writes copy 14 Dec 2017

3D Printed Prototypes Demonstrate Smart Windows That Can Keep Your Car Cool in the Sun

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When you’ve left your car in the sun all day, unless you were lucky enough to find a parking spot under a tree, getting back in and sitting inside immediately leads to feeling enveloped in an intense, stifling heat. But researchers from the University of Delaware may have found a solution, with their prototype windows that can switch from clear to reflective just by adding a liquid. The technology could be used not only to keep your car cool under the sun, but also for roof panels that keep houses warm in the winter and cool in the summer, and to make energy-efficient office buildings.

Switchable glass, which uses applied voltage to change from opaque to clear, is available, but at $100 per square foot, it’s not exactly a wallet-friendly option. But by combining smart technology with 3D printing, which has been done with mirrors and other products, the researchers developed an inexpensive, easy to manufacture prototype smart glass, made of a 3D printed plastic panel covered by a thin chamber. The panel has a pattern of retroreflective structures, which means the light reflects back to the direction it originally came from. These structures become transparent when the chamber is filled with fluid methyl salicylate, which matches the plastic’s optical properties.

Schematic view of variable transmittance prototype.

“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,”  Keith W. Goossen with the University of Delaware said.

“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 recognizing that such a simple concept could work.”

Operating modes of variable transmittance device, (left) air – reflective, (middle) water – diffuse transmittance, (right) index matched – specular transmittance.

Goossen and  fellow University of Delaware researcher Daniel Wolfe published a paper on their prototype in the Optical Society journal Optics Express, titled “Evaluation of 3D Printed Optofluidic Smart Glass Prototypes.” They used a Stratasys Objet30 Pro 3D printer to manufacture the plastic panels, complete with repeating retroreflective structures, for their switchable smart glass prototype, out of Stratasys’ transparent 3D printing photopolymer VeroClear.

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

The researchers also developed post-processing steps for the panels, which were constructed in varying sizes for testing purposes, to make sure that the plastic would stay highly transparent, and exhibit accurate corners, after printing was complete; both of these qualities are necessary for the structures to attain retroreflection.

“Prototyping the corner cube on the 3D printer indicated the optimum size was with a side wall of 4 mm,” the paper reads. “As the geometry reduces in size, printing defects increase and resolution decreases.”

Experimental setup for cycling.

The repeating structures were put through optical testing in order to find out if underlying characteristics, like the material’s light absorption or surface roughness, would cause any unexpected issues; luckily, the researchers were able to determine through the tests that the  retroreflective structures worked just as they should. In addition, they demonstrated that their smart window prototype could stand up under thousands of cycles, switching back and forth from reflective to transparent, without any signs of degradation. However, they also determined that a little of the fluid stays on the structure, and they are now developing coatings to help the fluid drain off without leaving residue.

“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.  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,” said Goossen.

Prototype smart glass is retroreflective (left) and becomes clear (right) when a liquid

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