Dirty ice can be ugly, but it has one advantage
What’s the worst thing about winter? Eternal snow shoveling? The bitter wind blowing across your face?
Some might say the real villain is ice, which causes slips and falls, spins cars, and delays flight. Engineers in Chicago have made it their mission to better understand the basics of ice cream and, better yet, how to get rid of it.
Their latest research, published in the journal Materials Horizonsshows that as water freezes, even small amounts of a contaminant dramatically reduce its tendency to adhere to a surface.
The discovery could one day lead to less harmful deicing salts – which corrode metals and infrastructure and harm the environment – or pave the way for alternative melting agents.
Researchers have long studied how ice adheres to surfaces, although their research almost always focuses on pure ice, says Sushant Anand, a mechanical engineer at the University of Illinois at Chicago who led the study.
“But water is not pure on the roads or in the oceans,” he said. “And when this water freezes on surfaces, it forms ice that contains all these contaminants.”
To study how this ‘dirty’ ice sticks to surfaces, Dr. Anand and his colleagues pure water with varying concentrations of table salt, soap or alcohol. They then placed droplets of the contaminated water on surfaces of copper, glass or silicon, and measured how much force it took to loosen the droplets after they had frozen.
All 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’s widely used to defrost sidewalks and roads.) What shocked the team was the extent to which the contaminants reduced the stickiness of the ice — somewhere between 100 and 1,000 times, said Dr. . Anand.
In general, reducing ice adhesion is difficult, and researchers are developing advanced techniques to do this. But the team managed to achieve this goal, only with common impurities.
“I didn’t really want to believe the results,” said Dr. Anand. “So we repeated the experiments for three or four years so that we could say with confidence that what we saw was correct.”
Chang-Hwan Choi, a mechanical engineer at 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 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 drop and into the semi-liquid layer between the drop and the surface to which it was attached. The presence of salt made the layer thicker, making it easier to release the ice.
This data could ultimately lay the foundation for deicing roads and solar panels, or better anti-icing coatings for power lines and airplane wings, said Dr. Choi. The findings could even lead to a method to make seawater drinkable by desalinating water through freezing.
But it will take a lot more work to get there, because the study’s results cannot be generalized to all impurities in nature, said Dr. Choi. He added that there are many more contaminants to consider – such as bacteria, oil and dirt – and all the different types of surfaces that ice sticks to in the real world.
So what do we think about all the sludge mixed with days-old snow on the streets this winter? The jury is still out.
“Contaminants such as salt are known to cause freezing delay,” said Dr. Anand. “Whether insoluble contaminants such as dirt do the same cannot be said at this time.”