Color-changing coating offers new responsive materials

08 Sep 2021

TUE research into how liquid crystals respond to wavelength could be used in energy-saving smart windows.

Influencing the color of an object through alterations to the way light interacts with its surface is an important factor for both engineers and designers, with surface coatings providing one attractive way to do so.

Research at Eindhoven University of Technology (TUE) has now led to the development of organic coatings that alter color with changing temperature, potentially useful for applications such as heat-regulating windows or temperature sensors.

The research, published in Advanced Photonics Research and incorporating the PhD research of TUE's Weixin Zhang, exploits the ability of cholesteric liquid crystals (CLCs) to show selective reflection properties arising from their internal form.

CLCs have a helical structure, with the dominant direction of molecules in each layer of liquid crystal, called the director axis, varying in a periodic manner. The pitch of this variation, ie. the cross-sectional distance in which the director axis of layers rotates through 360 degrees, determines the wavelength of light which the material reflects.

This opens up opportunities for large-scale fabrication of CLCs with responsive properties, thanks to their ability to self-assemble into photonic structures in which the internal alignment can be controlled through shear or capillary forces.

"Introducing a variation in CLC reflective color with change in temperature can be achieved via different mechanisms," noted TUE in its published paper. "But all these mechanisms rely on the change of essentially a single parameter, the pitch length,"

Temperature control for smart windows

Zhang has developed synthetic color-changing coatings whose structures are altered by phase transitions at specific temperatures, in ways which duly altered the reflective properties and hence visual color. At the same time these coatings maintain processability, stability, and a reversible temperature response.

The research successfully produced a photonic coating that was temperature responsive at visible wavelengths between 400 and 700 nanometers, and a second coating responsive in the infrared region between 700 and 2,500 nanometers.

"These photonic materials have key advantages over color-altering materials such as absorption-based dyes," commented Zhang. "The color exhibited by a photonic material can be precisely tuned by modifying the nanostructure of the material with minimal changes to the chemical composition of the material itself, making it quite easy to tune the color of the nanostructure over a wide range of wavelengths."

One significant application could be to regulate infrared light reflection from windows, a significant factor in energy consumption and the motivation for much recent research into ways to control the transparency of glass, although TUE said that further optimization of its materials will be needed before they are ready for deployment.

"The reflection band of the coatings shifts towards a lower wavelength in the IR range at higher temperatures, a characteristic favorable for smart windows as sunlight contains more energy in the lower wavelengths of IR," said the project team.

"Despite the potential of the materials, improvements will have to be made for the realization of commercial smart window products. Major improvements to focus on are increasing reflection efficiency, scale-up production, robustness, and cost prize."