Silk fibers may be the newest high-tech material on the market. Researchers from Purdue University have discovered that silk produces Anderson localization of light when electrons come to a halt in materials because of their scattering and defects. This could lead to new inventions and further understanding of heat transfer and light transportation.
The nano-architecture of silk has been proven to be capable of light confinement. Light confinement can be used in a variety of areas, including the medical industry for biosensing. The researchers discovered that silk’s light confinement effect works in biological and natural tissue due to the Anderson localization of light.
Synthetic metamaterials have been developed with the ability to ultra-efficiently control light, but they are limited to non-commercial production. On the other side, silk has a distorted nano-architecture that makes it easier and cheaper to produce for commercial and industrial production.
"This is fascinating because realizing Anderson localization of light is extremely challenging, yet we now know that it can be achieved using irregular, disordered nanostructures to create highly packed nanomaterials for strong light scattering as a silkworm produces a silk fiber and spins a cocoon shell in nature," said Young Kim, associate professor at Purdue University’s Weldon School of Biomedical Engineering.
A human hair is around 100 microns in diameter. Silk fibers are 10-20 microns in diameter, which is incredibly small. Silk fibers are made of thousands of tiny nanofibrils of around 100 nanometers wide. This size and structure is a huge player in silk’s ability to control light.
"Silk has many nanofibrils, which individually scatter light," Kim said. Anderson localization results from the “scattering centers” that are inside silk’s nanostructure.
In order for Anderson localization to happen there has to be scattering and interference between scattered light waves. In silk, there are densely packed irregular nanostructures that cause light waves to interfere with each other. This is sometimes either constructive and destructive. If the light waves are constructive, the light is intense. The scattering power is at its peak when a material has many scattering centers and when the size is similar to the wavelength of light, which is why silk is so successful.
"We found that most transmission of light disappears in most of the silk surface. However, counterintuitively, in a small area we found that the energy is confined, and this confined energy is transmitted through localized modes," Kim said. "The localized mode is a unique pathway for energy flow."
"Such a difference makes silk particularly interesting for radiative heat transfer," Kim said. The silk has a high emissivity for infrared light, meaning it readily radiates heat, or infrared radiation, while at the same time being a good reflector of solar light. Because the strong reflectivity from Anderson localization is combined with the high emissivity of the biomolecules in infrared radiation, silk radiates more heat than it absorbs, making it ideal for passive cooling, or "self-cooling."
To read more about this development, visit the Purdue site here.