In Brief
Harvard researchers have developed a new method of fogging up windows. It is far cheaper than any previous method, and takes just a second to transition.
Window Privacy

In a paper published in the journal Optics Letters, scientists from Harvard’s John A. Paulson School of Engineering and Applied Sciences assert that they have found a way to adjust the opacity of a clear glass window using geometry.

Although tunable windows aren’t exactly new, manufacturing them uses expensive electrochemical reactions to change them from clear to opaque; however, the new technology uses a clear sheet of glass or plastic with either side covered with soft elastomers, which are coated with silver nanowires.

In their normal state, these silver nanowires aren’t large enough to scatter light, but once voltage is applied the nanowires are energized and move toward each other. This squeezes and deforms the soft elastomer in an uneven fashion, scattering the light and giving the glass a smoky look.

This chain reaction all occurs in under one second

Foggy Windows
Image Credit: Harvard

Samuel Shian, who helped develop the technology, likens the process to a frozen pond, saying that “If the frozen pond is smooth, you can see through the ice. But if the ice is heavily scratched, you can’t see through.”

Shian and co-developer David Clark learned that voltage affected the roughness of the elastomer surface, and the amount of voltage applied affects the level of opacity of the glass.

The development makes an expensive technology more practical. “Because this is a physical phenomenon rather than based on a chemical reaction, it is a simpler and potentially cheaper way to achieve commercial tunable windows,” says Clarke.

The first generation of tunable windows used vacuum deposition to achieve the effect of opacity. Vacuum deposition involves depositing layers of a specific material one molecule at a time, a painstaking and expensive method.

Shian and Clarke’s nanowire method can be applied simply by spraying or peeling the nanowire solution onto the elastomer, making it practical for larger scale application.

The researchers are currently working with thinner elastomers that need lower voltages, while Harvard’s Office of Technology Development is filing a patent application on the breakthrough technology.