In contact observation of model synovial fluid lubricating mechanisms
This paper examines the fundamental mechanisms of synovial fluid lubrication in artificial joints. Film thickness measurements were made for bovine serum solutions in a model test device. In contact imaging was also carried out to aid interpretation of these results. The results indicated that two types of film are formed; a boundary layer of adsorbed protein molecules, which are augmented by a high-viscosity fluid film generated by hydrodynamic effects. The high-viscosity film is due to inlet aggregation of protein molecules forming a gel which is entrained into the contact preferentially at low speeds. As the speed increases this gel appears to shear thin, giving much lower lubricant film thickness. Results suggest that protein-containing fluids do not obey classical Newtonian EHL models.
The effect of transient conditions on synovial fluid protein aggregation lubrication
Little is known about the prevailing lubrication mechanisms in artificial articular joints and the way in which these mechanisms determine implant performance. The authors propose that interfacial film formation is determined by rheological changes local to the contact and is driven by aggregation of synovial fluid proteins within the contact inlet region. A direct relationship between contact film thickness and size of the protein aggregation within the inlet region has been observed. In this paper the latest experimental observations of the protein aggregation mechanism are presented for conditions which more closely mimic joint kinematics and loading. Lubricant films were measured for a series of bovine calf serum solutions for CoCrMo femoral component sliding against a glass disc. An optical interferometric apparatus was employed to study the effects of transient motion on lubricant film formation. Central film thickness was measured as a function of time for a series of transient entrainment conditions; start-up motion, steady-state and non-steady-state uni-directional sliding, and bi-directional sliding. The size of the inlet aggregations was found to be dependent upon the type of transient condition. Thick protective protein films were observed to build up within the main contact region for all uni-directional tests. In contrast the inlet aggregation was not observed for bi-directional tests. Contact film thickness and wear was found to be directly proportional to the presence of the inlet protein phase. The inlet phase and contact films were found to be fragile when disrupted by surface scratches or subjected to reversal of the sliding direction.
Friction and lubricant film thickness measurements on simulated synovial fluids
Abstract, This article is part of a research programme to study fundamental aspects of synovial fluid lubrication of artificial hip joints and focuses on the role of proteins (albumin and γ-globulin) in film formation. Friction and film thickness measurements have been carried out on a series of simple protein solutions (simulating synovial fluid SF) and bovine serum (BS). Protein solutions were prepared with different buffer solutions to simulate healthy and periprosthetic SFs. Simple bench-top tests using a ball-on-disc geometry were used to simulate hip joint articulation. All measurements were made under steady-state load and speed (5—50 mm/s) conditions., The friction results showed that BS and simple protein solutions demonstrate boundary properties by reducing friction in the slow-speed regime. The film thickness results with BS provide evidence of the formation of an adsorbed solid surface layer (<20 nm), which is augmented by a thicker hydrodynamic film. Film thickness was fairly constant over the speed range, but tended to decrease with increasing speed. The choice of buffer solution affected the lubrication properties; much thicker films were recorded for the phosphate-buffered saline compared to 2-amino- 2-(hydroxymethyl)-1,3-propanediol protein solutions. The friction and film thickness results were time and/or rubbing distance dependent, suggesting that the formation of the deposited film was pressure/friction related.
Frictional Properties of PVA Hydrogel
Total hip replacement is one of the most successful surgical treatments of modern medicine. Typically, at present, hard-on-hard bearing surfaces are widely used for components of artificial hip joints. Hard-on-hard means that both components have high modulus of elasticity in range of hundreds of GPa. However, these materials suffer from relatively high friction and wear rate. This is connected especially with occurring lubrication regime. To approach conditions presented in natural joints, it is necessary to think about artificial cartilage. One of the anticipated materials for artificial cartilage is polyvinyl alcohol (PVA) hydrogel. PVA hydrogel has water content about 85 % and its elastic modulus is approximately E ≈ 1.2 MPa, which is similar to natural cartilage. The main disadvantage of PVA hydrogel is its lower strength. In this study, commercial mini traction machine (MTM) was used to determine friction coefficient for various slide-to-roll ratios (SRR). Bovine serum was used as a lubricant and the tests were carried out under ambient temperature for three various speeds u1 = 25 mm/s; u2 = 50 mm/s; u3 = 100 mm/s and two different loads F1 = 5.2 N; F2 = 9.8 N, respectively. As expected, friction coefficient was very low, less than 0.05 under some conditions. In future, optical method based on the principle of fluorescent microscopy will be used for studying lubricant film thickness and protein adsorption on bearing surfaces.
