SPIN-COATING OF VISCOELASTIC AND NONVOLATILE FLUIDS OVER A PLANAR DISK

Spin coating of two commercially used polymer solutions is studied bot h theoretically and experimentally. Physical and theological character ization of these solutions indicates that under the spinning condition s 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 n umber, whereas their damping rate gets smaller as De increases. Since these oscillations dissipate very rapidly, and before substantial thin ning of the film takes place, the thinning rate, velocity, and shear s tress components do not deviate eventually from those of a Newtonian f luid. Such a complete explanation of similar experimental findings has not been offered before, The radial normal stress component does incr ease considerably over its Newtonian value, and this explains certain ''experimental practices.'' Similar oscillatory development early on o ccurs even at higher Re, as long as Re similar to De, but it is dissip ated again, this time because of the abrupt thinning of the film. The theoretical results are in good agreement with experimental measuremen ts of ''dry film'' thickness and with dynamical measurements of ''wet film'' thickness during spinning, which are reported herein for the fi rst time. Care must be taken in reporting ''dry film'' thickness becau se the commercial solutions under study retain part of the solvent aft er ''soft baking'' over a hotplate. Complete solvent removal produces dry films, but requires treatment in a vacuum oven, higher temperature s, and longer heating times.