Reflective Nanofilms Inspired by Butterfly Wings Create Vibrant Colors With Powerful Cooling Effects

During hot summer days, it feels more comfortable to wear white clothing than other colors like blue or black because they feel cooler. The heating effect that results from the absorption of light is also experienced in buildings, vehicles, and equipment.

Taking Inspiration From Nature

Objects appear to have different colors because they absorb certain wavelengths of light and reflect or transmit the others. In physics, visible light is described as a broad spectrum of electromagnetic waves with different wavelengths and frequencies. The wavelength of light absorbed by an object has an effect to its temperature, also known as color temperature.

For instance, a blue-painted car appears blue because it reflects blue light and other absorbs the other wavelengths of light. This means that the large amount of light that is absorbed heats the car.

A genus of butterfly, known as Morpho butterflies, possess wings with highly saturated blue color. These wings have unique surface nanostructure which is responsible for its ability to reflect light.

Passively Cooling Objects

In most buildings, huge amounts of energy have been used for cooling and ventilation. In electric cars, running the air conditioner reduces the driving range by almost half. To help solve this problem, a Chinese research team from the Shenzhen University developed cooling films that can help in achieving energy sustainability and carbon neutrality.

Led by Wanlin Wang, the research team created reflective nanofilms inspired by Morpho butterflies. To make this possible, they placed a disordered material in the form of a rough frosted glass below a multilayer material composed of titanium dioxide and aluminum dioxide. Then this structure was placed on a silver layer that reflects all wavelengths of light. This mechanism prevents the solar radiation from being absorbed as well as the heating effect associated with it.

The color of the film is determined by the way the components within the multilayered structure reflect light. For instance, the blue color is created by designing a multilayer material to reflect yellow light in a very narrow range of angles. This allows the disordered structure to diffuse the blue-colored light across a broad area.

This method of passive photonic thermal management has been previously done. However, it was accomplished only in white or clear objects due to the difficulty maintaining a wide viewing angle and high color saturation.

To test the effectiveness of their innovation, the research group created blue, yellow, and colorless nanofilms. These were placed outdoors in Shenzhen University on various surfaces from 9 a.m. to 4 p.m. during winter and summer. Then thermocouple sensors and infrared cameras were used in measuring temperatures.

It was found that during winter, the nanofilms were over 15 degrees Celsius cooler than the surfaces they were placed on, while they are about 35 degrees Celsius cooler in the summer. The result of the study also confirmed that the cooling films can effectively reduce the temperature of colored objects to about 2 degrees Celsius below the ambient temperature. When left outside all day, the blue films were 26 degrees Celsius cooler than the traditional blue car paint.

According to Wang, their new films can demonstrate excellent cooling performance regardless of the desired color, saturation or brightness. They can also be used in textiles to make clothes of any color which are comfortable to wear in hot temperatures.