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    Lubrication, Adsorption, and Rheology of Aqueous Polysaccharide Solutions

    Year: 2011

    Journal: Langmuir, 2011, 27 (7), pp 3474–3484, 20110525

    Authors: Stokes J.R. *§ , Macakova L. §, Chojnicka-Paszun A. †‡, de Kruif C.G. ‡, de Jongh H.H.J.†

    Last authors: Harmen H. J. de Jongh

    Organizations: † TI Food & Nutrition, P.O. Box 557, 6700 AN Wageningen, The Netherlands ‡ Van't Hoff Laboratory for Physical and Colloid Chemistry, Debye Institute, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands § Unilever Corporate Research, Unilever R&D Colworth, Colworth House, Sharnbrook MK44 ILQ, United Kingdom School of Chemical Engineering, University of Queensland, Brisbane, Queensland 4072, Australia Texture Department, NIZO Food Research, Kernhemseweg 2, P.O. Box 20, 6710 BA Ede, The Netherlands

    Country: Australia, The Netherlands, Holland, England, UK, United Kingdom, GB, Great Britain, Britain, U.K.

    Aqueous lubrication is currently at the forefront of tribological research due to the desire to learn and potentially mimic how nature lubricates biotribological contacts. We focus here on understanding the lubrication properties of naturally occurring polysaccharides in aqueous solution using a combination of tribology, adsorption, and rheology. The polysaccharides include pectin, xanthan gum, gellan, and locus bean gum that are all widely used in food and nonfood applications. They form rheologically complex fluids in aqueous solution that are both shear thinning and elastic, and their normal stress differences at high shear rates are found to be characteristic of semiflexible/rigid molecules. Lubrication is studied using a ball-on-disk tribometer with hydrophobic elastomer surfaces, mimicking biotribological contacts, and the friction coefficient is measured as a function of speed across the boundary, mixed, and hydrodynamic lubrication regimes. The hydrodynamic regime, where the friction coefficient increases with increasing lubricant entrainment speed, is found to depend on the viscosity of the polysaccharide solutions at shear rates of around 104 s−1. The boundary regime, which occurs at the lowest entrainment speeds, depends on the adsorption of polymer to the substrate. In this regime, the friction coefficient for a rough substrate (400 nm rms roughness) is dependent on the dry mass of polymer adsorbed to the surface (obtained from surface plasmon resonance), while for a smooth substrate (10 nm rms roughness) the friction coefficient is strongly dependent on the hydrated wet mass of adsorbed polymer (obtained from quartz crystal microbalance, QCM-D). The mixed regime is dependent on both the adsorbed film properties and lubricant's viscosity at high shear rates. In addition, the entrainment speed where the friction coefficient is a minimum, which corresponds to the transition between the hydrodynamic and mixed regime, correlates linearly with the ratio of the wet mass and viscosity at 104 s−1 for the smooth surface. These findings are independent of the different polysaccharides used in the study and their different viscoelastic flow properties.