The sun continually radiates infrared rays, so we can always feel its heat, especially in summer. But why is the beach a pleasant temperature while our car interior gets significantly hotter? When heat comes into contact with physical objects, such as the leather seats inside our cars (Fig. 1), it is absorbed. The objects then slowly release the heat back into the air. In a closed environment like a car’s interior, the temperature can become dangerously high because the re-emitted heat cannot exit the space. If car windows were able to reflect the infrared waves to prevent them from entering the car at all, the dangers and inconvenience of a hot day would no longer be a daily worry. The American Dermatological Association explains that infrared radiation from the sun is “readily transmitted through standard glass,” but protective glass can block the infrared radiation while still allowing for the passage of visible light . If car windows had such protective glass, a comfortable, controlled, and safe environment could be maintained in any car’s interior.
An Existing Solution
While low-emissivity technology was a significant development in the 1980s and is often implemented today in newer buildings (Fig. 2), energy costs are as cumbersome as ever, and we are still cringing at the thought of sitting on black leather seats in the middle of July. LbL deposition, a more recent technology, was first introduced in 1966 but developed into a practical process only 20 years ago . Today, LbL technology may be a more efficient, cost-effective solution to the problem .
The effect of alternating bilayers of high and low refractive indices causes constructive and destructive interference of incident light . This essentially means that light of a certain wavelength will be reflected off of the film’s surface, while all other light will be transmitted through the film. The true beauty of LbL films is that the wavelength of light that will be rejected by the film can be deliberately selected. The thickness of the film is what determines which wavelength of light will be reflected from the film, and the thickness of the film can easily be controlled . The thickness of the film corresponds directly to the number of bilayers in the film, just as the height of a multi-decker peanut butter and jelly sandwich is directly dependent upon the number of layers of peanut butter and jelly you have added; choosing the reflected wavelength is as easy as choosing the number of bilayers that are sprayed onto the film. Because there is high control over the thickness of the films (Fig. 5), the films can be engineered to reflect any wavelength of light, which opens up many practical applications for the films . Thus, the unwanted infrared rays that threaten the temperature of our car interior can be reflected when the appropriate number of bilayers is incorporated into a film, and our beach-day blues will be no more .
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