The dynamic contact angle formed when silicone oil displaces air from the surface of glass has been measured. Even though the glass was neither treated in any special way nor cleaned by an elaborate technique, the standard deviation associated with our measurements was approximately 1·5°. A sequence of experiments revealed that the dynamic contact angle depends on both the speed at which the oil spreads across the glass and the size of the characteristic length scale associated with the device within which it is measured. It is shown that the latter implies directly that: (i) the measured and the actual contact angles are not the same; (ii) the usual hydrodynamic model for fluids is inadequate when a moving contact line is present. These conclusions are consistent with recent theoretical studies.

Our main objective is to identify a boundary-value problem capable of describing the dynamics of fluids having moving contact lines. A number of models have been developed over the past decade and a half for describing the dynamics of just such fluid systems. We begin by discussing the deficiencies of the methods used in some of these investigations to evaluate the parameters introduced by their models. In this study we are concerned exclusively with the formulation of a boundary-value problem which can describe the dynamics of the fluids excluding that lying instantaneously in the immediate vicinity of the moving contact line. From this perspective, many of the approaches referred to above are equivalent, that is to say they give rise to velocity fields with the same asymptotic structure near the moving contact line. Part of our objecive is to show that this asymptotic structure has only one parameter. A substantial portion of our investigation is devoted to determining whether or not the velocity field in a particular experiment has this asymptotic structure, and to measuring the value of the parameter.More specifically, we use the shape of the fluid interface in the vicinity of the moving contact line to identify the asymptotic structure of the dynamics of the fluid. Experiments are performed in which silicone oil displaces air through a gap formed between two parallel narrowly-spaced glass microscope slides sealed along two opposing sides. Since we were unable to make direct measurements of the shape of the fluid interface close to the moving contact line, an indirect procedure has been devised for determining its shape from measurements of the apex height of the meniscus. We find that the deduced fluid interface shape compares well with the asymptotic form identified in the studies referred to above; however, systematic deviations do arise. The origin of these deviations is unclear. They could be attributed to systematic experimental error, or, to the fact that our analysis (valid only for small values of the capillary number) is inadequate at the conditions of our experiments.

A liquid spreading over a dry solid surface with a contact angle of 180° has always been regarded as a special case. It is commonly thought that this represents a unique situation in which a singularity does not arise in an analysis of the dynamics of the liquid when the usual hydrodynamic assumptions are made, i.e., when the liquid is Newtonian and incompressible and obeys the no-slip boundary condition at the solid. In fact, there are strong indications that it is not a special case.