3, Donghua University, Shanghai, , China
2, Lawrence Livermore National Laboratory, Livermore, California, United States
Smart windows that can switch transparency-opacity in response to surrounding environment have been demonstrated from chromogenic materials, polymer dispersed liquid crystals, and suspended particle devices driven by electrical field, light, and temperature. Previously, we show a stretchable smart window film that is responsive to mechanical stimuli. However, there is a need to fine-tune the range of the strain and the degree of the transparency change in a low-cost fashion. Here, we report a new design of smart window consisting of a elastomer film from polydimethylsiloxane (PDMS) with wrinkles on one side and silica nanoparticles (NPs) on the other side of the film. It allows us to take advantage of the transparency/opacity change of individual components and/or their combinations on demand. Since silica and PDMS have similar refractive indices, when stretching the composite film below the wrinkle pre-strain, the increase of transparency is mainly attributed to the change of wrinkle geometries. When the film is further stretched, air-pockets are created around the silica NPs, leading to increased opacity due to the scattering effect from air/silica NP interface. In the meantime, secondary wrinkles are created in the direction of stretching, thereby further increasing opacity. Together, a larger yet tunable drop in transparency is observed compared to that from either the pure wrinkled film or a simple NP/PDMS composite film. We also investigate how the transmittance of the films can be varied by the size of silica NPs and/or wavelength of the wrinkles.