February** Dynamic Incentive Program
Blueberries are a delicious fruit and a very healthy food, rich in antioxidants and vitamins. But have you ever wondered, why are blueberries blue? Is there anything special in their skin? Or is there some wonderful structure on their surface? Today, we're going to take a look at this question and see how physics can help us explain this natural phenomenon.
First of all, let's be clear that blueberries don't have a blue pigment in their skins. In fact, their skin contains a substance called anthocyanins, which are dark red in color, far from blue. So, why do we see blue? This is thanks to the surface of the blueberries, which is their wax layer.
The wax layer of blueberries is made up of many tiny crystals, and the shape and arrangement of these crystals are random and irregular. When light hits these crystals, they are scattered. However, not all light rays will be scattered equally, but depend on the wavelength of the rays. We know that visible light is made up of light rays of different colors, from red to violet, with decreasing wavelengths. These crystals are exactly the same size and shape as the wavelengths of blue or ultraviolet light, so they scatter these rays the strongest and the other colors less effectively. Therefore, when we look at blueberries, we mainly see the blue or ultraviolet light scattered from their surface, not the red pigment inside their skin.
This phenomenon is called structural color, that is, color is not determined by the properties of the substance itself, but by the structure of the substance. Structural colors are common in nature, such as peacock feathers, butterfly wings, and even rainbows, all of which are produced by structural colors. Structural colors are characterized by the fact that they do not change with the aging or damage of the substance, and it can produce very vivid colors that are more eye-catching than pigment colors. This is useful for some animals or plants, as it can help them attract mates, or warn enemies, or attract pollinators. For example, the blue color of blueberries can attract some birds that are sensitive to blue or ultraviolet light, let them eat blueberries, and then take the blueberry seeds to other places to help blueberries reproduce.
In order to further understand the mechanism of blueberry structural color production, a detailed optical and structural analysis of the epidermis of blueberries was carried out in a paper published in Science Advances.
The researchers first measured the reflectance spectra of their epidermis and found that they all had a blue or ultraviolet peak and no other colors. This shows that their color is produced by structural colors, not by pigments. To verify this, the researchers removed the wax layers with an ethanol solution and found that they all turned dark red or black in color. The researchers also looked at the topography of the epidermis with a microscope and found that their wax layer is made up of many irregular tiny structures that range in size from a few hundred nanometers to a few microns, comparable to the wavelength of visible light. The shape and arrangement of these structures are random, with no apparent periodicity or symmetry. This suggests that the structural color of the wax is produced by a disordered structure and not by an ordered structure.
In order to further understand the mechanism of the formation of the structural color of wax, the researchers used numerical simulation to calculate the scattering characteristics of the wax layer. The researchers used the random ball pile model to simulate the structure of the wax layer, which assumes that the wax layer consists of a number of randomly distributed spherical structures that are random in size, position, and orientation. The researchers used the finite-difference time-domain method (FDTD) to solve for the scattering field of the wax layer, and then used the scattering matrix method (SMM) to calculate the reflectance spectrum of the wax layer.
The researchers found that when the size of the spherical structure was 05 to 1At 5 μm, the reflectance spectrum of the wax layer can be in good agreement with the experimentally measured spectrum, while when the size of the spherical structure exceeds 2 μm, the reflectance spectrum of the wax layer will deviate significantly. This suggests that the structural color of the wax is caused by the scattering of spherical structures, and the size of the spherical structures must be within a suitable range to produce blue or ultraviolet reflection peaks.
The researchers also used the scattering theory method to analyze the scattering mechanism of the wax layer, and found that the scattering of the wax layer is mainly composed of two parts: form factor and structure factor. The shape factor describes the scattering properties of a single spherical structure, while the structure factor describes the coherence effect between spherical structures.
The researchers found that the shape factor contributed more to the scattering of the wax layer, while the structure factor contributed less to the scattering of the wax layer. This means that the scattering of the wax layer is mainly determined by the scattering of the individual spherical structures, and the coherence effect between the spherical structures is negligible. This is different from the case of ordered structural colors, which often need to consider the influence of structural factors because of the strong coherence effect of ordered structures.