Imagine a city building covered with colours, shapes and patterns that respond automatically to changes in the ambient air temperature. It would turn a lot of heads — but according to alumnus Raphael Kay (MIE MASc 2T2), it could also enable big savings on energy costs.
Kay is currently a PhD student at Harvard University, working under Professor Joanna Aizenberg. But before that, he completed his master’s degree at U of T Engineering, working with Professor Ben Hatton (MSE). That’s where he began work on a technology dubbed “liquid windows.“
Inspired by marine animals such as shrimp, cuttlefish and octopuses that use movements of fluid-like materials to change the colour of their skin, the window system is based on transparent panels filled with a thin layer of liquid. Low-cost tubes and pumps are then used to inject a second liquid into the first or pump it back out again.
This microfluidic system creates colourful patterns that can be modified to dynamically regulate the wavelengths and amount of light going through the panel. By optimizing light transmission — for example, by letting in more when it’s cold, or less when it’s hot — the system can help reduce the energy needed for heating and air conditioning.
“There are so many different factors you can play with: the underlying liquid types, what pigments you add, even how fast or slow you inject one into the other, which affects the shape of the colour burst you get,” says Kay.
“But one factor that turns out to be very important is the viscosity of each liquid, and that viscosity is going to change depending on temperature.”
Kay, Aizenberg and Hatton are all co-authors on a paper published in PNAS that describes in detail how this temperature-dependent mechanism can be harnessed to add life-like self-regulatory behaviours to the system.
“Let’s say you pick one liquid where the viscosity changes a lot based on temperature, and another one where it doesn’t change as much,” says Kay.
“When it’s colder, because of a fluid instability that depends on the ratio between fluid viscosities, injecting one into the other will give you a flower-like pattern, which lets in a lot of solar heat between the petals. When it’s hotter, and the difference in viscosities collapses, you’ll get more of a solid circle, which blocks more solar heat. So you can use this inherent temperature-dependence to self-correct how much solar heat enters a room.”
Kay believes that the combination of passive behaviors, such as temperature-dependent viscosity changes, and active behaviors, such as pumping liquids in and out, offers advantages over existing systems.
“Fully active systems provide great control, but they take energy to run,” says Kay.
“On the other hand, fully passive systems don’t take as much energy, but that also means you have less control. Our system, which combines the intrinsic temperature-responsive behaviors of fluids with their ability to be actively replaced and flowed only when needed, could provide the best of both worlds.”
Given that heating and cooling are responsible for some of the largest energy costs when it comes to the built environment, even a relatively mild improvement in efficiency could make a big difference.
Hatton says that it also represents a paradigm shift in building design.
“We tend to think of buildings as static, as monuments built to last a long time without changing,” says Hatton.
“But it’s interesting that surfaces and interfaces in nature are generally dynamic, able to change and respond to their environment. Trying to find smart materials and layers for smarter buildings is a new approach to an old problem, and one that we think holds a lot of promise.”
