Water Induced Reversible Switching of Optical Transparency of an Elastomer-Hydrogel Based Two-Phase Composite

Dynamically tunable optical transparency has many applications: smart windows, camouflage coatings, sensors, and displays. To this end, switchable optical properties of complex materials like metamaterials and nanostructured surfaces, induced essentially via scattering, refraction, and reflection of light, have been explored extensively. However, the cost of making these materials and their complex design limit their applications. To address this need, we have presented a two-phase composite (TPC) material in which a hydrogel, e.g., gelatin, is dispersed as particles in a continuous medium of poly(dimethylsiloxane) (PDMS). Reversible switching of optical transparency is achieved by selective swelling of the embedded gelatin phase inside a pool of water and deswelling it by drying. In essence, an aqueous solution of gelatin is dispersed as fine droplets in a pool of PDMS oligomers mixed with its curing agent (elastomeric phase) and is then cured. The cured film turns opaque. When immersed in water for long enough, water diffuses through silicone and gets absorbed by gelatin. At this, gelatin swells with a consequent change in transparency. The rate of change in transparency is found to be a function of gelatin loading in the TPC material, gelatin drop size, and the thickness of the film. Within a certain parameter range, this rate turns more pronounced than would have been expected for simple Fickian diffusion of water through the film.