MIT researchers have found a way to use the mechanical vibrations of sound waves to shake water molecules free from a storage medium. The breakthrough significantly speeds up the process of harvesting drinking water from thin air.
From special paints, to spongy aerogels, to materials with the highest-known surface areas, scientists around the world are working hard to find ways to pull water from the air around us.
Even in very dry climates, the air we breathe contains water, and finding a way to harvest it could provide a source of clean drinking water to millions of people, even if they don't have access to freshwater sources or municipal supplies.
The issue with many of the current water-harvesting concepts is that once a material grabs water from the air, heat needs to be applied to evaporate the water off of that material and condense it again for use as drinking water.
"Any material that’s very good at capturing water doesn’t want to part with that water," says MIT's Sevetlana Boriskina. "So you need to put a lot of energy and precious hours into pulling water out of the material."
Researchers at MIT have been trying to eliminate this energy-intensive process as we saw earlier this year with their passive water harvester, which collects water in a bubble-wrap-like material and uses the heat from the Sun to evaporate it out. While that eliminates the need to apply an energy source to the system, it can take awhile for the entire process to play out and deliver clean drinking water.
Now, Borinska – the principal research scientist at MIT's Department of Mechanical Engineering – and a new team of researchers have come up with a novel – and much faster – way to free trapped water that has been harvested from the air: sound waves.
The researchers developed a brand-new ultrasonic actuator shaped like a flat plate. They then placed a previously tested and saturated water harvesting material in its center. Next they turned on the actuator and targeted the material with ultrasonic waves, those that travel at frequencies in excess of 20 kilohertz (20,000 cycles per second).
The result was that all of the water the material contained was shaken out as droplets in just two to seven minutes. The plate design allowed those droplets to funnel through nozzles and drip into a collection area. You can see how the whole thing works in the following visualization.
"With ultrasound, we can precisely break the weak bonds between water molecules and the sites where they’re sitting," says study first author Ikra Iftekhar Shuvo. "It’s like the water is dancing with the waves, and this targeted disturbance creates momentum that releases the water molecules, and we can see them shake out in droplets."
According to the researchers, the device was able to free the captured water at a rate that is 45 times more efficient than using heat from the Sun. Even though the ultrasonic actuator requires a tiny bit of power to operate, the team says that electricity could be provided by an attached solar cell.
"The beauty of this device is that it’s completely complementary and can be an add-on to almost any sorbent material," says Boriskina. "Once the material is saturated, the actuator would briefly turn on, powered by a solar cell, to shake out the water. The material would then be ready to harvest more water, in multiple cycles throughout a single day."
Borsinka says that after further development, the new system could be deployed as a practical home-based system that could use a rapid-absorption material and a larger actuator, that would be about the size of a window, to pull free drinking water from the air.
"It’s all about how much water you can extract per day," she concludes. "With ultrasound, we can recover water quickly, and cycle again and again. That can add up to a lot per day."
The team's breakthrough has been published in the journal Nature Communications.
Source: MIT
