Synthetic trees could tap underground water in arid areas

They could also help coastal residents mine fresh water from salty sources

If you put a narrow straw in a glass of water, some of that fluid will climb up the tube. But without help, that water will climb only a small distance. Now, researchers have come up with a device that can mimic trees’ ability to make water climb great heights. That could bring drinking water to communities that don’t now have enough.

Described as a “synthetic tree,” this new device could help people more easily tap into groundwater, scientists say. It might also let people in coastal areas mine fresh drinking water from the sea.

Water molecules find each other really attractive. That’s why large numbers of them come together to form round droplets. Sometimes, however, water molecules are more attracted to other substances than to themselves. That’s why they climb up the sides of a glass. (Did you ever notice that the surface of the water in a glass curves upward where it meets the side of the glass?) In a very narrow tube, water can even climb a few centimeters (a couple of inches or so), notes Jonathan Boreyko. He’s a materials scientist at Virginia Tech in Blacksburg. The process driving this climb is called capillary action. But for water to rise more than a few centimeters, that capillary action needs help.

Plants know the secret to helping, Boreyko explains. Throughout the day, water evaporates out a plant’s leaves. This creates suction inside a tissue called xylem (ZY-lem). That suction pulls water and nutrients up through the roots to nourish leaves and new growth. Such suction can lift water to the tree tops. Now, Boreyko and his teammates have designed a “synthetic tree.” They say it can help people living in dry areas harvest water from even deep underground.
Ndidi Eyegheleme is a mechanical engineer. She grew up in a small coastal village in Africa. Today she works with Boreyko at Virginia Tech. “Where we lived was surrounded by salty water,” she recalls. “So, we had a lot of challenges getting fresh water to drink.” That’s why she’s so interested in making synthetic trees. They could help people in dry parts of the world, too, she adds. Worldwide, about 2.2 billion people lack reliable access to clean drinking water, the United Nations reports. Yet in some places, water may be just a few meters (yards) below their feet.

The Virginia Tech team recently built a prototype synthetic tree inside a clear plastic box. It’s small enough to sit on a benchtop. Nineteen small plastic tubes serve as this tree’s “trunk.” The inside diameter of each tube is about six times the diameter of the lead in a pencil. Each tube carries water up from a container to a ceramic “leaf” that’s a tad larger in diameter than a soda can. Just like a real leaf, this ceramic disk has many tiny pores. Evaporation from those pores creates the suction that helps pull water up the tubes.
Mimicking how a tree slurps water “is a wonderful approach,” says Xianming Dai. He’s a mechanical engineer at the University of Texas at Dallas. Synthetic trees might be able to tap water as much as 100 meters (328 feet) underground, he says. He advises bundling the tree trunk’s long fragile tubes inside a larger protective pipe. That way the team’s system should be easy to scale up to larger sizes.

Such devices also could be modified to help people generate fresh water from salty, says Dai. How? Install a membrane that removes dissolved salt from water before it enters the lower end of the tubes. That would help prevent salt crystals from forming in the tubes or clogging the leaf as water evaporates, he notes.