Unusual contact-line dynamics of thick films and drops

We report several novel phenomena in contact-line and fingering dynamics of macroscopic spinning drops and gravity-driven films with dimensions larger than the capillary length. It is shown through experimental and theoretica l analysis that such macroscopic films can exhibit various interfacial shap es, including multi valued ones, near the contact line due to a balance bet ween the external body forces with capillarity. This rich variety of front shapes couples with the usual capillary, viscous, and intermolecular forces at the contact line to produce a rich and unexpected spectrum of contact-l ine dynamics. A single finger develops when part of the front becomes multi valued on a partially wetting macroscopic spinning drop in contrast to a di fferent mechanism for microscopic drops of completely wetting fluids. Contr ary to general expectation, we observe that, at high viscosity and low freq uencies of rotation, the speed of a glycerine finger increases with increas ing viscosity. Completely wetting Dow Coming 200 Fluid spreads faster over a dry inclined plane than a prewetted one. The presence of a thin prewetted film suppresses fingering both for gravity-driven flow and for spin coatin g. We analyze some of these unique phenomena in detail and offer qualitativ e physical explanations for the others. (C) 1999 Academic Press.