SPIN COATING OF COLLOIDAL SUSPENSIONS

The coupled, unsteady Navier-Stokes, convective diffusion, and thermal energy equations that describe spin coating of colloidal suspensions are solved numerically. The theoretical model, absent of any adjustable parameters, is used to explore the effects of angular velocity, initial solvent weight fraction, solvent properties and spin coating protocol on the evolution of temperature and concentration profiles in the liquid film during spin coating. The predicted coated film thickness is found to be in excellent quantitative agreement with spin coating experiments performed with both hard-sphere and nonhard-sphere suspensions of monodisperse latex particles in water. The coated film thickness, determined by ellipsometry, is shown to depend on the inverse square root of the angular velocity except at high ionic strength when the dependence on angular velocity is weaker. Timescales that characterize spin coating of colloidal suspensions are shown to be quite different from those that characterize spin coating of polymer solutions, and consequently simple models for predicting the coated film thickness of polymer solutions (Bornside et al., 1991; Lawrence, 1989) are shown to be inadequate for colloidal suspensions. Rapid substrate acceleration, high rotation rates, partial saturation of the overlying gas phase, and high initial solids concentration are identified as spin coating protocols that suppress a convective instability that produces radial striations in the coated film.