2017 年 21 巻 1 号 p. 13-20
Colloidal nanosheets are obtained by exfoliation of inorganic layered crystals such as clay minerals in water. They form colloidal liquid crystals at high nanosheet concentrations due to their shape anisotropy. Liquid crystalline nanosheets can be aligned under external fields. For this purpose, electric field is advantageous owing to its facile instrumentation, controllability, quantitative reproducibility, and broad applicability. AC electric field is utilized for aligning liquid crystalline nanosheets because the samples are aqueous colloids of electrically charged particles. The 2D shape of nanosheets facilitates alignment of their longitudinal direction parallel to the electric field in many cases. When a colloid sample is incubated to grow liquid crystalline domains up to the size of sub-mm scale, hierarchical macroscopic structures are evolved under electric fields, where the grown domains play a role of secondary building blocks. The final structure varies depending on the relative direction of applied electric field and gravity. An electric field applied parallel to gravity gives a net-like assembly of the nanosheet domains, whereas orthogonal application of an electric voltage and gravity assembles nanosheets in stripes spanning over more than 1-mm length through unidirectional nanosheet alignment. Such flexible organization of nanosheet-based structures is a unique aspect of exfoliated nanosheets in colloidal liquid crystalline states.
