Colloidal Photonic Inks for Mechanochromic Films and Patterns with Structural Colors of High Saturation

Colloidal arrays show structural colors through wavelength-selective diffraction. The structural colors are dynamically tunable with mechanical deformation for a non-close-packed colloidal array embedded in an elastic matrix. However, such compositions usually render photonic materials transparent and structural color low saturated. In this work, we formulate colloidal inks to produce mechanochromic films and patterns that show consistent structural colors with high saturation. The inks are composed of a high-volume fraction of silica particles and a low fraction of polydopamine nanoparticles dispersed in an elastomer-forming resin. The silica particles have repulsive interparticle potential and form a non-close-packed array, whereas polydopamine nanoparticles are positioned in the interstitial areas. The colloidal arrays are captured in the elastomer by photopolymerization of the resin. As polydopamine nanoparticles reduce incoherent scattering and make the materials opaque, the structural color arisen from the colloidal array is pronounced and independent of the background. Moreover, the photonic materials show a dynamic and reversible change of structural color according to deformation. For large strains, the photonic effect is overwhelmed by absorption of polydopamine nanoparticles, rendering the materials dark brown. This unique mechanochromic property is used to make patterns that are reversibly color-tunable and hidable, which are appealing for user-interactive anti-counterfeiting and active camouflage.