Structural Inclination for Next-Generation Mechano-Responsive Smart Windows

By implementing smart window technologies to regulate temperature and lighting conditions, an estimated 50% reduction in energy consumption for building services can be achieved. Promising candidates for smart windows are mechano-responsive membranes that exhibit tunable transmission through strain-controlled light scattering.<br/><br/>However, the state-of-the-art light scatterers have encountered a bottleneck. Achieving appreciable contrast still necessitates large strains which require additional space, occupying more than 15% of the original window area. Furthermore, recent improvements in contrast and strain reduction have come at the cost of complexity in composition and fabrication. Therefore, a new design principle is urgently needed to simultaneously achieve high visibility, strong regulation of irradiation, low operational lateral strains, and simplicity in composition and fabrication at a low cost.<br/><br/>One potential solution lies in manipulating pore sizes within a 3D structure through out-of-plane compression. The reversible opening and closing of pores provide the scatterer with opacity when released and transparency when compressed, accommodating changes in the in-plane area. This working mechanism inherently addresses the issue of large areal changes. The performance of a compression-driven scatterer relies on the optical density in the released state and its sensitivity to compressive strains. To tackle this, submicron 3D structures with inclined pores are proposed for the new scatterer. The array of submicron pores induces strong multiple scattering of visible light, resulting in a high optical density in the released state and thus a high achievable transmittance contrast. A novel 3D patterning technique based on a slanted exposure angle enables the realization of tailorable inclination angles in submicron structures. Experimental and multi-physics simulation results demonstrate that the inclined structures redistribute local compressive strains, facilitating pore closure and significantly increasing transmittance for a given compression. Scatterers based on the inclined 3D structures achieve high transmittance of 96% and an unprecedented transmittance contrast of 95%, all while experiencing minimal areal strains during operation. Compared to conventional driving modes, the out-of-plane compression mode brings the scatterer closer to practical use, enabling localized modulation of transmittance by selectively compressing specific positions of the scatterer that need to be transparent. The design principle and fabrication technique presented in this work for the compressive strain-responsive scatterer are expected to drive advancements in smart windows, transparent displays, and flexible pressure sensors.