Angle- and strain-independent coloured free-standing films incorporating non-spherical colloidal photonic crystals.

Colloidal photonic crystals (CPCs) provide a convenient way to generate structural colour with high stability against degradation under environmental factors. For a number of applications including flexible electronic and energy devices, it is important to generate flexible structural colour that maintains its colour regardless of the angle of observation and the extent of mechanical deformation. However, it is challenging to simultaneously achieve these goals because anisotropy in typical CPC structures (e.g., CPC films) tends to lead to angle-dependent photonic properties and also changes in the lattice constant due to mechanical deformation lead to changes in the photonic properties of CPCs. To overcome these challenges, we present a means of fabricating large-area free-standing films of CPC structures that exhibit angle- and strain-independent photonic characteristics. First, monodisperse double emulsions encapsulating colloidal crystal arrays are prepared using a microfluidic device. By inducing crystallization of highly charged polystyrene particles in the core of double emulsions using osmotic annealing, we generate angle independent colloidal photonic crystal (CPC) supraparticles. Moreover, the shape and crystallinity of the CPC supraparticles can be tuned by changing the concentration of salt in the solution used for osmotic annealing. Subsequently, an array of CPC supraparticles is embedded inside an elastomeric matrix to form a flexible free-standing film, which exhibits structural colours that are independent of viewing angles and externally applied strain.