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

assembled on soft polymeric surfaces by the vertical deposition method. Both hydrophilic andhydrophobic polymers, polyvinyl alcohol ( PVA) and polydimethylsiloxane ( PDMS), were employed assubstrates 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 hexagonalalignment to those on glass. However, on the macroscopic scale, substrates were found to have a profoundinfluence on both the adhesion and structural behavior of silica colloidal crystals. Regarding interfacialadhesion, at low silica deposition concentrations, films of silica colloidal crystals would form uniformlyon 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, theyremained 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 onlyorderly macroscopic cracks were observed on PDMS. The high adhesion and stability of silica crystals onPDMS might be explained by the low elastic modulus of PDMS which allowed PDMS to deform to releasethe shrinkage stress that occurs during the self- assembly process. Water contact angle measurementsshowed that the colloidal crystals deposited on hydrophobic PDMS were unexpectedly hydrophobic, witha 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 silicaparticles during the deposition process. This work provides fundamental insights and the first step to thepossible application of silica colloidal crystals on soft materials. c 2014 Elsevier B. V. All rights reserved.