EXPERIMENTAL STUDIES ON THE PARAMETRIZATION OF LIQUID SPREADING AND DYNAMIC CONTACT ANGLES

The dynamics of a spreading liquid are controlled by the details of th e fluid motion very near the moving contact line. Modeling this motion is not trivial. The classical hydrodynamic model has a singular stres s field at the moving contact line. This singularity prevents the use of the contact angle in dynamic conditions and predicts that an infini te force would be needed to sink a solid into a fluid. A model, valid in the small capillary number (Ca) limit, describes the fluid motion a nd viscous interfacial deformation near the moving contact line. The m odel contains a single free parameter, omega(0), which can be related to material parameters and must be determined experimentally. Experime nts are reported that tested the range of validity of this asymptotic hydrodynamic model. The fluid-vapor interface shape and fluid velocity held produced by a glass tube entering a bath of polydimethylsiloxane at constant speed were measured near and far from the contact line. T hey were compared with the model using the free parameter omega(0) as a fitting constant. This procedure established the validity of the the ory and provided a means of measuring omega(0). The ranges (in capilla ry number and in space) of validity of the theory were established. Th e model fails near the contact line at Ca greater than or equal to 0.1 . This failure starts near the contact line, propagates out and increa ses in magnitude as Ca increases. For Ca less than or equal to 0.1, th e model with viscous deformation fails far from the contact line but d escribes the interface shape within similar to 400 mu m from the conta ct line. The model begins to fail at distances where the interface sha pe ceases to be controlled by geometry-independent viscous forces but responds instead to a competition between viscous and gravitational fo rces. At even larger distances from the contact line, viscous forces b ecome negligible and the interface looks static-like. The experiments showed that the contact angle formed by the extrapolation to the solid surface of the static-like interface far from the contact line equals omega(0) as predicted by the theory. Comparisons of omega(0) and appa rent dynamic contact angles based on meniscus height measurements, the ta(app), are presented. Small but systematic errors were found which i ncrease with Ca. In contrast to omega(0), theta(app) cannot be related to material parameters and hence cannot be used to generate archival modeling information for spreading dynamics.