Directed alignment of graphene-oxide liquid crystals in confined spaces: Effects of flow, pickering stabilization, and phase transition

Because of its versatility for processing, graphene oxide (GO) serves as a superior substance for two-dimensional carbon-based materials. Controlling GO alignment is crucial to augment its inherent anisotropic nature. Here, we report the controlled alignment of GO confined in the rectangular and circular capillary tubes. GO aligns in contrasting orientations according to the phase of GO dispersion during loading into a capillary. Upon GO liquid crystal (LC) loading, GO-layers are oriented tangential to the inner surface of capillary, resulting in a concentric organization of the GO-layers. In the LC phase, the GO orientation is mainly influenced by the flow so that the GOs are aligned tangential to the hydraulic shear plane. However, GO self-assembles parallel to the capillary cross-section when loaded in the isotropic phase and then concentrated into the LC phase. Surprisingly, by excluding flow-induced effects, the GO layer is aligned perpendicular to the inner surface throughout the capillary. This unexpected alignment of the GO-layers is initiated in the meniscus region by the “Pickering effect” (i.e., tangential adsorption of GO along the Air-Liquid interface). Upon solvent evaporation, GO dispersions experience a sequential isotropic to nematic phase transition from the meniscus to the central region. During the phase transition, the orientation of the GO layer is determined by the long-range ordering of the LC phase. As a result, the alignment effect caused by the Air-Liquid interface determines the GO alignment throughout the capillary. Our results provide useful insight into the delicate control of GO alignment in a confined space. Precise control of GO alignment in a confined space can be achieved based on the appropriate directional design of the Air-Liquid interface and the corresponding sequential phase transition.