MIT ultrasonic tech pulls drinking water from air in minutes

Extracting the captured water usually involves heat -- and a long wait. Most current systems rely on sunlight to warm these materials until the trapped moisture evaporates and condenses into liquid. This slow step can take many hours or even stretch into days.

MIT engineers have now identified a much faster method for recovering this water. Instead of relying on solar heating, the team uses ultrasonic vibrations that shake the moisture loose.

Ultrasonic Vibrations Offer a Faster Alternative

The researchers created a high-frequency ultrasonic device that vibrates rapidly. When a water-absorbing material, or "sorbent," sits on top of the device, it sends out ultrasound waves tuned to break the bonds holding water molecules in place. Their tests showed that this approach frees the water within minutes, while heat-driven systems typically require tens of minutes or several hours.

Because it does not use heat, the device needs a power source. The team suggests that a small solar cell could supply electricity and also act as a sensor to detect when the material is saturated. The system could even be set to activate automatically whenever enough water has accumulated. Such automation would allow the setup to collect and release water repeatedly throughout the day.

A Step Toward Practical Air-to-Water Systems

"People have been looking for ways to harvest water from the atmosphere, which could be a big source of water particularly for desert regions and places where there is not even saltwater to desalinate," says Svetlana Boriskina, principal research scientist in MIT's Department of Mechanical Engineering. "Now we have a way to recover water quickly and efficiently."

Boriskina and her coauthors describe the device in a study published on November 18 in Nature Communications. The paper was led by first author Ikra Iftekhar Shuvo, an MIT graduate student in media arts and sciences, along with Carlos Díaz-Marín, Marvin Christen, Michael Lherbette, and Christopher Liem.

Improving Atmospheric Water Harvesting

Boriskina's research group develops materials that interact with environmental conditions in innovative ways. Recently, they explored atmospheric water harvesting (AWH) and how materials can be engineered to pull moisture from the air efficiently. The long-term goal is to provide a reliable source of drinking water for communities that lack both freshwater and saltwater supplies.

Like many other teams, they initially assumed that AWH systems placed outdoors would take in moisture overnight and then rely on sunlight during the day to release it through evaporation and condensation.

"Any material that's very good at capturing water doesn't want to part with that water," Boriskina explains. "So you need to put a lot of energy and precious hours into pulling water out of the material."

A New Direction Sparked by Ultrasound Research

The idea for a faster method emerged after Ikra Shuvo joined the group. Shuvo had been working with ultrasound for wearable medical devices, and during discussions with Boriskina, they realized that ultrasonic vibrations might dramatically speed the water-release step in atmospheric water harvesting.

"It clicked: We have this big problem we're trying to solve, and now Ikra seemed to have a tool that can be used to solve this problem," Boriskina recalls.

How Ultrasound Shakes Water Free

Ultrasound refers to acoustic pressure waves that exceed 20 kilohertz (20,000 cycles per second). These high-frequency waves are invisible and inaudible to humans. The team found that at the right frequency, ultrasound can shake water molecules free from the material holding them.

"With ultrasound, we can precisely break the weak bonds between water molecules and the sites where they're sitting," Shuvo says. "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."

Designing a High-Frequency Actuator

Shuvo and Boriskina built an ultrasonic actuator specifically for atmospheric water harvesting. At its center is a flat ceramic ring that vibrates when voltage is applied. Around it is another ring containing tiny nozzles. As droplets shake loose, they fall through the nozzles into collection containers placed above and below the vibrating ring.

The team tested the device using previously developed AWH materials. They placed small, quarter-sized pieces of the sorbent in a humidity chamber at different humidity levels until each sample became fully saturated. Each sample was then placed on the actuator and vibrated at ultrasonic frequencies. In every test, the actuator released enough moisture to dry the material within minutes.

Efficiency Gains and Practical Potential

The researchers estimate that the ultrasonic method is 45 times more efficient than relying on solar heat when extracting water from the same material.

"The beauty of this device is that it's completely complementary and can be an add-on to almost any sorbent material," Boriskina says. She imagines a household system that uses a fast-absorbing material paired with an ultrasonic actuator, each roughly the size of a window. When the material becomes saturated, the actuator would briefly activate using power from a solar cell, shake out the water, and then reset for another cycle.

"It's all about how much water you can extract per day," she says. "With ultrasound, we can recover water quickly, and cycle again and again. That can add up to a lot per day."

This work was supported, in part, by the MIT Abdul Latif Jameel Water and Food Systems Lab and the MIT-Israel Zuckerman STEM Fund.