The colorful black, the ever-changing white, the brilliant green... have been passed down for 100,000,000 years?

Audit expert: Luo Huiqian

Associate Researcher, Institute of Physics, Chinese Academy of Sciences

The natural world is full of colorful colors. Whether it is lush green trees, clusters of red flowers, or the birds and butterflies flying among them, they all add a lot of beauty to our world. Their colors are fresh, bright, lively, and dazzling. So how do these beautiful colors appear?

Sources of Color Generally speaking, there are three sources of color in nature: bioluminescence, chemical color and structural color. Bioluminescence refers to the conversion of chemical energy in organic matter into light energy and release of it by some organisms under aerobic conditions through the action of catalysts. Fireflies, jellyfish and other bioluminescent organisms belong to this category.

Source: unsplash

Chemical colors rely on pigments. In nature, most of the colors we see are chemical colors, which will fade or change color due to the loss or change of pigments. For example, the color of flamingos comes from their food, algae and plankton, which contain carotenoids. After flamingos eat them, the accumulation of carotenoids makes their feathers appear scarlet. Once flamingos stop eating this kind of food, their color will slowly change back to gray or white.

Source: unsplash

Structural color depends on the surface structure of organisms. When the viewing angle changes or the surface structure changes, the colors we see will be slightly different. For example, the color of butterflies and birds' wings will change during vibration. Another example is that when people are grinding and polishing amber and insects, if they accidentally damage their structure, the metallic color preserved in amber and insects for hundreds of millions of years will be destroyed.

Source: pixabay

In simple terms, the principle of structural color is the feedback of the surface structure of organisms to light. Each structural color is the product of an optical phenomenon. The surface structure of organisms is different, so the colors displayed are also ever-changing.

There are three types of structural color: thin film interference, diffraction effects, and selective scattering.

The structural colors of organisms mostly come from thin film interference. The colors produced by thin film interference are related to the number of layers of the biological surface structure and the structural morphology of each layer. The more layers there are and the more complex the structure, the more obvious the interference between the reflected light and the emitted light after the light hits the surface, and the brighter the structural color on the surface.

Thin film interference source: Baidu Encyclopedia

The structural color produced by the diffraction effect mainly comes from the Bragg diffraction effect produced by the regular structure of biological surfaces. When light encounters the regular surface structure, it is partially absorbed, and the remaining light changes its propagation direction, refracting or reflecting, and after superimposing and interfering with the light in the same direction, a new optical effect is produced, which is the structural color.

Schematic diagram of surface structure Bragg diffraction Source: Baidu Encyclopedia

Irregular biological surface structures scatter when illuminated by light. Scattering that can produce structural colors is usually Rayleigh scattering or Mie scattering. When the particle size is smaller than the wavelength of visible light, Rayleigh scattering mainly occurs, and the scattered color component is mainly blue. When the particle size is approximately equal to or larger than the wavelength of visible light, Mie scattering occurs, and the scattered colors are mainly red and green.

The application of structural color is based on a special color rendering principle. Structural color has many advantages, such as clean and environmentally friendly, long-lasting color fixation, and controllable color. People can control the surface structure of objects through various means to make them show specific colors. This type of technology has been applied in many fields.

In the military field, by spraying special dyes on various equipment to make them absorb or scatter light of specific wavelengths, the equipment can be made invisible in front of radar. In addition, by making full use of the multi-layer microporous structure that produces structural colors, smart wearable devices can be manufactured that have both camouflage functions and breathable and heat-insulating properties.

In the field of industrial production, structural color fibers can be made by carefully designing the surface structure. Clothes woven from such fibers do not need to be dyed. On the one hand, this can avoid the harm to the environment caused by dye wastewater in production, and on the other hand, it can prevent clothes from fading after repeated washing.

Horizontal observation of Winter Olympics commemorative coins

Vertical observation of Winter Olympics commemorative coins Source: Internet

In the field of life, structural colors are also widely used. For example, the Winter Olympics commemorative banknotes issued previously demonstrated the development concept of "Green water and green mountains are gold and silver mountains" by implanting nano-microstructures. When observed horizontally, the banknotes show "green water and green mountains", while when observed vertically, "green water and green mountains" become "gold and silver mountains". This technology not only enhances the aesthetic value of banknotes, but also integrates the concept of sustainable development into it, giving full play to the environmental protection characteristics of structural colors.

In addition, a study published in the journal Science Advances showed that a Chinese and American research team developed a new type of structural color material, which may be used in the field of transportation in the future. The researchers conducted an experiment to cover the intersection with this material. Due to the different observation angles of pedestrians and drivers, the signals received were also different, which could play a warning effect.

The colors created by nature always amaze us, and while we explore these colors step by step, we are also constantly learning the magic of nature's creation. In this process of exploration-learning-exploration, we have added many gorgeous colors to the world.