Structural color: Magical presentation without pigmentation

●It is a color expression without pigment ● It has magical characteristics such as high saturation, never fading, and color controllability ● It has broad application prospects in military camouflage and military stealth

Without ink or paint, you can paint a colorful and realistic picture! Is this a fantasy, or is it the rebirth of Ma Liang?

Neither. It is a color rendering technique called "structured color". The magic of it is that without any pigment, it can express colorful colors, and the brightness is higher, the sense of layering is stronger, and the color is richer.

How are structural colors different from ordinary colors, and what scientific mysteries are hidden in them? Now, let's unveil its mystery.

Hidden in nature, it comes from discovery

A thousand worlds, colorful. Since its birth, human beings have been full of love for color. As early as 40,000 years AGO, ancestors began to paint murals using primitive methods such as heating loess and grinding colored ore or plants to make colorful pigments. But in the development process of the following tens of thousands of years, people did not have a clear and profound understanding of color.

One day in the mid-17th century, a ray of sunlight shone through a window into the physicist Newton's laboratory, and when this beam of light penetrated into the small prism that Newton was holding, an important discovery was made - the colorful colors of nature are actually the response of the human eye to different wavelengths of light, and the original color is associated with light. After the birth of the microscope, Newton and Hooke two physicists observed the relationship between peacock feather color and light through it, and found that in the peacock's beautiful feathers, in addition to having pigments similar to those in traditional pigments, there are a large number of branches that reflect light, and the reflected color is closely related to the arrangement and thickness of these branches. At the end of the 19th century, the British zoologist Frank first fully explained the mystery of the color of different animals in nature: the color of animals is either the presence of a definite pigment in the skin, or the optical effect caused by the scattering, diffraction or uneven refraction of light - the former is called pigment color, the latter is called structural color.

Pigment color is the color that is intuitively presented after the absorption or reflection of light by a single substance, while the structural color is a variety of colors produced by a large number of ordered structures scattering, diffraction or interfering with light at different wavelengths. Like a pigment color, it originally existed in nature, but it was discovered later because it was hidden deeper. Also at the end of the 19th century, the French physicist Gabriel Lippmann used the principle of structural color to invent the color photographic interference method, that is, the original color of the object can be highly restored on black and white photos without dyes, and its "natural color photography technique using interference phenomena" won the Nobel Prize in Physics in 1908.

In the long history of more than a century, with people's deep understanding of light and the maturity of modern micro-nano scale processing technology, this "structural color", which scientists call subversive color presentation technology, began to show its magical and fascinating scientific brilliance.

Magical features Subvert tradition

Science is about discovery. The discovery of structural color has made people understand that the colorful colors in nature are both pigment colors obtained through the absorption or reflection of light by pigments, and structural colors obtained through the combination of scattering, diffraction and interference of light.

Compared with traditional pigments, structural color is a color expression that does not require pigments, which is based on the principle of physical optics, processing materials into periodic structures at the micro-nano scale. Due to the resonant properties of micro-nano structures, the resonant wavelength is affected by the size and period of the structure, and can scatter light of a specific color on the surface of the material under the illumination of white light. Recently, a Japanese research institute can depict a high-definition picture without ink and pigment by changing the structure of the drawing "paper" (a polymer that can be synthesized artificially), and its pattern resolution is 3 times that of traditional inkjet printing.

Compared with the traditional pigment color, the unique coloring principle of the structural color makes it have unique magical characteristics, which are mainly reflected in the following three aspects:

Bright colors and high saturation. Structural colors are highly wavelength-selective, so the display of specific colors can be achieved by controlling the surface structure of the material. Traditional drawing or screen display is generally based on the three primary color mixing scheme, that is, through the appropriate collocation, the construction of various other colors, but this color method is essentially a kind of "false color". Because on the colored surface, there is no real scattering of light corresponding to the wavelength of the color under view. Structural colors, on the other hand, can scatter any high-purity color as needed to achieve true "full color", so that the rendering effect is more vivid and full.

Clean and environmentally friendly, never fade. The production of structural colors is based on the processing of raw materials at the microscopic scale, and common manufacturing techniques include: electron beam lithography, magnetron sputtering radio frequency method, vacuum nano-evaporation method, solution coating method and physical deposition method. These processing methods completely abandon the traditional coloring method using dyeing cylinders or coatings, and by improving the properties of raw materials, the structural colors can be more durable to cope with harsh environments such as strong light radiation and acid and alkali corrosion. Therefore, the surface processed by structural color can not only maintain the original luster for a long time, but also greatly reduce the harm of chemical paint to the environment and human body during its production process.

Color controllable, adjustable polarization. Different from the characteristics of chemical dyes "coloring is stereotyping", structural colors use the influence of the small structure on the surface of the material on the beam to achieve the presentation of different colors. Because the tiny units in the structural color can be flexibly controlled by means of external force deformation, electromechanical control, etc., so that the light waves scattered on the surface of the material can be flexibly controlled. In particular, the tiny structural units arranged in cycles can also realize the polarization control of the light field, similar to letting the scattered photons "hold hands" and vibrate together in a specified direction to form an "optical fingerprint" unique to the material. This magical property of structural colors will open up new avenues for optical anti-counterfeiting, three-dimensional imaging and other technologies.

Military applications have great potential

As a subversive color presentation technology, the uniqueness of structural color makes it a broad application prospect in printing, display, spraying, anti-counterfeiting and other fields, and its application in the fields of national defense and military 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, the battlefield 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. At present, a technology called "nano-coloring" has been successfully used, and its products have unique gradient colors, angle colors, double-color colors, metallic colors, etc., and have waterproof, antibacterial, sunscreen, antioxidant, acid and alkali resistance and conductive shielding functions. In addition, this feature can also be applied to the manufacturing of medical wearable detection equipment to achieve real-time monitoring of the physiological state of battlefield personnel.

Using the characteristics of high brightness, high saturation and controllable polarization, we develop holographic color printing anti-counterfeiting technology to improve the anti-counterfeiting performance of documents and protect identity information security. According to reports, a research team in Singapore used the principle of structural color to achieve color image display under white light by designing nanotubes of different heights on the surface of the material. Compared with traditional ink printing, this holographic color printing anti-counterfeiting technology not only has the advantages of ultra-high printing resolution and never fades, but also more amazingly, when this printing material is illuminated by the laser, it can project 3 set images on the distant screen. This technology has broad application prospects in the field of military security such as identity information protection and anti-counterfeiting of secret-related documents.

Expert biography: Yang Junbo, professor and graduate supervisor of the College of Arts and Sciences of the National University of Defense Technology, standing committee member of the Optoelectronic Technology Professional Committee of the Chinese Society of Astronautics, member of the China Society of Micro and Nanotechnology. He has presided over more than 20 national and military key scientific research projects, published more than 120 papers, and selected the research results as important achievements in Chinese optics.

Above: Structural color on bird wings.