A serendipitous observation in a Chemical Engineering lab at Penn Engineering has led to a surprising discovery: a new class of nanostructured materials that can pull water from the air, collect it in pores and release it onto surfaces without the need for any external energy.
The research, published in Science Advances, describes a material that could open the door to new ways to collect water from the air in arid regions and devices that cool electronics or buildings using the power of evaporation.
The interdisciplinary team includes Daeyeon Lee, Russell Pearce and Elizabeth Crimian Heuer Professor in Chemical and Biomolecular Engineering (CBE); Amish Patel, Professor in CBE; Baekmin Kim, a postdoctoral scholar in Lee's lab and first author; and Stefan Guldin, Professor in Complex Soft Matter at the Technical University of Munich.
The research describes a material that could open the door to new ways to collect water from the air in arid regions and devices that cool electronics or buildings using the power of evaporation
"We weren't even trying to collect water," says Lee. "We were working on another project, testing the combination of hydrophilic nanopores and hydrophobic polymers when Bharath Venkatesh, a former Ph.D. student in our lab, noticed water droplets appearing on a material we were testing. It didn't make sense. That's when we started asking questions."
Those questions led to an in-depth study of a new type of amphiphilic nanoporous material: one that blends water-loving (hydrophilic) and water-repelling (hydrophobic) components in a unique nanoscale structure. The result is a material that both captures moisture from air and simultaneously pushes that moisture out as droplets.
Water-collecting nanopores
When water condenses on surfaces, it usually requires either a drop in temperature or very high humidity levels. Conventional water harvesting methods rely on these principles, often requiring energy input to chill surfaces or a dense fog to form to collect water passively from humid environments. But Lee and Patel's system works differently.
Instead of cooling, their material relies on capillary condensation, a process where water vapor condenses inside tiny pores even at lower humidity. This is not new. What is new is that in their system, the water doesn't just stay trapped inside the pores, as it usually does in these types of materials.
"In typical nanoporous materials, once the water enters the pores, it stays there," explains Patel. "But in our material, the water moves, first condensing inside the pores, then emerging onto the surface as droplets. That's never been seen before in a system like this, and at first we doubted our observations."
A material that defies physics
Before they understood what was happening, the researchers first thought that water was simply condensing onto the surface of the material due to an artifact of their experimental setup, such as a temperature gradient in the lab. To rule that out, they increased the thickness of the material to see if the amount of water collected on the surface would change.
"If what we were observing was due to surface condensation alone, the thickness of the material wouldn't change the amount of water present," explains Lee.
But, the total amount of water collected increased as the film's thickness increased, proving that the water droplets forming on the surface came from inside the material.
Even more surprising: the droplets didn't evaporate quickly, as thermodynamics would predict.
"According to the curvature and size of the droplets, they should have been evaporating," says Patel. "But they were not; they remained stable for extended periods."
With a material that could potentially defy the laws of physics in their hands, Lee and Patel sent their design off to a collaborator to see if their results were replicable.
"We study porous films under a wide range of conditions, using subtle changes in light polarization to probe complex nanoscale phenomena," says Guldin. "But we've never seen anything like this. It's absolutely fascinating and will clearly spark new and exciting research."
