VISCOELASTIC FREE-SURFACE FLOWS - SPIN-COATING AND DYNAMIC CONTACT LINES

The effect of elasticity on the spreading of a spinning drop of fluid is investigated in the context of lubrication theory. It is shown that the Oldroyd-B constitutive equation permits a solution in which the f ree surface of the central part of the drop thins uniformly in space. Perturbation results for small effects of elasticity indicate an incre ased thinning rate of the free surface compared to Newtonian results f or the central part of the spinning drop, and that this enhanced thinn ing rate persists only over a few characteristic relaxation times. Ela stic effects in the capillary region near the moving contact line are also investigated by perturbation theory for small elasticity. Two met hods for resolving the contact line singularity are chosen: matching t he free surface profile to a precursor film of thickness b, and introd ucing slip at the spinning plate. For the precursor film model, the fr ee surface correction changes character from a net enhancement of the capillary ridge near the contact line for large b, to a negative corre ction over most of the profile for small b. With the slip model, the f ree surface correction gives a net enhancement of the capillary ridge for all values of the slip parameter alpha. The difference between the models for thin precursor films or slight slip is explained by examin ing the manner in which the stress relaxes near the contact line. The results suggest that viscoelastic contact line dynamics may be more se nsitive to the local molecular physics than the Newtonian counterparts .