新型纳米结构材料通过自发吸附-释放机制实现大气水收集和无源冷却的功能，其核心在于材料的多级孔隙结构与表面能的协同作用。研究发现，这类材料在夜间利用环境湿度波动吸附气态水分子，白天通过光热效应或温差触发毛细作用力释放液态水。例如，金属有机框架材料（MOFs）因超高比表面积（如HKUST-1达692-1600 m²/g）和可调控的亲水官能团，可在15-30%相对湿度下实现0.4-1.2 g/g的吸水量。这种吸附过程遵循Type V等温线特征，通过水分子簇形成完成相变。
在冷却应用方面，材料的蒸发焓利用效
"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 on 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."
A Stabilized Cycle of Condensation and Release
It turns out that they had created a material with just the right balance of water-attracting nanoparticles and water-repelling plastic -- polyethylene -- to create a nanoparticle film with this special property.
"We accidentally hit the sweet spot," says Lee. "The droplets are connected to hidden reservoirs in the pores below. These reservoirs are continuously replenished from water vapor in the air, creating a feedback loop made possible by this perfect balance of water-loving and water-repelling materials."
A Platform for Passive Water Harvesting and More
Beyond the physics-defying behavior, the materials' simplicity is part of what makes them so promising. Made from common polymers and nanoparticles using scalable fabrication methods, these films could be integrated into passive water harvesting devices for arid regions, surfaces for cooling electronics or smart coatings that respond to ambient humidity.
"We're still uncovering the mechanisms at play," says Patel. "But the potential is exciting. We're learning from biology -- how cells and proteins manage water in complex environments -- and applying that to design better materials."
"This is exactly what Penn does best, bringing together expertise in chemical engineering, materials science, chemistry and biology to solve big problems," adds Lee.
The next steps include studying how to optimize the balance of hydrophilic and hydrophobic components, scale the material for real-world use and investigating how to make the collected droplets roll off surfaces efficiently.
Ultimately, the researchers hope this discovery will lead to technologies that offer clean water in dry climates or more sustainable cooling methods using only the water vapor already in the air.
This work was supported by National Science Foundation grants NSF-2309043 and NSF-1933704, a Department of Energy grant (DE-SC0021241), a Semilab UCL Chemical Engineering Impact Ph.D. Studentship, a National Science Foundation Graduate Research Fellowships Program grant (DGE-2236662), an Alfred P. Sloan Research Foundation grant (FG-2017-9406) and a Camille & Henry Dreyfus Foundation grant (TG-19-033).