The absence of a rheological effect on the spreading of small drops

The fluid mechanics of the movement of the contact line region for non-Newt onian fluids has been analyzed using the method of Joanny and de Gennes [C. R. Acad. Sci., Ser. II: Mec., Phys., Chim., Sci. Univers Terre 299, 279 (1 984)] where under lubrication theory approximation the rate of viscous diss ipation is equated to the rate of surface work to get the expression for th e rate of spreading. Two kinds of fluids are considered, one where the visc osity is dependent on shear rate but there are no normal stresses, and anot her where there are normal stresses but the coefficients are not dependent on the shear rate. Both show the spreading rates to be independent of the n on-Newtonian nature of the fluids to a good approximation, with the zero sh ear viscosity replacing the Newtonian viscosity in the result of Joanny and de Gennes. This appears to be in keeping with the available experimental d ata. (C) 2001 American Institute of Physics.