Water Induced Reversible Switching of Optical Transparency of an Elastomer-Hydrogel based Two-phase Composite

We have examined reversible switching of optical transparency of a two-phase material consisting of hydrogel micro-spheres dispersed inside a continuous medium of crosslinked poly(dimethylsiloxane) (PDMS). Here, aqueous solution of gelatin is dispersed as drops in PDMS oligomers mixed with the crosslinker. Both phases get crosslinked to form an elastomeric material. Films of this material remain optically opaque in the unswelled state of the embedded hydrogel, but, when placed inside water for prolonged period, water diffuses through the silicone phase and gets absorbed by gelatin. At this, gelatin swells with consequent change in transparency with time. We show that the size of dispersed microspheres and the extent of crosslinking of the PDMS, both affect the kinetics of this process. In fact, for some range of parameter values, the rate of change in transparency turns more pronounced than would have been expected for simple Fickian diffusion of water through the film. We show also that selective swelling of the dispersed phase is necessary for the film to get transparent. Furthermore, it is possible to dye the dispersed phase with a water-soluble dye implying that the dye molecules can co-diffuse with water through the network. We highlight also few potential applications of this material.