Water Induced Reversible\nSwitching of Optical Transparency\nof an Elastomer-Hydrogel Based Two-Phase Composite

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