Authors
Borkar A.V., Tsamopoulos J.A., Gupta S.A., Gupta R.K.
Abstract
Spin coating of two commercially used polymer solutions is studied both theoretically and experimentally. Physical and rheological characterization of these solutions indicates that under the spinning conditions currently used they behave as nonvolatile, viscoelastic fluids with constant viscosity and elasticity. The corresponding Reynolds (Re) and Deborah (De) numbers are up to order unity. The theoretical analysis demonstrates and explains why, at very short times after the inception of impulsive spinning, the velocity and stress fields in such fluids develop in an oscillatory manner. The amplitude of these oscillations increases with the ratio of the retardation parameter to the Deborah number, whereas their damping rate gets smaller as De increases. Since these oscillations dissipate very rapidly, and before substantial thinning of the film takes place, the thinning rate, velocity, and shear stress components do not deviate eventually from those of a Newtonian fluid. Such a complete explanation of similar experimental findings has not been offered before. The radial normal stress component does increase considerably over its Newtonian value, and this explains certain “experimental practices.” Similar oscillatory development early on occurs even at higher Re, as long as Re-De, but it is dissipated again, this time because of the abrupt thinning of the film. The theoretical results are in good agreement with experimental measurements of “dry film” thickness and with dynamical measurements of “wet film” thickness during spinning, which are reported herein for the first time. Care must be taken in reporting “dry film” thickness because the commercial solutions under study retain part of the solvent after “soft baking” over a hotplate. Complete solvent removal produces dry films, but requires treatment in a vacuum oven, higher temperatures, and longer heating times. © 1994 American Institute of Physics.
