Formation of optically anisotropic films from spherical colloidal particles.

Colloidal silica films, formed by the drop evaporation method, showed birefringent spherulite optical properties. They displayed a Maltese cross pattern under crossed polarizers, and interference colors, such as blue and orange-red, under crossed polarizers with a compensator. The difference in refractive index was estimated to be 9x10(-4) from the interference colors. Scanning electron microscopy (SEM) results revealed anisotropic structures in the colloidal films. Particles formed radially ordered hexagonal arrays. The drop evaporation method used in this report, which dries from the edge to the center, resulted in a radially ordered colloidal film. When a colloidal silica film was prepared using a unidirectional drying method, particles were packed in an ordered structure corresponding to the drying direction and the resulting film showed different birefringent optical properties. Our results show that a variety of birefringent films can be obtained from spherical colloidal dispersions through control of the drying method.