#hydrophobic

Fabrics that resist water are essential for everything from rainwear to military tents, but conventional water-repellent coatings have been shown to persist in the environment and accumulate in our bodies, and so are likely to be phased out for safety reasons. That leaves a big gap to be filled if researchers can find safe substitutes.

Now, a team at MIT has come up with a promising solution: a coating that not only adds water-repellency to natural fabrics such as cotton and silk, but is also more effective than the existing coatings. The new findings are described in the journal Advanced Functional Materials, in a paper by MIT professors Kripa Varanasi and Karen Gleason, former MIT postdoc Dan Soto, and two others.

“The challenge has been driven by the environmental regulators” because of the phaseout of the existing waterproofing chemicals, Varanasi explains. But it turns out his team’s alternative actually outperforms the conventional materials.

“Most fabrics that say ‘water-repellent’ are actually water-resistant,” says Varanasi, who is an associate professor of mechanical engineering. “If you’re standing out in the rain, eventually water will get through.” Ultimately, “the goal is to be repellent—to have the drops just bounce back.” The new coating comes closer to that goal, he says.

Two key physical phenomena take place at the surfaces of materials: catalysis and wetting. A catalyst enhances the rate of chemical reactions; wetting refers to how liquids spread across a surface.

Now researchers at MIT and other institutions have found that these two processes, which had been considered unrelated, are in fact closely linked. The discovery could make it easier to find new catalysts for particular applications, among other potential benefits.

“What’s really exciting is that we’ve been able to connect atomic-level interactions of water and oxides on the surface to macroscopic measurements of wetting, whether a surface is hydrophobic or hydrophilic, and connect that directly with catalytic properties,” says Yang Shao-Horn, the W.M. Keck Professor of Energy at MIT and a senior author of a paper describing the findings in the Journal of Physical Chemistry C. The research focused on a class of oxides called perovskites that are of interest for applications such as gas sensing, water purification, batteries, and fuel cells.

Since determining a surface’s wettability is “trivially easy,” says senior author Kripa Varanasi, an associate professor of mechanical engineering, that determination can now be used to predict a material’s suitability as a catalyst. Since researchers tend to specialize in either wettability or catalysis, this produces a framework for researchers in both fields to work together to advance understanding, says Varanasi, whose research focuses primarily on wettability; Shao-Horn is an expert on catalytic reactions.