IMBIBITION OF OIL IN FILM FORM OVER WATER PRESENT IN EDGES OF CAPILLARIES WITH AN ANGULAR CROSS-SECTION

In this paper it is shown, for the first time, that a nonspreading oil can form films over water films present in the edges of pores in a wa ter-wet porous medium. Both the water and the oil wet the solid surfac e preferentially to air, but water wets the solid preferentially to oi l. The mechanism of the film formation is capillary imbibition, i.e., spontaneous displacement of air by the oil, and it is similar to the f ormation of water films in the edges of a clean glass capillary of ang ular cross section filled with air. The principal difference is that, whereas the water imbibes over a solid surface, the oil imbibes over a surface consisting partly of water and partly of a solid wetted by oi l. It is shown, by a combination of applications of Laplace's equation of capillarity with the condition of minimum surface free energy, tha t if a ''critical'' oil film thickness is exceeded by the addition of excess oil, then this excess oil will imbibe over the water film in th e edge until the equilibrium critical oil film thickness is establishe d. The equations have been solved by iteration for the critical oil fi lm thickness, using representative values of the parameters involved. Further, it is shown that an oil blob placed in a capillary of polygon al (e.g., square or equilateral triangle) cross section, containing wa ter films in the edges of the capillary, will imbibe in the form of fi lms over the water present in the edges, subject to certain limitation s, specified in the paper. The theoretical predictions are supported b y photomicrographs showing imbibition of benzene equilibrated with wat er and a paraffin oil over water present in the edges of a 2D glass mi cromodel. The spreading coefficient is negative for both systems. (C) 1995 Academic Press, Inc.