Just Physics: Dirty ice may be ugly, but it has one advantage

KATRINA MILLER

What’s the worst part of winter? The perpetual shovelling of snow? The bitter wind that whips across your face? Some might say the real villain is ice, which causes slips and falls, sends cars spinning and delays flights. Engineers in Chicago have made it their mission to better understand the basics of ice and, better yet, how to get rid of it. Their latest research, published in the journal Materials Horizons, reveals that while water freezes, even tiny amounts of a contaminant dramatically decreases its tendency to stick to a surface.

The discovery could someday lead to less damaging de-icing salts — which corrode metal and infrastructure and harm the environment — or pave the way for alternative melting agents. Researchers have been studying how ice adheres to surfaces for a long time, though their research almost always focuses on pure ice, said Sushant Anand, a mechanical engineer at the University of Illinois Chicago who led the study.

“But water isn’t pure on the roads or in the oceans,” he said. “And when this water freezes on surfaces, then it forms this ice that has all these contaminants inside.” To study how this “dirty” ice sticks to surfaces, Dr. Anand and his colleagues mixed pure water with varying concentrations of table salt, soap or alcohol. They then put droplets of the contaminated water on surfaces made of copper, glass or silicon, and measured how much force it took to unstick the droplets after they froze.

All of the contaminants weakened the strength of the ice’s grip, but salt and alcohol did the best job. (Perhaps salt’s effectiveness isn’t so surprising: It is widely used to de-ice walkways and roads.) What shocked the team was how much the contaminants decreased the ice’s stickiness — anywhere from 100 to 1,000 times, Dr. Anand said.

In general, decreasing ice adhesion is difficult, and researchers are developing advanced techniques to do so. But the team was able to accomplish this goal just with common impurities. “I didn’t really want to believe the results,” Dr. Anand said. “So we spent actually three or four years repeating the experiments so that we could say with confidence what we were seeing was accurate.”

Chang-Hwan Choi, a mechanical engineer at the Stevens Institute of Technology in New Jersey who edited a book on ice adhesion, said the results were “really timely, and will open a new avenue for research in this area.” Little information has been published about contaminated ice, added Dr. Choi, who was not involved in the new research.

Simulations led by Subramanian Sankaranarayanan, a chemical engineer at Argonne National Laboratory in Illinois and an author of the study, gave the team insight into what was happening at the molecular level. As the ice formed, salt particles were pushed to the edges of the droplet and into the semi-liquid layer between the droplet and the surface it was attached to. The presence of salt thickened the layer, making it easier to loosen the ice. This data could eventually lay the groundwork for ice-repellent roads and solar panels, or for better anti-ice coatings for power lines and aircraft wings, Dr. Choi said. The findings may even lead to a method of making seawater drinkable by desalinating water through freezing. But it will require a lot more work to get there, since the results of the study cannot be generalized to all impurities in nature, Dr. Choi said.