Viscous Boundary Lubrication of Hydrophobic Surfaces by Mucin Gleb E. Yakubov,* ,† James McColl, ‡ Jeroen H. H. Bongaerts, †,§ and Jeremy J. Ramsden ‡ UnileVer Corporate Research, Colworth Science Park, Bedfordshire MK44 1LQ, U.K., Department of Materials, Cranfield UniVersity, Bedfordshire MK43 0AL, U.K., and SKF Engineering and Research Centre, KelVinbaan 16, 3439 MT Nieuwegein, The Netherlands ReceiVed June 13, 2008. ReVised Manuscript ReceiVed NoVember 28, 2008 The lubricating behavior of the weakly charged short-side-chain glycoprotein mucin “Orthana” (M w ) 0.55 MDa) has been investigated between hydrophobic and hydrophilic PDMS substrates using soft-contact tribometry. It was found that mucin facilitates lubrication between hydrophobic PDMS surfaces, leading to a 10-fold reduction in boundary friction coefficient for rough surfaces. The presence of mucin also results in a shift of the mixed lubrication regime to lower entrainment speeds. The observed boundary lubrication behavior of mucin was found to depend on the bulk concentration, and we linked this to the structure and dynamics of the adsorbed mucin films, which are assessed using optical waveguide light spectroscopy. We observe a composite structure of the adsorbed mucin layer, with its internal structure governed by entanglement. The film thickness of this adsorbed layer increases with concentration, while the boundary friction coefficient for rough surfaces was found to be inversely proportional to the thickness of the adsorbed film. This link between lubrication and structure of the film is consistent with a viscous boundary lubrication mechanism, i.e., a thicker adsorbed film, at a given sliding speed, results in a lower local shear rate and, hence, in a lower local shear stress. The estimated local viscosities of the adsorbed layer, derived from the friction measurements and the polymer layer density, are in agreement with each other. 1. Introduction Movement is a survival prerequisite for almost all living organisms. Efficient lubrication of moving internal and external surfaces is vital to minimize wear and loss of energy during motion. Examples are numerous and diverse: mammals lubricate their joints using synovial fluid and coat their respiratory airways, gastro-intestinal, ocular, and many other surfaces with a mucosal layer, while cephalopods utilize a mucus-coated foot on which they move. 1 Biolubrication is also crucial for processing of foods in the mouth, which has a large bearing on oral health, mastication, swallowing, and mouthfeel. 2,3 For example, an astringent mouthfeel is believed to be caused by the interaction of polyphenols with the salivary film on oral surfaces, resulting in a aggregation of salivary proteins at those surfaces, which in turn reduces oral lubrication. 2-5 Adsorption of highly hydrated hydrophilic and amphiphilic polymers onto surfaces reduces friction, most notably between hydrophobic surfaces. 6-9 A natural example of such an am- phiphilic polymer is lubricin, 10 a mucin-like glycoprotein with a high content of sulfonated sugars and sialic acid residues. Another much-studied example is hyaluronic acid (HA), 11-15 one of the main components of synovial fluids. In both cases the lubricating properties are thought to originate from dense brush- type coatings and/or high charge densities. Attempts have been made to understand the mechanism of biolubrication at the molecular scale. Adsorbed biopolymers and (glyco-)proteins form a barrier against bare surface-surface interaction through steric, brush-brush, and/or electrostatic repulsion. The ubiquity of charged biopolymers at biological interfaces might imply that polyelectrolye polymers facilitate lubrication across the majority of biological lubricating systems. 16,12,11 However, the creation of such a barrier is not sufficient to ensure efficient boundary lubrication as it does not necessarily provide an efficient sliding mechanism. 17 It has been proposed that hydrated salt ionsstypically accumulated as the counterions to the polyelectrolyte coatingssact as molecular ball bearings if they are in some way ‘trapped’ close to the surface and cannot be squeezed out. 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