First, the color
In biological systems, there are two main sources of color, pigment colors and structural colors.
For structural colors, the most common example in life is probably a disc: looking at a disc from a different angle, we can see different colors. This color that changes with the angle of observation is called rainbow color. By placing the disc under the microscope, we can see the tiny structures regularly arranged on the surface of the disc, which are the data recorded on the disc. It is precisely because of these tiny structures that the structural color of the disc is caused.
But not all structural colors are rainbow colors. For example, the color of the blue feathers of the Eastern Blue Plover also comes from the structural color, but does not change with the viewing angle. This blue color is caused by the scattering of light by an irregular microstructure.
Four ways to form a structural color:
The mechanism by which the first structural color is produced is the interference of a single-layer film of light, and the example often seen in life is the color of the oil film on the surface of the water. Different colors of light interfere with different angles of incidence and different thicknesses of the oil film, resulting in a rich color of the oil film.
The second mechanism of structural color is that light interferes in a multilayer film, which is alternately superimposed by two substances with different refractive indices.
The third structural color is caused by photonic crystals, the so-called photonic crystals are microscopic structures formed by the periodic arrangement of two substances with different refractive indexes. For example, the structural color of peacock feathers comes from the photon crystal structure inside the feathers
The fourth structural color is caused by grating diffraction, and the grating structure is usually prepared by laser direct writing, holographic interference, ion/electron beam lithography, and numerical control machining (CNC). The spacing of the grating lines is usually in the order of a few microns or even hundreds of nanometers, and the diffraction and scattering of light are achieved by regulating the parameters of the grating period, angle, depth, refractive index, duty cycle and so on.
Second, the application status of structural colors
1. The team of Wu Limin, professor of the Department of Materials Science of Fudan University, assembled polymer colloidal microspheres with a diameter of several microns to more than ten microns into a single-layer microsphere array on the viscose layer of ordinary transparent polymer tape, and for the first time developed an intelligent response structural color film material with both reverse reflection and intelligent response structural color with angle-dependent heterochrome and non-coloring with angle, and revealed the formation mechanism of its intelligent response structural color.
This kind of reverse reflection structural color film can be applied to the night traffic reflective signs or billboards, because the direction of the headlight lighting and the driver's viewing angle are on the same axis, the driver can observe a uniform, bright reflective color from far to near; and the pedestrians on the side of the road due to the viewing angle and the direction of the lamp lighting are in different axes, with the vehicle from far and near can observe the changing reflective color, as shown in the following figure, so that pedestrians (especially wearing headphones or hearing impaired) can actively avoid the vehicle behind them, 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 1. Schematic diagram of the application of traffic reflective signage prepared by the inverse reflection structural color film on the night road, as well as photos of the pedestrian perspective and the (E-G) driver perspective when the vehicle is at different distances from the sign L (80 meters, 50 meters, 30 meters).
Figure 2. The moving vehicle is away from the billboard by the driver's perspective and (E-H) pedestrian perspective at a distance and close to 80 meters, 40 meters, 15 meters and 10 meters .)
The reason for this novel optical phenomenon is that the polymer colloidal microspheres and the transparent tape form a unique double-layer microstructure based on the air cushion microspheres / polymers, when the white light beam is incident from the microsphere-free side of the film, the light will be in the microsphere into the part and the micro-immersion part of the hemispherical interface, respectively, film interference and total internal reflection effect, so as to achieve the effect of intelligent structural color generation and reverse reflection in turn. By adjusting the particle size of the microspheres, the thickness of the air layer can be controlled, resulting in structural colors of different colors.
2, textile structure coloring structure color textiles, because there is no participation in the coloring process of pigments or dyes, there is no fading phenomenon, no need to consume a lot of water resources for rinsing, but also to avoid the waste of resources caused by the wastewater discharge of rich pigments, dyes and printing and dyeing aids, providing a new idea for ecological green pollution remediation.
