Concentric/bipolar ordering of liquid crystalline graphene oxide nanosheets confined in microfluidically synthesized spherical droplets

Abstract

Liquid crystalline (LC) two-dimensional (2D) nanomaterials have been exploited for designing exquisitely assembled structures or for reinforcing LC properties in a mix with conventional LC materials. Graphene oxide (GO) nanosheets can be regarded as one representative example of these 2D LC materials. As a means to control the LC alignment characteristics, the strategy of imposing physical confinement has been widely investigated for LC species. Thus, the understanding of LC GO ordering within a confined geometry becomes salient for developing GO-based nanostructures or for synthesizing novel functional materials. However, previous attempts have mainly utilized a viscous polymerisable medium to capture the LC-ordered phase of GOs. As a result, a fundamental understanding of the LC ordering of a pure GO suspension under confinement is lacking. In this study, we present a novel method for elucidating the LC alignment characteristics of GOs under confinement using microfluidically synthesized microdroplets of a water-in-oil-in-water double emulsion system. The internal LC ordering of GOs inside the highly monodispersed microdroplets with an oil shell is elaborately controlled by varying the experimental variables, such as GO concentration, microdroplet size, and GO nanosheet size. In particular, under strongly confined conditions within spherical droplets, the GO alignment generally realizes concentric ordering, reflecting the curvature of the microdroplets. However, when the confinement effect is relatively weak, bipolar ordering with parallel alignment of the GO nanosheets predominates. As a result, the phase diagram of the effects of geometric and concentration factors on LC GO ordering can be successfully constructed. Therefore, it is highly anticipated that the presented approach to improve the LC alignment selectivity of 2D nanomaterials will be a solid platform for implementing highly controlled LC architectures with different functionalities.