Elastic Photonic Microbeads as Building Blocks for Mechanochromic Materials

Colloidal assemblies have been used to make mechanochromic materials. As interparticle spacing of colloids in the elastomeric polymer matrix can be changed by mechanical strain, the structural colors are also dynamically tunable. In common, such mechanochromic colloidal structures have been prepared in a film format. Here, we design elastic colloidal structures with a microbead format to provide high reconfigurability over a macroscopic shape and optical property. As the assembly of microbeads can fit into any macroscopic templates, we can utilize them as building blocks for secondary assembly to produce mechanochromic materials with any formats. To produce the elastic photonic microbeads, we use microfluidic technology to prepare oil-in-water emulsion droplets, where the dispersion of silica particles in an elastomer-forming photopolymerizable resin with acrylate group is used as a dispersed phase. The silica particles spontaneously form a non-close-packed colloidal array in each droplet due to repulsive interparticle potential, which is stabilized by in situ photopolymerization. The resulting microbeads display structural color depending on the size of silica particles. The mechanochromic microbeads are further assembled to form 2D and 3D structures, where the interstitial voids of the microbeads are filled with another elastomeric silicone-based polymer with higher modulus than the microbeads. These macroscale assemblies show rotation-independent structural colors and low angle dependency. More importantly, the assemblies show the mechanochromic property as individual microbeads dynamically deform under macroscopic strain.