The Flow of Thin Viscous Liquid Films on Rotating Disks

Abstract

This paper reports a study of the flow of thin viscous liquid films on the surface of rotating silicon disks. The two liquids studied are a low-molecular-weight linear-chain perfluoropolyether polymer (Demnum S20), deposited in films 4 to 25 nm thick, and a star-shaped alkyl ester of the orthosilicic acid (tetrakis(2-ethylhexoxy)silane), deposited in films 7 to 10 nm thick. The disks are spun at high speeds and the radial thickness profiles of the liquid films are measured as a function of spin time by ellipsometry. Comparison of the measured thickness profiles with the theory of Emslie et al. [ J. Appl. Phys. 29, 858 (1958)] shows that the flow of these thin liquid films can be modeled as that of a continuum, Newtonian liquid with an apparent viscosity 50 to 80% higher than that of bulk. The increase above bulk viscosity may be attributed either to collective molecular motion effects or to the layer of molecules in contact with the disk surface being immobile, resulting in an increase of the apparent viscosity assigned to the complete film.