The research team directed by Professor Wang Shimin of Hubei University and Professor Xu Weilin of Wuhan Textile University has developed an effective and easy-to-operate carbon fiber fabric coloring method, which is not only color adjustable, but also has excellent washing resistance. The study has been published in ACSNano, a top international journal in the field of materials science.
Compared with the traditional colorants, these methods of giving textiles color completely by structure have the advantages of color water not fading and environmental friendliness, and can be used as a new type of colorant, which has a very wide application prospect in textile dyeing and finishing, coatings, printing, etc., which is of great significance for achieving a green and healthy life. External environmental factors, such as acidity and alkalinity, humidity, temperature, stress, etc., will have a certain regulatory effect on the special periodic physical structure of structural coloring textiles, which produces structural colors of different colors with changes in external environmental factors. This technology of dynamically regulating colors in the visible light range by using changes in external environmental factors provides new ideas for the development of sensor humidity, temperature, and pH detection equipment. In addition, the influence of light intensity on structural color generation in light scattering and the dependence of structural color on the viewing angle can also be used to provide a new idea for the preparation of stealth materials, such as structural color fibers based on photonic crystal structures that can be used as banknote authenticity marks.
3. Structural color oil droplets
Researchers at Pennsylvania State University in the United States discovered this new structural color mechanism while studying a special class of oil droplets. They dispersed two water-insoluble but mutually insoluble organic liquids into water, forming oil droplets that are not uniformly spherical, forming a curved liquid level at the junction of the two organics in the oil droplets. They then found that under the illumination of light, the edges of the droplets that should have been colorless showed brilliant colors. The researchers confirmed the cause of the color through experimental and theoretical simulations: because the refractive index of the liquid with a smaller density above the oil droplet is higher than that of the dense liquid below, when the light is irradiated into the oil droplet from top to bottom, it will be totally reflected at the interface of the two liquids, that is, the incident light is all reflected and will not refract into another medium. Due to the presence of surfaces, after multiple total reflections, the light that hits different parts of the droplet will have a poor optical path, which will interfere, resulting in the oil droplet taking on a specific color.
The oil droplets exhibit a structural color. The rulers are 100 microns (larger) and 2 centimeters (smaller)
The researchers further found that when changing the shape of the surface at the junction of the two liquids in the oil droplet, the structural color of the oil droplet also changed, which made the new display technology possible. For example, they added a special surfactant to the water, the chemical structure of which changes under ultraviolet radiation, which further affects the curved shape of the oil droplets. If you selectively irradiate certain areas of the liquid, you can get two different colors, thus presenting a specific pattern.
Changing the shape of the liquid level in the oil droplet changes the structural color of the oil droplet, thus presenting a certain pattern on the liquid surface. The rulers are 2 cm (center) and 50 μm (right)
4. Professor Xu Ting and Professor Lu Yanqing of Nanjing University led their research group, together with researchers from the National Institute of Standards and Technology, to complete an experiment using natural light structured colors to paint, and they reproduced with ultra-high precision the legendary painting "The Girl with the Pearl Earring" by the 17th-century Dutch artist Jan Vermeer.
The actual size of this "painting" is only about one millimeter, and it is necessary to observe the details through the microscope; in addition, the "painting" does not use any pigments, and only relies on the refraction and transformation of the micro-nano structural material to accurately paint the whole painting with the desired color; and for the first time, the control of the light and dark transition between light and color is realized, making the whole painting look more three-dimensional.
Researchers said that first of all, this is a special way of storing encrypted information, which can be stored on a glass sheet by controlling the output and projection conversion of different wavelengths of light, because each pixel is nanoscale, and the information saved will be very rich.
Secondly, the experiment needs to be adjusted by a special polarizing filter to show a clear and beautiful picture, if this filter is removed, the pattern presented is completely different, and because the original design and production of this micro-nano structure is quite complex, it is difficult to imitate, to a certain extent, the technology can be used to do high-value items anti-counterfeiting, to some items to encode images and text, this "anti-counterfeiting" logo only through some special means, can see its true appearance.
