Photochemical transformation of topological defects formed around colloidal droplets dispersed in azobenzene-containing liquid crystals

We investigated photochemical transformation of asymmetric topological defects formed around water droplets dispersed in the azobenzene-containing liquid crystals. It was found that initial structures and the photo-induced phenomena of the defects were clearly affected by chemical structures of the azobenzene compounds. In quite strong homeotropic anchoring obtained by using the azobenzene compound with a long alky chain, the hedgehog was formed around the droplets at the initial state. When the anchoring strength was decreased by changing the carbon number of the alkyl chain, a novel asymmetric defect which will be an intermediate structure between the hedgehog and the Saturn ring was observed. The ionic groups had an influence on the photochemical transformation of the defects. The azobenzene compound with a smaller ionic group induced only a change in the anchoring strength, and a disappearance of the hedgehog was observed on the irradiation with ultra-violet light. In the case of the azobenzene compound with a larger ionic group, photochemical switching of the anchoring and the transformation into the boojums was achieved. The photo-induced phenomena could be qualitative explained by an effectiveness of the cis-trans photoisomerization depending on molecular areas of the azobenzene compounds determined by the ionic groups.