Formation of silica colloidal crystals on soft hydrophobic vs rigid hydrophilic surfaces

Monodisperse silica nanoparticles (NPs) were synthesized via the modified Stِber method and then self-assembled on soft polymeric surfaces by the vertical deposition method. Both hydrophilic and hydrophobic polymers, polyvinyl alcohol (PVA) and polydimethylsiloxane (PDMS), were employed as substrates and were investigated for their influences on silica colloidal crystal formations. In SEM micrographs, silica colloidal crystals formed on both PVA and PDMS surfaces presented similar hexagonal alignment to those on glass. However, on the macroscopic scale, substrates were found to have a profound influence on both the adhesion and structural behavior of silica colloidal crystals. Regarding interfacial adhesion, at low silica deposition concentrations, films of silica colloidal crystals would form uniformly on glass and PVA but only form partially on PDMS due to the hydrophobic nature of the PDMS substrate. At high silica deposition concentrations, films of colloidal crystals formed on glass, PVA and PDMS surfaces; however, the colloidal crystal film delaminated from both glass and PVA surfaces; in contrast, they remained intact on PDMS. In terms of structure, as the silica concentration increased, random microscopic to orderly macroscopic cracks were observed on samples deposited on glass and PVA, while only orderly macroscopic cracks were observed on PDMS. The high adhesion and stability of silica crystals on PDMS might be explained by the low elastic modulus of PDMS which allowed PDMS to deform to release the shrinkage stress that occurs during the self-assembly process. Water contact angle measurements showed that the colloidal crystals deposited on hydrophobic PDMS were unexpectedly hydrophobic, with a water contact angle (WCA) around 120°, while the WCAs were <50° for those on hydrophilic surfaces. This phenomenon might be attributed to the migration of hydrophobic constituent in PDMS to the silica particles during the deposition process. This work provides fundamental insights and the first step to the possible application of silica colloidal crystals on soft materials.