Influence of load and elastic properties on the rolling and sliding friction of lubricated compliant contacts
Lubricated “soft” contacts, where one or both contacting solids have a low elastic modulus, are present in many practical engineering and biological applications including windscreen wipers, wet tyres, elastomeric seals, contact lenses and the tongue/palate system. In such contacts, the prevailing lubrication mode is “isoviscous EHL” (elastohydrodynamic lubrication). Unlike in steel–steel contacts, rolling friction can be considerable and this originates in part from the viscoelastic properties of the compliant surfaces. In this paper the influence on friction of both applied load and the elastic properties of the solids is studied using a mini traction machine. In this machine, the rolling and sliding friction can be separately determined. The viscoelastic properties of the polymers employed are measured using a dynamic mechanical analysis apparatus. The measured friction is compared to theoretical models for soft EHL and the viscoelastic energy losses arising from the contact deformation. Consideration of the frequency dependence of the substrate viscoelasticity enables reasonably accurate predictions of the rolling friction coefficient, especially within the mixed and boundary lubrication regimes.
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.
Influence of load and elastic properties on the rolling and sliding friction of lubricated compliant contacts
Lubricated “soft” contacts, where one or both contacting solids have a low elastic modulus, are present in many practical engineering and biological applications including windscreen wipers, wet tyres, elastomeric seals, contact lenses and the tongue/palate system. In such contacts, the prevailing lubrication mode is “isoviscous EHL” (elastohydrodynamic lubrication). Unlike in steel–steel contacts, rolling friction can be considerable and this originates in part from the viscoelastic properties of the compliant surfaces.
In this paper the influence on friction of both applied load and the elastic properties of the solids is studied using a mini traction machine. In this machine, the rolling and sliding friction can be separately determined. The viscoelastic properties of the polymers employed are measured using a dynamic mechanical analysis apparatus. The measured friction is compared to theoretical models for soft EHL and the viscoelastic energy losses arising from the contact deformation. Consideration of the frequency dependence of the substrate viscoelasticity enables reasonably accurate predictions of the rolling friction coefficient, especially within the mixed and boundary lubrication regimes.
Influence of Temperature on the Frictional Properties of Water-Lubricated Surfaces
The influence of temperature on the lubricating properties of neat water for tribopairs with varying bulk elasticity moduli and surface hydrophilicity, namely hard-hydrophobic interface (h–HB), hard-hydrophilic interface (h–HL), soft-hydrophobic interface (s–HB), and soft-hydrophilic interface (s–HL), has been investigated. With increasing temperature, the coefficients of friction generally increased due to the decreasing viscosity of water. This change was more clearly manifested from soft interfaces for more feasible formation of lubricating films. Nevertheless, dominant lubrication mechanism appears to be boundary and mixed lubrication even for soft interfaces at all speeds (up to 1200 mm/s) and temperatures (1 to 90 °C) investigated. The results from this study are expected to provide a reference to explore the temperature-dependent tribological behavior of more complex aqueous lubricants, e.g., those involving various additives, for a variety of tribosystems.
End-grafted Sugar Chains as Aqueous Lubricant Additives: Synthesis and Macrotribological Tests of Poly(l-lysine)-graft-Dextran (PLL-g-dex) Copolymers
Comb-like graft copolymers with carbohydrate side chains have been developed as aqueous lubricant additives for oxide-based tribosystems, in an attempt to mimic biological lubrication systems, whose surfaces are known to be covered with sugar-rich layers. As adopted in the previous studies of the graft copolymer poly(l-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG), which showed both excellent lubricating and antifouling properties, a similar approach was chosen to graft dextran chains onto the same backbone, thus generating PLL-g-dex. PLL-g-dex copolymers readily adsorb from aqueous solution onto negatively charged oxide surfaces. Tribological characterization at the macroscopic scale, either under pure sliding conditions or a mixed sliding/rolling contact regime, shows that PLL-g-dex is very effective for the lubrication of oxide-based tribosystems. The relative lubricating capabilities of PLL-g-dex copolymers compared with PLL-g-PEG copolymers were observed to be highly dependent on the molecular structure of the copolymers (in particular, side-chain density along the backbone) and the measurement conditions (in particular, time between tribocontacts); the PLL-g-dex copolymers with a low degree of grafted side chains (≤20% grafting of available protonated primary amine groups along the backbone) showed better lubricating performance than their PLL-g-PEG counterparts at high tribocontact frequency (≥ca. 0.32 H
Methodological study on the removal of solid oil and fat stains from cotton fabrics using abrasion
Solid fats are one of the most difficult stains to remove at low temperatures. Mechanical action is beneficial for stain removal, but the potential and limitations of such an essential part of washing are not known. Fabric abrasion has been studied for the first time in a systematic and controlled manner using a tribometer. The efficiency of cotton–cotton abrasion, to simulate the rubbing of clothes, was studied in the absence of detergents using models of liquid and solid oils (hexadecane, octadecane, and undecanoic acid) and real fats (lard and buttermilk fat). In model oils, abrasion is not very effective at any temperature, whereas in typical fats abrasion significantly improves cleaning in a wide range of temperatures. The different behavior is caused by the temperature-dependent solid fat content of lard and butter. Fluorescence microscopy is introduced as a novel methodology for the quantification of the fat content in soiled fabrics