Smart Window Based on Electric Unfolding of Microwrinkled TiO2 Nanometric Films

Flat glass is clear while frosted glass with a microrough surface appears translucent by light scattering. Motivated by this optical surface scatterer, a transparent soft media with electrically tunable surface roughness could make a low-cost smart window. Recently, a dielectric elastomer actuator with micro wrinkled transparent compliant electrodes was proposed to make such a smart window, but its active microwrinkled area occupied only a small fraction of the window device, as it needs room for large-strain unfolding. To solve this issue, this paper first shows transparent-to-translucent switching by a low-strain-induced microwrinkling and the reverse by electric-field induced unfolding. This is possible by using an electrically tunable optical surface scatterer that consists of a TiO2 nanometric thin film of high modulus and high refractive index that is sandwiched between a nanometric conductive overcoat of transparent conductive polymer, that is, poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) and a dielectric elastomer substrate. In-line transmittance of these optical metasurfaces decreases from 81% to 1.85% under a 4-5% radial compression. These microwrinkled surfaces can return to being clear by reversible unfolding upon voltage-induced areal expansion. This tunable window device survives 1000 cycles of microwrinkling and unfolding despite the fact that bulk TiO2 is brittle and harder than an elastomeric substrate. In addition, these TiO2 interfaces greatly reduce the current leakage and enable high-voltage activation of the device at a strikingly low power of 0.83 W/m(2). This excellent tunability of optical diffusion even exceeds that reported by a commercial polymer-dispersed-liquid-crystal smart window.