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RESEARCH PAPERS

Viscosity Ratio Effect in the Emulsion Lubrication of Soft EHL Contact

[+] Author and Article Information
J. de Vicente1

Food Research Centre, Unilever R and D Colworth, Colworth House Sharnbrook, MK44 1LQ, UK; Tribology Section, Department of Mechanical Engineering, Imperial College London, London, SW7 2BX, UKjvicente@ugr.es

H. A. Spikes2

Tribology Section, Department of Mechanical Engineering, Imperial College London, London, SW7 2BX, UKh.spikes@imperial.ac.uk

J. R. Stokes

Unilever Corporate Research, Unilever R and D Colworth, Colworth House Sharnbrook, MK44 1LQ, UKJason.Stokes@unilever.com

1

Present address: Department of Applied Physics, Faculty of Sciences, University of Granada, C/ Fuentenueva s/n, 18071 – Granada, Spain.

2

Corresponding author.

J. Tribol 128(4), 795-800 (Jun 07, 2006) (6 pages) doi:10.1115/1.2345400 History: Received June 20, 2005; Revised June 07, 2006

Many foodstuffs and personal care products consist of two-phase systems which, during use, are rubbed between compliant biosurfaces to form thin lubricating films. It is important to understand the nature and properties of the films thus formed since these contribute to the user’s sensory perception, and thus appreciation, of the products concerned. In this paper, the lubrication properties of simple oil-in-aqueous phase emulsions are studied in a steel/elastomer “soft-EHL” contact. It is found that overall behavior is strongly dependent on the ratio of the viscosities of the two phases. When the viscosity of the dispersed oil phase is lower or comparable to that of the continuous aqueous phase, the latter enters the contact and controls film formation and friction. However, when the dispersed phase has viscosity at least four times larger than the dispersion medium, the former enters the contact and determines its tribological properties. This effect is believed occur because at high viscosity ratios the droplets are nondeformable and are thus forced into the contact inlet region, where collisions occur that result in shear-induced coalescence. Once a pool of viscous fluid is formed, the lower viscosity bulk fluid is unable to displace it because the viscous shear stress is too small, so the pool acts as a reservoir to supply the contact.

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Copyright © 2006 by American Society of Mechanical Engineers
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Figures

Grahic Jump Location
Figure 1

Master Stribeck curve for Newtonian fluids on hydrophobic elastomer surfaces

Grahic Jump Location
Figure 2

Friction coefficient as a function of entrainment speed for crude sunflower oil-in-water emulsions (no surfactant added) at seven different concentrations: 0.5, 1, 5, 7.5, 10, 20, and 30wt% on hydrophobic elastomer surfaces. Also represented is the friction behavior of pure sunflower oil, SFO, using hydrophobic (HB) and hydrophilic (HL) elastomer surfaces.

Grahic Jump Location
Figure 3

Friction coefficient as a function of entrainment speed for a range of different viscosity ratio emulsions on hydrophobic elastomer surfaces. All contain 20wt% oil. The polar phase was formulated by mixing glycerol and water. No surfactant was employed.

Grahic Jump Location
Figure 4

Different attempts at achieving master Stribeck curve for emulsions on hydrophobic elastomer surfaces: (a) Using the viscosity of the polar phase as the scaling factor for the entrainment speed, (b) using the viscosity of the emulsion predicted from Palierne model (18), and (c) using the viscosity of sunflower oil and the polar phase as needed to force collapse. Solid line represents the Newtonian master curve (cf. Fig. 1). Viscosities used to force collapse are shown in brackets.

Grahic Jump Location
Figure 5

Friction coefficient as a function of entrainment speed for unity viscosity ratio emulsions at different oil concentrations (10, 20, 30, 40, and 60wt%) on hydrophobic elastomer surfaces. In order to match the viscosity of the two phases, the composition of the polar phase is 82.5wt% glycerol and 17.5wt% water: No surfactant is employed.

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