A colloidal gel developed at the National Institute of Standards and Technology (NIST) has demonstrated that it can control structural color and light transmission. Called SeedGel by its creators, the material was originally developed for industrial and bioengineering use. The researchers have now found a new use for the gel as a temperature-sensitive light filter. According to researcher Yun Liu, a scientist at the NIST Center for Neutron Research (NCNR) and a professor at the University of Delaware, their previous work found that SeedGel can transform from clear to opaque and back again, though at the time they didn’t look at what it could do with color. When white light is shined on the gel, a specific wavelength is allowed to pass through it. The wavelength that is allowed through depends on the gel’s temperature. A temperature change of less than one-tenth of a °C can be enough to change the wavelength that can pass through the gel. The gel’s temperature can be adjusted to provide dynamic color response over an extensive range of wavelengths. It can transmit any color in the visible range and some colors from the ultraviolet (UV) and infrared (IR) spectra. In part, the gel’s versatility of potential uses is owed to its complex set of interconnected microscopic channels that form within it. The gel is a system of charged silica nanoparticles dispersed in a binary solvent of 2,6-lutidine and water. The mixture becomes a solid gel above 26 °C and interlocking microscopic channels form within it. These channels offer passageways for other materials to travel through and give the gel a high amount of internal surface area. When it is initially formed, the gel is transparent. As its temperature rises, it becomes opaque to all but individual colors. When first heated, the gel allows shorter, bluer wavelengths to pass through. As it grows hotter, it transmits progressively longer, redder wavelengths. Once the temperature range is exceeded, the gel grows opaque to all visible light. Neutron scattering experiments performed at the NCNR showed that the change in temperature causes an exchange of liquid molecules between the microscopic channels in the gel. This alters the overall refractive index of the channels, allowing one wavelength of light to be transmitted based on temperature and causing all other wavelengths to scatter. To demonstrate that the color is dynamically tunable, the researchers designed an experimental setup with both a scattering mode and a transmission mode. In the scattering mode, the researchers placed a white-light source on one side of a sample and positioned the camera at an angle on the other side to capture the scattered light. The sample remained transparent while its color transitioned from blue to yellow when the temperature decreased from 28.5 to 27.5 °C; but it became increasingly opaque when temperatures went above 29.7 °C or below 26.9 °C. In the transmission mode, the researchers placed red and blue light on one side of the sample and a camera on the opposite side to capture the transmitted light. Long-wavelength (red) light passed through the gel at high temperatures, and short-wavelength (blue) light was permitted to pass through at low temperatures. At temperatures below the gelation point, the SeedGel reverted to a liquid sample, transparent to all visible light. The coloration capabilities of the gel are not a result of absorption or luminescence of the chemicals used, but due to the structures formed within the sample. The phenomenon is an example of the Christiansen effect. Filters that rely on the Christiansen effect already exist, but the researchers believe that the SeedGel offers distinct advantages. SeedGel is more sensitive to temperature changes and operates at a broader range of temperatures. It can be tuned to cover wavelengths ranging potentially from UV to near-infrared (NIR). More light is permitted through with Seed-Gel than with a typical Christiansen filter. The gel is made of inexpensive, readily available materials. “The approach is versatile with great tunability, and the manufacturing process can be easily scaled up,” researcher Yuyin Xi said. “It is a promising candidate for use in a range of smart optical devices and new classes of materials that have color applications.” The research was published in Nature Communications (www.doi.org/10.1038/s41467-022-31020-0).