Numerical Study of Liquid Film Microstructure During Spin Coating of Ag Colloidal Suspension

A multiple time-step discrete-element approach is employed to model the transport, collision and adhesion of small silver (Ag) colloidal particles in a spin coating process. The three-dimensional (3D) particle motions are computed using a combination of fluid-induced forces and torques, and forces and torques induced by the particle collision and adhesion forces. These analysis are used to predict the final aggregate size distribution and microstructure during the non-evaporative phase of spin coating of a thin film, which is important for controlling the abrasiveness, opacity, conductivity, and other properties of the film, as well as for using the deposited particles for growing new materials (e.g., nanotubes). The computations examine the effect of adhesion potential, volume concentration, and spin speed effects on the distribution of particle aggregate sizes and their deposition during spin coating. The aggregates are observed to be primarily governed by these parameters, as has previously been observed in experiments. The results provide a fundamental understanding of the physics of thin film spin coating processes and insight in to microstructure control during the coating process.