Protein and colloidal crystals have been compared with inorganic crystals in view of hierarchical defect structures. Defects in proteins are more hierarchical and because of the larger molecule size than inorganic crystals. The perfection of protein molecules themselves is more important than the structural imperfection of the crystal. Because of much smaller elasticity of colloidal crystals, defects in colloid crystals are grain boundaries, stacking faults or twin in normal cases. The points of interests in colloidal crystals lie on the fact that a great advantage of colloidal building particles, in comparison to atoms and molecules, is that the interactions between them are tunable. The building colloid particles have been replaced from simple spherical latex particles to particles with various size, different physical properties like the structure of magnetism in the building particle and morphologies. Special emphasis could be placed on the defect structures of building colloid particles, which are responsible for the formation of multiple twining or quasicrystals that be would utilized as new building units for colloidal crystals with special physical properties. Changing bond strength between building units with DNA strands with structure would also be an important issue as reported (W.B. Rogers and V.N. Manoharan, Science, 2015).

  Crystallization of colloidal systems has been studied for past few decades as models to study phase transition in general. Complex crystal structures formed in multi-component colloids have attracted considerable attention. The fabrication of 2D area-filling colloidal eutectics, however, has not been reported till date. Here, we report spontaneous formation of eutectic structures in binary and ternary aqueous dispersions of colloids due to depletion attraction by using linear polyelectrolytes. Microscope observations showed that the eutectic structures were formed because of interplay of crystallization of constituent components and accompanying fractionations in the multi-component colloids. A binary phase diagram, defined by a mixing ratio and inverse area fraction of the particles, was analogous to that for atomic and molecular eutectic systems. These structures could be immobilized in polymer gel to produce self-standing materials. The present findings will be useful in the design of the optical properties of colloidal crystals.

  A review is given on gravity-induced processing of colloidal crystals. In particular, Monte Carlo simulation studies for gravitational tempering of colloidal crystals are focused. In this method the gravitational condition is weakened for a certain period after the sedimentation in order to reduce defects included in the colloidal crystals grown by sedimentation. Some extended defects, which could not disappear by a simple gravitational annealing, could successfully be kicked out or diminished in the gravitational tempering. We understand that due to alleviation of gravitational tightness some defect structures become mobile and then some extended defects disappear. There is a trade-off: thermodynamic driving force of colloidal crystallization is reduced simultaneously when the gravitational condition is weakened. That is, if the gravitational condition is overly weakened, crystallinity decreases. Moreover, it is suggested that there is an optimum period of treatment at weakened gravitational condition.

  To obtain high quality colloidal crystals, control of impurity behavior during crystallization is required. We have investigated impurity partitioning quantitatively for the colloidal crystal. We applied conventional partition model of normal crystal growth such as Thurmond and Struthers model and Burton Prim Slichter (BPS) model to colloidal crystal growth, which provides equilibrium partition coefficient (k₀). Discussion on the basis of free energy derived from volume fraction of colloidal particles well account for variation of k₀s for impurity of different size. In-situ observation revealed detailed partition behavior, including orientation-dependency, the effect of the grain boundary, and the morphology of solid-liquid interface. These new findings contribute to constructing methodology for growing high quality colloidal crystal as well as understanding general concept of partitioning that takes place during phase transition.

  Growing process and lattice strain of thin-film colloidal crystals were investigated with reflectance and transmission spectroscopy, optical stop-motion microscope and Kikuchi-Kossel line optics. Dispersions of crystallites were poured between two parallel plates to be melted by shear stress and the flow of dispersions was ceased abruptly for the recrystallization of larger colloidal crystals. It was found that formed crystal lattices were tending to be compressed along the flow direction.

  Synchrotron X-ray topography is one of the most powerful methods for the observation and characterization of crystal defects, especially dislocations, in crystals. For protein crystals, larger crystals with millimeter size are required to obtain clear topographic contrasts of dislocations. In this review, various types of dislocations such as straight, loop, and curved ones are identified for large crystals of hen egg-white lysozyme and glucose isomerase (GI). Pendellösung fringes which have been seen in high quality Si crystals are also observed in GI crystals with high quality. In addition, digital topography with X-ray CCD camera is shown with the analysis of local rocking curves in protein crystals. The digital topography shows promising for the evaluation of not only the distribution but also the misorientation of subgrains with strain in protein crystals.

  We have been studying numerical models of the protein depletion zones (PDZ) and the impurity depletion zones (IDZ) around crystals in terrestrial gravity and in microgravity and have shown that all sections of the crystals are surrounded by different supersaturation levels of protein and different concentrations of impurities when growing in the steady state model and in the transient state model. This suggests that the extent of molecular order may depend on the position in the crystal. Therefore, we prepared three kinds of lysozyme samples which contained different amounts of impurities and crystallized them in terrestrial gravity and in microgravity. Then we ran grid scans using these crystals to obtain diffraction data and found that the unit cell volume of the outer layer of the crystal increased when 50% purified lysozyme was crystallized in terrestrial gravity. The results were consistent with the numerical analysis. From these results, we conclude that the quality of diffraction data may depend on which part of the crystal the focused X-ray beam is applied. Therefore, it may be important to select the position in the crystal to aim the X-ray beam for a better diffraction dataset collection.