Imaging stress and strain in the fracture of drying colloidal films

For three-dimensional photonic crystals, made either by top-down microfabrication or by bottom-up self-assembly approaches, to comply with the stringent requirements of optical telecommunication applications, their degree of structural perfection and optical quality must meet an exceptionally high standard. Only with such superior quality photonic crystals can their unique optical properties be harnessed in optical devices and circuits constructed from micrometer-sized optical components. In this paper, we present a new strategy for making silica colloidal crystal films with a sufficiently high level of structural perfection and optical quality to make it competitive as a practical route to photonic crystal optical components. The attainment of this goal takes due cognizance of three key synergistic factors in the film formation process. The first recognizes the necessity to prepare high-quality silica spheres, which are highly monodisperse, with a polydispersity index significantly better than 2%, and the second recognizes that the population of spheres must be devoid of even the smallest fraction of substantially smaller or larger spheres or sphere doublets. The latter turns out to have a minimal effect on the polydispersity index, and yet a major detrimental effect on the overall structural order of the film. The third concerns the film-forming method itself, which necessitated the development of a novel process founded upon isothermal heating evaporation-induced self-assembly (IHEISA) of spheres on a planar substrate. This new method has several advantages over previously reported ones. It is able to deposit very high-quality silica colloidal crystal film rapidly over large areas, with a controlled thickness and without any restrictions on sphere sizes.

It has long been known that thick films of colloidal dispersions such as wet clays, paints, and coatings crack under drying. Although capillary stresses generated during drying have been recently identified as the cause for cracking, the existence of a maximum crack-free film thickness that depends on particle size, rigidity, and packing has not been understood. Here, we identify two distinct regimes for crack-free films based on the magnitude of compressive strain at the maximum attainable capillary pressure and show remarkable agreement of measurements with our theory. We anticipate our results to not only form the basis for design of coating formulations for the paints, coatings, and ceramics industry but also assist in the production of crack-free photonic band gap crystals.