Dip coating of charged colloidal suspensions onto substrates with patterned wettability: Coating regime maps

Aqueous dispersions of silica nanoparticles were dip coated onto Si substrates that contained patterned wettability. The patterns were prepared by photolithography and consisted of groups of hydrophilic lines (5-100 μm wide) separated by hydrophobic areas (5-100 μm wide). Coating were made from two aqueous silica dispersions: a cationic dispersion in which particles have positive surface charge, and an anionic dispersion in which particles have negative surface charge. Coating morphology, thickness, and pattern quality were characterized. For a pattern containing 25 μm wide hydrophilic stripes separated by equally wide hydrophobic spaces, coating regime maps were created to show the effect of process variables on pattern features and morphology. Within the map there is a critical concentration for both dispersions, above which uniform stripes are formed and below which a segregated non-uniform structure results. Coatings prepared at withdrawal rates of 0.1 mm/s or lower resulted in a monolayer of coatings in the case of cationic silica and no deposition in the case of anionic silica. A maximum withdrawal rate was also found; above a critical speed, excess liquid is entrained and results in nonuniformity in the bottom of the pattern. The physical origin of the regimes and differences between the two types of particles are discussed.