Free-standing mesoporous silica films; morphogenesis of channel and surface patterns

Extraordinary channel patterns have been observed in an oriented mesoporous silica film that is synthesized at the boundary between air and water. The basic channel designs comprise concentric circles, herringbones, fingerprints and hairpins. It is suggested that these patterns are spatio-temporal silicified recordings of the polymerization and growth of a silicate liquid-crystal seed emerging in two spatial dimensions at the air/water interface. The origin of the channel patterns may be from defects, such as disclinations and dislocations, that spontaneously form in a surface confined precursor silicate liquid-crystal film to give a liquid-crystalline texture to the resulting mesoporous silica film. The reaction-diffusion processes and defects that are believed to contribute to the curved three-dimensional morphologies and channel patterns of mesoporous silica free to grow in solution, may also occur in the surface version, except that the isotropy of solution is replaced by the anisotropy of the air/water interface together with proximity effects from neighboring growth centers. Polymerization induced radial stresses produced in the assynthesized mesoporous silica film before drying, are relieved through warping of the film into micrometer-scale hillock-shaped protuberances. These mounds are arranged into patterns that appear to reflect the channel designs within the film. A linear radial stress model is found to successfully account for the warp patterns. A possible connection between the two-dimensional channel patterns in mesoporous silica films and the two-dimensional stripe domain patterns found in modulated phases is discussed.