New gel proves itself as a highly tunable color filter

Color these scientists happy. An exotic gel they studied at the National Institute of Standards and Technology (NIST) has an unexpected property: the temperature of the material determines the color of light that can pass through it.

The material, which the research team calls “SeedGel,” has already shown promise as a multi-purpose tool, with applications ranging from batteries to water filters to tissue engineering. The team’s new newspaper, which appears in nature communication, highlights the gel’s new ability as a temperature-sensitive light filter. Shine white light on the gel, and depending on the temperature of the gel, only a specific wavelength, or color, will pass through. A temperature change of less than a tenth of a degree Celsius can be enough to change the allowable wavelength, which can be any color in the visible range, as well as parts of the ultraviolet and infrared.

“Our previous work showed that the SeedGel can transform from clear to opaque and back again, but we haven’t explored what it can do to color,” said Yun Liu, who is both a scientist at the NIST Center for Neutron Research (NCNR) and a professor at the University of Delaware. “The ability to precisely control the color was a new discovery.”

The team’s creation stands out from others that may be familiar from the market. Do not confuse it with a mood ring, whose thermochromic liquid crystals change color with temperature. Nor is it a variation of photochromic sunglass lenses, which darken when exposed to ultraviolet rays. Instead, the gel functions as a temperature-sensitive gate for a particular wavelength of light.

Their gel starts out as a transparent liquid made from water and liquid solvents with silica nanoparticles added. When this mixture is heated to a certain temperature, the liquids and nanoparticles will form a physical gel that initially remains transparent but now has a different internal structure. Instead of a formless liquid, the liquids form interlocking microscopic channels, with the nanoparticles trapped in one of them.

As it is heated through a specific range of higher temperatures, the newly discovered effect appears: the gel becomes opaque to all colors except individual colors, letting shorter, bluer wavelengths through first and then gradually longer, redder wavelengths. Eventually, once this temperature range is exceeded, the gel becomes opaque to all visible light.

Neutron scattering experiments performed at the NCNR explain this unusual behavior. Changing the temperature causes an exchange of liquid molecules between the microscopic channels, increasing the overall refractive index of these channels. One wavelength of light comes through, but other colors are scattered.

The behavior is an example of the Christiansen effect, which was established in 1884. Filters exist that rely on the Christiansen effect, but the researchers indicate that their new gel offers clear advantages to the industry: Not only is their gel more sensitive to temperature changes, but the potential temperature range in which it works is wider, because it can be adjusted to anywhere between 15 and 100 degrees Celsius. It can be tuned to cover a wide range of wavelengths, possibly from ultraviolet (from just under 400 nanometers) to near infrared (up to 2500 nanometers). And it lets in more light than typical Christiansen filters.

Because the gel, regardless of the modifications, is made of inexpensive, readily available materials, it offers benefits to the industry, said Yuyin Xi, a team member at the University of Delaware.

“The approach is versatile with great tunability and the manufacturing process can be easily scaled up,” he said. “It is a promising candidate for use in a range of smart optical devices and new classes of materials with color applications.”