Mechanically tunable opacity effect in transparent bilayer film: Accurate interpretation and rational applications

Abstract Transparent bilayer films, consisting of a stiff layer and a compliant layer, may gradually be tuned into an opaque state under mechanical stretching, which is interesting and is useful in intelligent window. However, an accurate interpretation of this effect is still absent, and there is no rational guidance for well realizing its applications at present. Here, we demonstrate that this effect is mainly induced by the diffractive behaviors of the wrinkles emerging and evolving in stiff film under stretching. Changes in the 0th order diffracted beam efficiency, which depend on mechanically tunable wrinkle amplitude, are the primary reason for the tunable property of this effect. The tunable rate of wrinkles amplitude change is proportional to the stiff layer thickness. Therefore, a guided approach to the application of this kind of bilayer film involves rationally combining stiff layer thickness and stretching percentage. We show the practical applications of this kind of bilayer films in window display. The films with different stiff layer thicknesses need different minimum stretching percentages to tune into completely opaque, while the wrinkle patterns induced by these stretching with the similar amplitude (1100 nm). This work accurately interprets the opacity effect in bilayer films and further proposes theoretical guidance for its application, which may efficiently promote the fabrication of new bilayer films and their rational use in light tuning field.