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A reflective smart-window prototype (left) becomes transparent when a liquid with a refractive index similar to the reflective structure is pumped into a chamber in front of the structure (right). [Image: Keith Goossen / University of Delaware]

A team of researchers led by Keith Goossen and Daniel Wolfe from the University of Delaware, USA, has proposed a new technique for manufacturing optofluidic smart windows that switch from reflective to transparent without constant electric stimuli and at a lower cost than commercially available smart windows (Opt. Express, doi: 10.1364/OE.26.000A85). The new design consists of a 3-D-printed retroreflective polymer panel backed by a flat glass panel. When a liquid with the same refractive index (RI) as the polymer is pumped in between the two panels, the smart window transitions from reflective to transparent.

Results from prototype tests show that the new design can modulate visible light transmittance from 8 percent when reflective to 85 percent when transparent—and can do so consistently over time. The team says their new design could eventually lead to more affordable smart windows that are energy efficient and offer privacy, as well as roofing panels that keep buildings warm in the winter and cool in the summer.

Improving smart-window technology

Smart windows are a relatively new technology that could, through visible light transmittance, reduce energy costs by as much as 30 percent. Existing commercial smart-window designs, including electrochromic, suspended particle and polymer-dispersed liquid crystal devices, switch from transparent to translucent or opaque with the application of electricity, light or heat. However, these designs are expensive to manufacture, may require a constant flow of electricity, and can produce unwanted heat from light absorption.

Goossen and Wolfe’s team claims that its optofluidic smart window could be manufactured at a lower cost than those currently on the market because their design switches the glass from reflective to transparent using a simple 3-D-printed reflective polymer sheet and an inexpensive RI-matching liquid. Furthermore, heat from light absorption isn’t an issue because the polymer sheet reflects light—and the heat that comes with it—back to its source (i.e., retroreflection).

The optofluidic smart-window prototype

The University of Delaware smart-window prototype measures 54×55×8.5 mm and consists of a transparent glass panel and a reflective polymer panel separated by a thin cavity. The researchers used VeroClear—a transparent photopolymer with an RI of 1.52—to create the polymer sheet patterned with 4-mm-high reflective corner cubes. They chose methyl salicylate for the cavity liquid because its RI nearly matches VeroClear’s RI and because it has a low viscosity, which makes it easier to pump in and out of the thin cavity. 

The smart window appears reflective when the cavity between the polymer panel and glass panel is filled with air. In this state, the pattern of corner cubes reflects back normally incident light rays to the light source because of the RI difference at the polymer–air interface. The RI difference at the interface disappears when the chamber is filled with methyl salicylate, making the window appear transparent.

High reflectance, high transmittance

The researchers used a spectrophotometer to measure the smart-window prototype’s ability to modulate visible light transmittance. The combined prototype results showed that the new design is capable of modulating visible light transmittance from 8 percent when the chamber is filled with air to 85 percent when the chamber is filled with methyl salicylate. Goossen and Wolfe’s team say data from these prototype tests “fit well” with their earlier theoretical smart-window simulations, thereby validating the design.

To test cycling performance, the researchers used a pumping station to move methyl salicylate in and out of a smart-window prototype while continuously measuring visible light transmittance. After 1,000 cycles with an average transition time of 10 seconds, the researchers report that the prototype only showed a slight deviation in light transmittance. Specifically, during the transparent phases, the transmittance went from 85 percent to 86 percent after 1,000 cycles; and during the reflective phases, transmittance went from 34 percent to 38 percent.

Although the design is still in the prototype stage, Goossen says “… the innovation here is mostly in recognizing that such a simple concept could work.”