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 microwrinkled 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/m². This excellent tunability of optical diffusion even exceeds that reported by a commercial polymer-dispersed-liquid-crystal smart window.