Scientists Made Water Move Using Only Light
Engineers from MIT and Saudi Arabia have worked together to develop a system that could control the way water moves over a surface, and all they use is light.
The impetus for the study came from the desire to separate oil from water, which is needed during the treatment of briny water and crude oil from oil wells. Once they intermingle, they're difficult to separate. Electricity can currently get the two apart, but that's energy intensive and doesn't work when the water has salt in it, which is pretty common on Earth.
Instead of focusing on the water or the light, the researchers from MIT and the King Fahd University of Petroleum and Minerals looked at the surfaces where the two would interact, which can be measured using a metric called wettability. Everyone's familiar with the concept of getting wet, but "wetting" in a geophysical sense is defined as the tendency of one fluid to spread on, or adhere to, a solid surface.
The researchers set about making a surface where titanium dioxide or titania, a photovoltaic compound used as a thickener in sunscreen, was responsive to visible light. They painted their surface, which also included glass, with an organic dye.
"We were inspired by the work in photovoltaics, where dye sensitization was used to improve the efficiency of absorption of solar radiation," says MIT mechanical engineering professor Kripa Varanasi. "The coupling of the dye to titania particles allows for the generation of charge carriers upon light illumination. This creates an electric potential difference to be established between the surface and the liquid upon illumination, and leads to a change in the wetting properties."
When the wetting properties changed, the water seeped out of the oil. Even if the water was saline, it seeped out.
"By systematically studying the relationship between the energy levels of the dye and the wettability of the contacting liquid, we have come up with a framework for the design of these light-guided liquid manipulation systems," Varanasi says. "By choosing the right kind of dye, we can create a significant change in droplet dynamics. It's light-induced motion – a touchless motion of droplets."
Since light and sun are two pretty common resources, there are many efforts underway towards increasing their cooperation with each other. Another MIT team recently worked on a way for sunlight to create water out of the air.
Source: MIT
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