A capillary-based model system for hydrodynamic spreading in porous media

CHARACTERIZING THE WETTING PROPERTIES OF SOFT POROUS MATERIAL USING A WASHBURN-LIKE APPROACH Q. Zhang1,2, F. Mugele1, P.M. Lugt2,3 and D. van den Ende1 1Physicis of Complex fluids, University of Twente 2Laboratory of Surface Technology and Tribology, University of Twente 3SKF Research & Technology Development, Nieuwegein, the Netherlands Lubricating grease is a two-phase soft material consisting of a thickener matrix that provides mechanical strength and mobile base oil. The matrix acts as a reservoir that slowly releases the oil during years of bearing operation. Despite the fact that the reliability of most mechanical machinery crucially depends on the lifetime of bearings and their lubrication, the process of oil retention and release by the grease matrix is very poorly understood[1]. In this project, we facilitate the oil-release by extracting the oil from the grease using filter paper. Taken into account the heterogeneity of the pores sizes in the paper[2], the mass of oil imbibed by the paper is quantified by measuring the transmitted light intensity[3] using a modified Lambert-Beer relation. The imbibition timescale from the grease is found to be of 2 orders of magnitude longer than that of a drop of the same base oil. The slow oil release results from the high affinity between the grease matrix and the oil, leading to a pressure gradient (~ 5kPa) opposite to the direction of the extraction flow. To quantify this wetting affinity, we developed a Washburn-like model in which both grease and paper are described as porous media. The creeping oil flow is driven by the difference in wetting properties of the grease matrix and the paper. Furthermore, we found that the imbibition dynamics depends on the oil content of the grease. Even a slight increase (~0.2%) of the wetting energy volume can lead to an increase of a factor 2 in the timescale of imbibition. Figure 1 The amount of oil adsorbed in the paper versus time from an oil drop and from de-saturated grease patches (from top to bottom the saturation level decreases from 100% to 65%) REFERENCES: 1. Lugt P. M. ‘A review on grease lubrication in rolling bearings’ Tribology transactions, 2009, 4, 470-480. 2. Danino D. and Marmur, A. ‘Radial capillary penetration into paper: limited and unlimited liquid reservoir’ Journal of colloid and interfaces science, 1994, 166(1), 245-250. 3. Gillespie T. ‘The spreading of low vapour pressure liquids in paper’ Journal of colloid science, 1958, 13(1), 32-50.