contact to values below 0.03. Thus, we show that quince mucilage is a native nanocellulose material with a notable ability to lower friction.     

Soft-tribology: Lubrication in a compliant PDMS–PDMS contact

We investigate the influence of surface roughness and hydrophobicity on the lubrication of a soft contact, consisting of a poly(dimethylsiloxane) (PDMS) sphere and a flat PDMS disk. The full Stribeck curves, showing boundary, mixed and elasto-hydrodynamic (EHL) lubrication, are presented for varying surface roughness and hydrophobicity. It is found that neither surface roughness nor hydrophobicity influence the friction coefficient (μ) within the EHL regime. However, increasing surface roughness decreases μ in the boundary regime, while extending the limits of the boundary and mixed lubrication regimes to larger values of the product of velocity and lubricant viscosity (Uη). The transition from the mixed lubrication to EHL regime is found to take place at lower values of the film thickness parameter Λ for increasingly rough surfaces. We found Λ=0.7 in the case of a root mean square (r.m.s.) surface roughness of 3.6 μm, suggesting that the effective surface roughness in a compliant compressed tribological contact is lower than that at ambient pressures. Rendering the PDMS surface hydrophilic promotes full-film lubrication and dramatically lowers μ in the boundary regime by more than an order of magnitude. This influence of surface wetting is also displayed when examining a range of lubricants using hydrophobic tribopairs, where the boundary μ decreases with decreasing lubricant–substrate contact angle. Implications of these measurements are discussed in terms of the creation of model surfaces for biotribological applications.     

A Method of Calculation for Contact Pressure between Femoral Head and Cup of Artificial Hip Joint

Abstract

A method of calculation for contact pressure between a hard femoral head and soft plastic cup of a hip joint was studied. Providing that the contact pressure is proportional to the radial deformation of the cup, an equation of equilibrium in terms of a nondimensional parameter was derived. Once the magnitude of the parameter is determined by solving the equation, the contact pressure distribution is easily obtained. An effect of the radial clearance between a femoral head and cup on contact pressure was evaluated using the equation. Furthermore, the effect of Young's modulus and thickness of a plastic cup on contact pressure were also evaluated. According to the results, contact pressure increased with an increment of clearance and Young's modulus. The contact pressure increased with a decrement of thickness of the plastic cup. The analytical solution was compared with the finite element method (FEM) analysis and the agreement was confirmed. An equation of frictional torque was also derived     

Effect of operational conditions and environment on lubricity of hydrophobins in water based lubrication systems

Abstract

In this study, the effect of operational conditions (normal load, sliding velocity) and environment (pH and ionic strength) on the lubrication properties of two different hydrophobin proteins were investigated using pin on disc tribometry and ellipsometry. The studied proteins were wild type HFBI and the glycosylated hydrophobin FpHYD5. It was observed that the friction of a stainless steel versus stainless steel contact lubricated with either of the hydrophobins did not depend on the normal load. However, increased sliding velocity occasionally led to a decrease in friction when the surfaces were lubricated with the glycosylated FpHYD5. The tribological behaviour of FpHYD5 was studied at pH values ranging from 3 to 9 and generally lowered friction by 31–38% and wear by 40–65% compared to the corresponding buffer solutions. An exception was pH 9, where FpHYD5 increased friction and wear compared to the buffer solution. Ionic strength affected both the amount of protein that was adsorbed and the lubrication properties of glycosylated hydrophobins.     

Friction and Adhesion Hysteresis between Surfactant Monolayers in Water

We briefly review the model that correlates friction between two surfaces in adhesive contact with the loading–unloading adhesion hysteresis between them. We then examine in light of this model the observed low friction between two mica surfaces coated with a double-chained quaternary ammonium surfactant in intimate adhesive contact in water. This enables us to propose a mechanism for surfactant boundary lubrication in water that is rather different from the classic boundary lubrication in air: in this mechanism, adhesion takes place at the interface between the opposing surfactant hydrocarbon tails, whereas frictional sliding takes place at the interface between the hydrated surfactant headgroups and mica. The implications of our findings to biolubrication processes are discussed.