Contribution of water droplets to defect array formation in water-in-cholesteric liquid crystal emulsions

The quadrilateral defect array is spontaneously formed after termination of the shear to the cholesteric emulsions, which are composed of water, surfactants, and cholesteric liquid crystals. We have here investigated the motion of water droplets in cholesteric media during the formation process of the defect array by using the laser scanning confocal fluorescence microscopy. A small amount of water-soluble fluorescent probe, Safranin O, was mixed in the cholesteric emulsions to detect the position of water droplets. After stopping the shear, the fluorescence intensity near glass surfaces increases rapidly, in contrast to the reduction in the midway between glass plates, which indicates that water droplets become preferably settled on the glass surface, not in the liquid-crystal medium. The spatial distribution of the droplets on the glass surface tends to be two-dimensionally uniform, independent of the period of the defect array and the cholesteric pitch. The condensation of water droplets onto the glass surface occurs only within 10 s after the shear, much more quickly than the formation of the defect array. On the basis of these experimental results, we discussed the effect of water droplets on the formation of the defect array, particularly focusing on the growth of the nuclei of point defects, which are initially produced on the glass plates by the homeotropic anchoring of the substrate surface.