5, micro-nano structure color packaging (no ink prints, micro-nano texture) any product is inseparable from packaging, any packaging is inseparable from printing, and excessive printing is often contrary to green environmental protection, some are even harmful. For example, organic solvents such as ethanol, toluene, and xylene are often used in printing inks. Most of the above organic solvents will be discharged into the air after drying to pollute the environment, and the residual parts will also cause harm to the consumer's body. In particular, prints with a large inking area and a thick ink layer have more residual solvents, and the toxic substances released during use pollute the air and endanger people's health.
Inkless printing believes that the use of environmentally friendly materials, processes, and simplifying the process, reducing environmental pollution, reducing manufacturing costs, is the biggest demand of enterprises.
Inkless printing is a phenomenon based on the reflection, refraction, diffraction, scattering and other phenomena of light generated by the surface micro-nano structure, which is used to express the color, contrast, dynamic, three-dimensional and other information of the image text, which is the structural color mentioned above. This microstructure is carried on glass, plastic and even sheet metal and can partially or completely replace the traditional ink printing process. It not only has the advantages of technology, fashion, environmental protection, low cost, etc., but also makes the texture more refined, rich and flexible. This is not only a breakthrough at the technical level, but more importantly, a change in concept.
Ink-free printed medicine packs
Inkless printed maca sachets
Inkless printed e-cigarette pack
Ink-free printed cigarette packs
Suzhou Impression Technology will be environmental protection, fashion, anti-counterfeiting perfect integration, abandon the kind of overemphasization of the product design of the thick makeup ink printing, with a more responsible attitude and method to create a new form of packaging prints, with a more concise, durable shape to make the product as long as possible to extend its service life, while conveying the green, humanistic spirit concept, for environmental protection printing and packaging, to ensure consumer safety to make their own contribution.
Third, about the future of structural colors
The application of structural color is not limited to this, as a subversive color presentation technology, the uniqueness of structural color makes it in the fields of printing, display, spraying, anti-counterfeiting and other fields will usher in a broad application prospect, in the field of national defense and military, its application is even more potential.
Using the principle of unique color, we will promote military technological changes such as stealth and camouflage. Structural color is a color expression that can be finely controlled by light waves (that is, electromagnetic waves), which can show important military value in stealth, camouflage, three-dimensional imaging, helmet-mounted display, artificial intelligence, virtual enhancement and virtual reality, light information processing and other aspects of the regulation of electromagnetic wave frequency (wavelength), amplitude, polarization, spin and orbital angular momentum. A foreign research institute can achieve stealth when detected by changing the spacing of nanoparticles in the dye so that it only absorbs or scatters light of a specific color. This new type of technology, known as "photon dye", if widely used in military equipment spraying, will bring about technological changes such as military stealth and camouflage, thereby greatly improving the self-protection ability of military equipment and the concealment of military operations.
Through the fine design of the structure, wearable intelligent equipment is developed. The structural color usually belongs to the multi-layer microporous structure, through the fine design, this special structure can allow liquids or gases to flow in, and let it achieve internal circulation, so that the body equipment in different temperature, humidity conditions, still has excellent thermal insulation and breathability. At the same time, periodic hydrophobic or oil-trapping particles can be introduced on the surface of military uniforms and camouflage materials to create functional clothing with both camouflage ability and waterproof and oilproof ability.
Sources for this article:
1. Zhu Xiaowei, Xing Tieling, "Research Progress of Structural Colored Textiles", Journal of Textile Science and Engineering, Vol. 37, No. 4 (No. 138 in total);
2. "Scientists Develop New Intelligent Display Structural Color Film Materials", China Science News, No. 11, 2019: 111;
3. Some of the content is reproduced from the network, if there is infringement, please contact the editor.