Switchable passive radiative cooling via mechanical stress for stretchable smart window

Abstract

We report a mechanically stretchable smart window passive radiative cooler (SW-PRC) that can reversibly switch between opaque and transparent states in response to external mechanical stress while maintaining efficient heat dissipation through the atmospheric window in both optical states. To fabricate the SW-PRC, we modify the surface of SiO2 nanoparticles (NPs) using trimethylchlorosilane (TMSCl). The surface-modified SiO2 nanoparticles (SM-SiO2 NPs) exhibit hydrophobic properties, preventing agglomeration of SiO2 NPs within a polydimethylsiloxane (PDMS) matrix and achieving a homogenous SW-PRC with a low haze factor (20–30%) and high visible-light transmittance (≥ 90%). These composites also demonstrate high emissivity in the long-wave infrared region, confirming their radiative cooling capabilities. When the SW-PRC is stretched by more than 5%, air voids form around the SiO2 NPs, causing visible-light scattering and resulting in hazy, non-transparent films. This optical conversion between transparent and hazy states can be repeated over 20 times. The switchable SW-PRC has shown a temperature drop of 3.25 °C in its unstretched state and up to 4.75 °C when stretched by 30%. We believe that our switchable SW-PRC represents an advancement in overcoming the limitations of current smart window applications.