(163f) Flow-Assisted Assembly of Multilayer Colloidal Crystal Arrays through Spin Coating

Spin coating colloidal suspensions is a robust, efficient method for creating colloidal crystal arrays with good long-range order for applications (Jiang et al., 2004, Journal of the American Chemical Society 126(42) 13778) including optical materials. To evaluate the potential for broad application of this method, we used direct visualization with confocal microscopy to investigate the effects of strain, radial position, and Peclet number on the local crystallinity of 3D arrays assembled by spin coating. Specifically, our method uses charge stabilized, refractive index matched poly (12-hydroxystearic acid) stabilized poly(methyl methacrylate) colloids of size about one micron suspended in the viscous solvent dioctyl phthalate. To quench assembled structures for interrogation by 3D confocal microscopy, the suspensions (colloid volume fraction ~ 0.35) are spin coated on glass substrates with a small fraction of photopolymer that is subsequently gelled by UV exposure. Upon initiating spin coating, we find that excess suspension is expelled to the outside edge of the substrate by centrifugal forces. The result is a thin, microscopically level film with complex, spatially varying crystallinity. The full thickness of the colloidal crystal array is imaged at a number of radial positions using confocal microscopy. Particle centroids are located in 3D by means of quantitative image processing. Local crystallinity is quantified by application of local bond order parameter criteria developed by ten Wolde et al. (ten Wolde et al., 1996, Journal of Chemical Physics 104 9932). We quantify the effect of radial position, strain and strain rate on the variability of local crystal quality along an axis perpendicular to the substrate.