Smart windows use liquid to switch from clear to reflective

2 min read

Researchers from the University of Delaware have developed prototype windows that switch from reflective to clear with the simple addition of a liquid. The switchable windows are said to be easy to manufacture and could help keep parked cars cool in the sun or make office buildings more energy efficient.

Although glass that uses an applied voltage to switch from clear to an opaque or tinted state is commercially available, its high cost – around $100 per square foot – has hindered widespread use.

Keith Goossen, who led the research team, said: “We expect our smart glass to cost one tenth of what current smart glass costs because our version can be manufactured with the same methods used to make many plastic parts and does not require complicated electro-optic technology for switching.”

The Delaware team’s smart windows contain a plastic panel with a pattern of structures that is retroreflective. Meaning that rather than reflecting light in all directions, it reflects light back in the direction it came from.

The researchers demonstrated a prototype of their smart glass consisting of a 3D printed plastic panel covered by a thin chamber. When the chamber is filled with the fluid methyl salicylate – which matches the optical properties of the plastic – the retroreflective structures become transparent.

“Although we had to develop new ways to process 3D printable plastics with good optical performance, develop inexpensive refractive index-matching fluids and come up with highly reflective optical structures, the innovation here is mostly in recognising that such a simple concept could work,” Goossen explained.

3D printing the prototype

To make their switchable glass, the researchers 3D printed the plastic panels with repeating retroreflective structures of various sizes for testing. They used a commercially available clear 3D printable material and developed post-processing steps to ensure the plastic remained highly transparent after printing and exhibited accurate corners, which were important to achieve retroreflection.

“Without 3D printing, we would have had to use a moulding technology, which requires building a different mould for every different structure,” said Goossen. “With 3D printing, we could easily make whatever structure we wanted and then run experiments to see how it performed. For commercial production, we can use standard injection moulding to inexpensively make the retroreflective panels.”

Once the researchers figured out the optimal size to use for the repeating structures, they performed optical testing to determine whether characteristics such as surface roughness or the material’s light absorption would cause unexpected optical problems. These optical tests showed that the structures worked exactly as indicated by optical simulations.

They also demonstrated that the device can undergo thousands of cycles from transparent to reflective without any degradation. They did, however, find that some fluid stays on the structure instead of draining off. To solve this issue, the researchers are developing coatings that will help the fluid drain off the plastic without leaving any residue.

“To further demonstrate the technology's usefulness, we are building an office door that incorporates the new smart glass as a switchable privacy panel,” said Goossen. “These types of panels are currently made with much more expensive technology. We hope that our approach can broaden this and other applications of smart glass.”