Effect of Temperature and Pressure on Diffraction of Photonic Crystals of Microgel Suspensions

At suitable conditions, sub-micron sized colloidal particles self assemble into crystalline arrays (known as colloidal crystals) with lattice constants in the visible range. These crystals exhibit Bragg diffraction of visible light, as certain frequencies cannot propagate through, hence can serve as photonic crystals. Highly diffracting photonic crystals (PCs) can be prepared through colloidal route by self-assembly of monodisperse particles. However size polydispersity (SPD) is inherent to all colloidal suspensions: hard-sphere, charge stabilized and stimuli responsive microgels. Disorder introduced by temperature and SPD have important implications on optical properties of these crystals. First and second-type disorder in these crystals are known to arise from thermal motion and SPD, respectively. In the case of photonic crystals prepared through hard-sphere or charge stabilized suspensions, the SPD should be less than 11% and is not tunable. Thermo-responsive microgels particles in aqueous medium are known to offer wide tunability of particle size by varying temperature and pressure. However the influence of these parameters on SPD is not reported and this talk discusses several new results obtained from dynamic light scattering, confocal laser scanning microscopy and UV-visible spectroscopy techniques on photonic crystals of poly(N-isopropylacrylamide) (PNIPAM) stimuli-responsive microgel particles. Here we show that osmotic pressure not only compresses particles but also decreases SPD significantly, resulting in self-assembly of ordered structure of microgel particles, which otherwise remain disordered at ambient conditions due to high size polydispersity. The Bragg peak of PNIPAM microgels crystals, recorded using UV-visible spectroscopy, are found to exhibit a blue shift upon increasing the osmotic pressure and are understood to arise from the increase in the number density as well as due to decrease in particle size.