Originally published 2016-07-29.
In 2016, chemists at the US Pacific Northwest National Laboratory (PNNL) reported carbon-based nanorods that soak up water vapour when the air is fairly dry, then push much of that water back out once relative humidity climbs to about 50 to 80 percent. Almost every other absorbent material does the opposite and keeps taking on moisture as the air gets wetter. The rods were made by accident, and they hinted at new low-energy ways to collect water or move sweat away from skin, although a working device is still a long way off.
Here is what the team saw, the physics they think explains it, and what the finding can and cannot do.
An experiment that went the wrong way
The PNNL group was trying to grow magnetic nanowires. What came out instead were tiny carbon-rich rods. When chemist Satish Nune placed them in an instrument that tracks how a sample's weight changes with water vapour, the result looked like a fault: as humidity rose, the rods got lighter instead of heavier.
Suspecting the equipment, the researchers looked at the rods under a high-powered microscope while raising the humidity. They could see water condense around the rods and then ooze out at the points where neighbouring rods crossed. The weight of the material dropped by roughly half right after this release. When conditions changed again, the rods took water back up, so the cycle was reversible.
Postdoctoral researcher David Lao, who made the material, described it with a kitchen comparison: "Our unusual material behaves a bit like a sponge; it wrings itself out halfway before it's fully saturated with water."
The physics: capillary condensation and a 1.5-nanometre squeeze
Two ideas help explain the effect.
The first is capillary condensation. In very narrow spaces, water vapour can turn into liquid at lower humidity than it would in open air, because the curved surface of the liquid in a tight gap is more stable. That is why porous materials such as silica gel can pull moisture from air that feels dry.
The second is what happens when the gap gets even smaller. According to the PNNL team, liquid water collecting between two rods forms a curved bridge whose surface tension pulls the rods toward each other. When the space between crossing rods shrinks to about 1.5 nanometres (roughly the width of a few water molecules), the water is no longer stable as a liquid and suddenly evaporates, leaving the rods to relax and start again.
The idea was not invented for these rods. Scientists studying crystallised proteins in the 1990s had proposed that water trapped between water-repelling (hydrophobic) parts of proteins could vaporise on its own. Papers in 2012 and 2013 then described liquids spontaneously evaporating in confined gaps of about 1.5 nanometres, a process one paper called "solvent cavitation under solvo-phobic confinement". The nanorods appear to be one of the first materials where this was seen in action at a scale people could weigh.
Watch the effect
The short video below walks through the discovery. As you watch, look for the sequence the researchers described: water gathering between the rods, the rods drawing together, and the water being expelled.
Possible uses, from desert air to sportswear
The researchers and science writers quickly listed ideas. Most are early-stage possibilities rather than products:
- Water harvesting. A material that collects moisture and then releases it with little added energy could, in theory, help gather drinking water from humid night air in dry regions.
- Purification and separation. Because water is pushed out as vapour in a controlled way, the team suggested the mechanism might help separate water from other substances. PNNL went as far as calling it a potential paradigm shift in purification and separation.
- Breathable fabrics. Clothing that draws sweat away from the body and releases it as vapour was another suggested use.
- Other liquids. The team also wondered whether similar materials could capture other liquids, such as methanol.
Heldebrant summed up the mood in the lab: "Now that we've gotten over the initial shock of this unforeseen behavior," the team could start imagining applications.
The limits: why this is not yet a water supply
It is easy to read headlines like "nanorods that harvest water from the air" and imagine a machine on a roof. The 2016 results do not get there yet, for several reasons:
- Only part of the material works. At the time, only about 10 to 20 percent of the rods expelled water. The rest behaved normally.
- Tiny quantities. The lab could make only a few grams at a time. Scaling up is a separate challenge.
- Collection and energy. Water released as vapour still has to be captured and condensed. A practical device must show that the whole cycle uses less energy than existing options such as refrigeration-based dehumidifiers.
- Durability. A useful material must repeat the cycle thousands of times without wearing out or clogging with dust.
Satish Nune named the next hurdle plainly: "before we can put these nanorods to good use, we need to be able to control and perfect their size and shape." Older copies of this story sometimes credit that line to Heldebrant; PNNL's own release attributes it to Nune.
Where this fits in the search for water from air
Pulling water from the atmosphere is an old idea. Fog nets in Chile and Morocco catch droplets from passing clouds, and electric atmospheric water generators work like dehumidifiers. In the years after 2016, researchers also developed sorbent materials such as metal-organic frameworks that absorb water at night and release it when warmed by the sun.
The PNNL rods are interesting because they release water when humidity rises, not when they are heated. That is unusual behaviour and a useful clue for materials scientists, even if it has not become a commercial technology. Accidental findings like this often matter most as a new principle that other teams can borrow.
Key facts
- Who: Satish Nune, David Lao, David Heldebrant and colleagues at Pacific Northwest National Laboratory.
- Published: "Anomalous water expulsion from carbon-based rods at high humidity", Nature Nanotechnology, June 2016.
- What happens: water is absorbed at lower humidity and expelled at about 50 to 80 percent relative humidity.
- Proposed mechanism: capillary condensation, then evaporation when rods are pulled within about 1.5 nm of each other.
Sources: PNNL news release (June 2016); Nune et al., Nature Nanotechnology (2016); Wikipedia: Capillary condensation; earlier coverage by ScienceAlert and Gizmag (New Atlas).
COMMENTS