Can a dynamic contact angle be understood in terms of a friction coefficient?

The kinetics of capillary rise of pure water and ethanol as well as mixture s thereof that under static conditions wet glass capillaries has been studi ed by a high-speed imaging technique. To adequately describe the experiment al data, a rate-dependent dynamic contact angle must be added to the Washbu rn-Lucas equation. This result is discussed in terms of a molecular frictio n coefficient at the front of the liquid flowing over the substrate. Dynami c inertia effects are significant at the initial stages of spreading and ma y have further importance for capillaries of larger radii. In addition, it was found that the preparation and cleaning of glass capillaries, which are essential for the liquid substrate interaction, have a significant effect on the initial rate of rise of the liquid. (C) 2000 Academic Press.