Columbia Scientists Discover That Water Molecules Define the Materials Around Us

The new findings emerged from Professor Sahin’s ongoing research into the strange behavior of spores, dormant bacterial cells. For years, Sahin and his students have studied spores to understand why they expand forcefully when water is added to them and contract when water is removed. (Several years ago, Sahin and colleagues garnered media coverage for harnessing that capability to create small engine-like contraptions powered by spores.)

Around 2012, Sahin decided to take a step back to ask why the spores behave the way they do. He was joined by researchers Michael S. DeLay and Xi Chen, authors on the new paper, who were then members of his lab. Their experiments did not provide a resolution to the mysterious behavior of spores. “We ended up with more mysteries than when we started,” Sahin remembers. They were stuck, but the mysteries they encountered were hinting that there was something worth pursuing.

After years of pondering potential explanations, it occurred to Sahin that the mysteries the team continually encountered could be explained if the hydration force governed the way that water moved in spores.

The team had to do more experiments to test the idea. In 2018, Harrellson, who is now a software engineer at the data analytics firm Palantir, joined the project.

“When we initially tackled the project, it seemed impossibly complicated. We were trying to explain several different effects, each with their own unsatisfying formula. Once we started using hydration forces, every one of the old formulas could be stripped away. When only hydration forces were left, it felt like our feet finally hit the ground. It was amazing, and a huge relief; things made sense,” he said.

The results of those experiments led the team and their collaborators to this paper. In addition to Harrellson, DeLay, Chen, and Sahin, the paper’s other authors are Ahmet-Hamdi Cavusoglu, Jonathan Dworkin, and Howard A. Stone. Adam Driks of Loyola University Chicago, who also contributed research, passed away before the completion of the work. Funding for the research was provided by the U.S. Department of Energy’s Office of Science and Basic Energy Sciences; by the Office of Naval Research; by the National Institutes of Health; and by the David and Lucile Packard Fellows Program.

The paper’s findings apply to huge amounts of the world around us: Hygroscopic biological materials–that is, biological materials that allow water in and out of them–potentially make up anywhere from 50% to 90% of the living world around us, including all of the world’s wood, but also other familiar materials like bamboo, cotton, pine cones, wool, hair, fingernails, pollen grains in plants, the outer skin of animals, and bacterial and fungal spores that help these organisms survive and reproduce.

The term coined in the paper, “hydration solids,” applies to any natural material that’s responsive to the ambient humidity around it. With the equations that the team identified, they and other researchers can now predict materials’ mechanical properties from basic physics principles. So far that was true mainly of gases, thanks to the well-known general gas equation, which has been known to scientists since the 19^th century.

“When we take a walk in the woods, we think of the trees and plants around us as typical solids. This research shows that we should really think of those trees and plants as towers of water holding sugars and proteins in place,” Sahin said: “It’s really water’s world.”