Optofluidic Smart Glass Heats and Cools Efficiently, Inexpensively

We may not be able to control the weather, but new switchable window technology using optofluidic smart glass could someday be used to keep our vehicles, businesses and homes comfortable regardless of the temperature outside. This newly developed smart window technology contains a plastic panel with a retroreflective pattern of structures. Rather than reflecting light in all directions like a mirror, the retroreflective panel reflects light back in the direction it came from. For example, if this smart window were used on the outside of a skyscraper, it would direct light up toward the sun rather than down to the street.

Researchers created a prototype smart glass that is retroreflective (left) and becomes clear (right) when a liquid with optical properties similar to the reflective structure is pumped into a chamber in front of the structure. Courtesy of Keith Goossen, University of Delaware.

Switchable glass, or smart windows, are devices capable of modulating light transmittance when voltage, light or heat is applied. These devices allow for the state of the glass to switch from transparent to translucent, or vice versa. This transition can occur passively or actively, depending upon the device technology.

Researchers from the University of Delaware have demonstrated a prototype retroreflective smart window capable of modulating visible light transmittance from 8 percent to 85 percent at normal incidence. Solar-weighted energy transmittance modulation ranged from 22 percent when the device was filled with air to 79 percent when the device was filled with index-matching fluid.

The prototype was made from a plastic panel covered by a thin chamber. When the chamber was filled with methyl salicylate, a fluid that matches the optical properties of the plastic, it caused the retroreflective structures in the panel to become transparent.

Researchers used 3D printing to make the panels with repeating retroreflective structures. They used a commercially available, clear, 3D-printable material and developed post-processing steps to ensure that the plastic remained highly transparent after printing and exhibited very accurate corners, which were important for successful retroreflection.

“Without 3D printing, we would have had to use a molding technology, which requires building a different mold for every different structure,” said researcher Keith 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 molding to inexpensively make the retroreflective panels.”

For testing purposes, the retroreflective structures were made in various sizes. Once the researchers determined the optimal size to use, they performed optical testing on the structures to determine whether characteristics such as surface roughness or the material’s light absorption might cause unexpected problems. Testing showed that the structures worked as optical simulations indicated they would.

The team demonstrated that the optofluidic smart glass could undergo thousands of cycles from transparent to reflective with only slight deviation in light transmittance. Prior to cycle 1, the transmittance averaged 10 percent in the reflective mode. After cycle 1, the transmittance averaged 34 percent, indicating a 24 percent increase in transmittance when operating in the reflective mode. After 1000 cycles, transmittance averaged 38 percent in the light blocking state, and 86 percent in the transmitting state.

Tests also revealed that some fluid stayed on the panel structure instead of draining off. To resolve this issue, the team is developing coatings that will help the fluid drain off the plastic without leaving any residue.

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

“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,” said Goossen.

One of the most promising applications for optofluidic smart glass could be in cars, where it could be used to change the windshield to a reflective state when the car is parked in the hot sun.

“Because our glass is retroreflective in the nontransparent state, almost all the light is reflected, keeping the glass, and thus the car, from getting hot,” said Goossen.

The researchers have also shown that the plastic retroreflective panels could be used as an inexpensive switchable roofing structure that could be used to reduce heating and cooling costs. For roofing applications, a layer of material placed under the panels would be used to absorb light when the panels are in their clear state. Although the methyl salicylate used in the prototype could freeze in very cold climates, freeze-resistant fluids could be developed.

“To further demonstrate the technology’s usefulness as switchable glass, 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.”

The research was published in Optics Express, a publication of OSA, The Optical Society (doi: 10.1364/OE.26.000A85).

A new type of smart window uses liquid to switch from reflective to clear. The new technology is inexpensive to make and could help make buildings and homes more energy-efficient. Courtesy of Keith Goossen, University of Delaware.