Surface Wettability Drives the Crystalline Surface Assembly of Monodisperse Spheres in Evaporative Colloidal Lithography

The wettability nature of substrates has been found to profoundly influence the surface assembly of monodisperse spherical particles for colloidal suspensions that are dried by evaporation to spontaneously form either periodic or disordered packing arrangements. The self-assembly of spheres has consequences when preparing surface masks for evaporative colloidal lithography. When a droplet of an aqueous suspension of monodisperse latex particles was dried by evaporation on flat substrates that are hydrophilic, a close-packed arrangement was formed spontaneously. However, when a similar aqueous suspension was deposited and dried on relatively hydrophobic substrates such as silicon and glass, a disordered arrangement was produced, revealing that there were negligible regions of hexagonal packing. The wettability of silicon wafers can be rendered to be hydrophilic by certain surface treatments. For example, after ozone exposure, silicon surfaces became hydrophilic as evidenced by changes measured for water contact angles. For silicon surfaces that were exposed to UV/ozone, crystalline arrangements of monodisperse latex spheres were generated with relatively few defects. Such physical or chemical treatments which tailor the wettability of surfaces can be used to improve reproducibility and to lower the density of defects when preparing surface masks for emerging manufacturing processes based on colloidal lithography.