The effect of induced air-flow on the spin coating of viscous liquids

In the spin-coating process for producing thin films, air flow induced by disk rotation provides a shear stress at the liquid-air interface which enhances the rate of thinning of the liquid film on the disk. An approximate analysis using Cochran’s classical solution [Proc. Cambridge Philos. Soc. 30, 365 (1934)] for the induced air flow is presented. Regions wherein air flow significantly enhances thinning are delineated. As film thickness becomes very small, in accordance with a dimensionless criterion developed as part of the analysis, the external air flow dominates the centrifugal effect and controls the final rate of thinning. A set of data obtained under conditions appropriate to application of this new theory shows enhanced thinning, but not to the degree predicted.