Shear-induced alignment of low-aspect-ratio nanorods for modulations of multiple optical properties†
Dynamic modulations of multiple optical properties in one system could bring new opportunities for applications in various areas. Here we report rheological modulations of birefringence and photonic bandgap in crystalline colloidal arrays of nanorods with an aspect ratio as low as 1.7. The tuning mechanism lies in the flow-induced alignment of nanorods along the shear strain profile, as confirmed by ultra-small-angle X-ray scattering (USAXS) studies. In the case of CCA, the strong interparticle electrostatic interaction provides the additional driving force for the nanorod alignment. Both the birefringence and photonic bandgap of the nanorod CCA can be tuned by regulating the flow rate and direction. This study not only reveals the feasibility of flow-induced alignment of low-aspect-ratio nanorods but also provides a new smart optical platform with interesting implications for a variety of applications, such as anti-counterfeiting, camouflage, and continual monitoring of flow rates in microscopic systems, manifested by our successful demonstrations of the detection of blockage in microfluidic chips.