By adjusting the opacity of the glass on existing photochromic windows, users can control how much sunlight passes through the window and into the room. In most cases, the glass partially blocks the sunlight's visible spectrum – keeping the room from getting too bright – along with its infrared spectrum, keeping the room from getting too warm.

On hot summer days, however, people might want the brightness of the visible light, but not the heat of the infrared. In winter, they'd probably want both. Additionally, they may wish to soften the visible light, so they don't have to squint all day long. That's where the new "liquid window" comes in.

Developed by a team of scientists at the University of Toronto – led by Prof. Ben Hatton – it's inspired by the color-changing skin of squids, cuttlefish and krill. Those animals are able to move pigments around in cells beneath their skin, changing it back and forth between transparent and opaque states.

Last year, the researchers announced a tintable window that was inspired by this capability. The prototype liquid window takes the concept further by incorporating multiple stacked sheets of transparent plastic, each one of which has a network of millimeter-thick microchannels running through it.

By pumping liquids containing different pigments (or other molecules) into or out of the channels in each sheet, it's possible to select different combinations of optical qualities for the window as a whole.

For instance, by pumping a visible-light-blocking pigment out of one sheet, while pumping an infrared-blocking pigment into another, the window can be set to let visible light through while blocking infrared light. Additionally, pumping a light-diffusing pigment in or out of another sheet adjusts the softness/harshness of the visible sunlight within the room.

Utilizing computer models based on the performance of the prototypes, the scientists estimate that even if liquid windows were only used to modulate the transmission of infrared light, a building would use about 25% less heating, cooling and lighting energy per year. If the windows were also used to control visible light, that figure would jump to around 50%.

"Buildings use a ton of energy to heat, cool and illuminate the spaces inside them," says recent U Toronto grad Raphael Kay, lead author of a paper on the study. "If we can strategically control the amount, type and direction of solar energy that enters our buildings, we can massively reduce the amount of work that we ask heaters, coolers and lights to do."

The paper was recently published in the journal PNAS.