Wu Limin research group of Fudan University has made important progress in the research of color materials for intelligent display structures

Compared with the traditional chemical color (such as pigment, dye and so on), structural color has the characteristics of high color reflectivity, high saturation, not easy to fade, environmental protection and so on However, the existing reflection types of natural or synthetic structural color materials are basically diffuse reflection or specular reflection, and few of them have the third type of material surface reflection - retroreflection, that is, the phenomenon that reflected light shoots back to the light source direction Moreover, these structural color materials usually only have the characteristics of color change with or without angle, which is difficult to have the characteristics of color change with angle and without angle at the same time However, the color of existing commercial retroreflective materials mainly comes from chemical pigments, which not only causes serious pollution in the process of production and use, but also fades and has no intelligent response function To solve this problem, Fudan University Wu Limin's team assembled polymer colloidal microspheres with diameters ranging from several microns to more than ten microns onto the viscose layer of common transparent polymer tape to form a single-layer microsphere array For the first time, a kind of intelligent response structured color film material with both reverse reflection and angular heterochromism and angular non discoloration was reported The formation mechanism of intelligent response structured color was revealed, and its intelligent response structure color was explored Display and other applications The related results were published online on science advances (2019, 5, eaaw8755), a major subsidiary of science, under the title of idridescence controlled and flexible tunable retroreflective structural color film for smart displays at 2:00 p.m EST on August 9 German Public Radio Science and technology channel conducted a telephone interview on the achievement, and Xinhua news agency, Washington branch and other media also reported on the achievement The large-area retroreflective structural color film developed by Professor Wu Limin's team shows colorless on the film surface with sphere and the film surface without sphere in indoor light, but under the illumination of white light source, the film surface without sphere shows retroreflective intelligent structure color : that is to say, uniform and bright retroreflective structural color can be observed from the direction of illumination, and it does not change color with the angle; while when observing from the direction of non illumination, the film shows different color with the change of illumination angle or observation angle, and there is no report about the optical properties of nature and synthetic materials The research shows that the reason for this new optical phenomenon is that a unique double-layer structure based on air cushion microspheres / polymer is formed between polymer colloidal microspheres and transparent tape The air layer is formed between the colloidal microspheres and the adhesive layer due to the stress contraction of the colloidal microspheres and the adhesive layer polymer due to the different elasticity The microspheres are suspended in the air layer through the connection of the viscose microfilaments and the viscose layer to form an array When the white light beam is incident from the non microsphere side of the film, the light will have film interference and total internal reflection effects on the hemispherical interface of the immersed part and the non immersed part of the microsphere respectively, so that the generation of intelligent structure color and the effect of reverse reflection can be realized in turn The thickness of the air layer can be controlled by adjusting the particle size of the microspheres, so that different color structure colors can be produced Fig 1 Preparation steps of (A-D), (E-I) structure and optical properties of retroreflective structural color films, and schematic diagram of (J) no color change with angle when illuminating / observing coaxial and (k) no color change with angle when illuminating / observing different axes (source: Science Advanced) the retroreflective structural color film has important application value in intelligent display, traffic safety reflective facilities, anti-counterfeiting labels, structural color coating and decoration and other fields For example, when the film is applied to traffic reflective signs or billboards at night, the driver can observe uniform and bright reflective colors from far to near because the direction of light illumination and the driver's perspective are in the same axis; while the pedestrians on the roadside can effectively remind the pedestrians because the angle of view and the direction of light illumination are in different axes and the changing reflective colors can be observed with the vehicle from far to near (especially those with headphones or hearing impairment) actively avoid vehicles in the rear to avoid traffic accidents Through certain lighting and observation methods, the film or coating can also realize other intelligent traffic display functions, such as traffic signal signs with interactive color change and flashing functions Figure 2 (a) schematic diagram of the application of traffic reflective signs made of retroreflective structural color film on the road at night, as well as the photos of pedestrian angle of view (B-D) and driver angle of view (E-G) when the vehicle is at different distance L (80m, 50m, 30m) from the signs (source: Science Advanced) Figure 3 The distance between the moving vehicle and the billboard changes from 80 meters, 40 meters, 15 meters and 10 meters (A-D) driver's perspective and (E-H) pedestrian's perspective Zeng Jing, a postdoctoral student and doctor of the Wulin research group, and Professor Gan qiaoqiang, a professor of the State University of New York at Buffalo, are the co first authors of the paper, Professor Wu Limin is the corresponding author of the paper, and the team of Professor Shi Lei from the Department of physics has participated in the discussion for many times The research work has been jointly supported by the national key research and development plan, the National Natural Science Foundation of China and the State Key Laboratory of polymer molecular engineering of Fudan University The research has applied for one patent from China and one patent from the United States.