CHAPTER 6. Tunable Colloidal Crystals Immobilized in Soft Hydrogels

Abstract

Colloidal crystals, three-dimensional periodic arrays of monodisperse colloidal particles, are expected to be mass-producible photonic crystalline materials. In particular, colloidal crystals immobilized in hydrogels have attracted considerable attention because their optical properties can be tuned by external stimuli. This chapter provides an overview of our recent work on gel-immobilized colloidal crystals with high optical quality, including their preparation and tuning properties. Two preparation methods for gel-immobilized colloidal crystals that combine microfluidics and a photopolymerization technique are described. The first method uses an air-pulse-drive system for preparing a large single-crystal-like colloidal crystal film. We found that polycrystalline charge-stabilized colloidal crystals could be converted into single-crystal-like colloidal crystals by running the suspension in a flat capillary cell, according to a flow-induced shear effect. The second uses emulsion droplets as templates for preparing spherical colloidal crystals, such as colloidal crystal microspheres and shells. Although colloidal crystals cannot form single crystals in a spherical shape, they exhibit useful characteristics for various applications. These crystals can be immobilized in a hydrogel network by a photopolymerization technique that preserves crystalline quality. Next, the tuning of properties by external stimuli, including temperature, mechanical compression, and solvent exchange using ionic liquids, is described. In addition, the control of the effective bandwidth in gel-immobilized colloidal crystals is shown. Finally, the conversion of the gel-immobilized colloidal crystal film into a dry film of densely packed colloidal crystals without cracks is demonstrated.