Rheo-optical study of colloidal crystals

The relationship between rheological (macroscopic) and optical (microscopic) properties has been measured simultaneously for the colloidal crystals of silica spheres in exhaustively deionized aqueous suspensions. Shear stress of colloidal crystals increases linearly as strain increases, which supports the fact that the colloidal crystals are elastic. In the range of large strains, the crystals show the yielding and flow resulting in a constant value of shear stress irrespective of strain. The intersphere spacing estimated from the peak wavelength is insensitive to the small strain applied, however, it decreases approaching a constant value when the strain further increases. These results support the fact that the crystals are deformed slightly but keep their original form of single crystals by the shear at small strains. However, the sliding of the lattice planes may occur accompanied with the deformation of the electric double layer from spherical to flame-like. Furthermore, partial melt of the crystal structure occurs when the shear rate is high enough, which is confirmed from a decrease in the peak intensity in the reflection spectra.