Thursday, September 27, 2018

Fighting Ice With...Ice?

If you live in a part of the world with cold winters, you probably know the awful feeling that comes with an unexpectedly early frost or snow—one that covers your car in a layer of ice before you’ve pulled out your gloves and ice scraper for the season. The one that makes your fingers freeze in anticipation as you blast the defrost and pull out a credit card so that you can begin chipping away at the windshield.

You know the kind.
Even if you don’t live in a place like that, you’ve probably experienced an airplane de-icing delay, a frozen air conditioner, or some other ice-induced inconvenience. The unwanted accumulation of ice and frost isn't just an annoyance, it’s a safety issue and an economic issue—costing billions of dollars each year in lost productivity and repair work.

But in a proof-of-concept experiment recently reported in the journal ACS Applied Materials and Interfaces, a team of researchers from Virginia Tech and Oak Ridge National Laboratory introduced a new strategy for preventing ice buildup. This “passive anti-frosting technology” uses a simple design approach to keep surfaces mostly dry—no surface coating, salt, heat, or credit cards required. Considering that the United States alone uses 20 million tons of salt for deicing highways each year and that it takes 4,000 liters of antifreeze to deice a large aircraft, that’s also really good news for the environment.

Top: The team's anti-frosting technology applied to a small sheet of aluminum. Bottom: Project leader Farzad Ahmadi tests the anti-frosting surfaces inside a humidity chamber.
Image Credit: Virginia Tech / Farzad Ahmadi.
Project leader Farzad Ahmadi, a graduate student who works with Jonathan Boreyko in the Nature Inspired Fluids and Interfaces Lab at Virginia Tech, loves winter—but he admits that he’s not a fan of scraping windshields. If his research pays off, someday he might not need to. For the last few years, Ahmadi has been part of a team studying how frost spreads along a surface. (His experiments even earned him the nickname, the Iceman, as he is frequently seen carrying a cooler of ice to his lab.) In this recent project, the team demonstrated that patterning a surface with thin stripes of ice can prevent dew and frost formation on the rest of the surface.

The team started with several pieces of plain aluminum. On each sample (except for the controls) the researchers created a patterned array of thin grooves. The grooves were evenly spaced along the sample, covering 10% of its surface area. After patterning, the researchers filled the grooves with water, chilled the samples until the water froze, and put the samples in a chamber supersaturated with water vapor.

Normally, samples in this environment gets frosty as moisture from the air condenses onto the surfaces and freezes. That’s what happened to the control samples (i.e., a smooth aluminum surface and a superhydrophobic one), which were frosted within few minutes. However, the ice stripes disrupted the usual behavior. Instead, on the grooved samples, the ice created areas of low pressure that pulled the moisture to them, leaving the areas between the stripes free from dew and frost for more than three hours.

On some of the samples, the grooves were etched directly into the surface, but on others, the grooves were elevated on fins, as shown below. Over time, the ice in the grooves patterned directly on the flat surfaces started to spread out and reach into the dry space. However, this process slowed way down in the samples with elevated grooves. There, the ice stripes grew upward, away from the surface. Interestingly, even after 24 hours in a supersaturated environment, the ice stayed contained to the elevated areas. The remaining 90% of the surface was still dry, as well as the sides of the fins. According to the researchers, any layer of ice formed on the grooves is easy to shed, thanks to air trapped between the suspended ice and the surface.

This series of images (click to enlarge) shows an aluminum sample with elevated grooves after 0, 1, 2, and 3 hours in a supersaturated environment. You can see that the ice is contained to the elevated grooves and grows upward over time.
Image Credit: Farzad Ahmadi, Virginia Tech.
Following their experimental success, the team modeled the situation and used computer simulations to explore the optimal geometry—the arrangement that gives you the largest dry area. This, along with recent theoretical work, suggests that it’s possible to keep a surface even more than 90% dry with the technique. It sounds great, right? But will it work in real life? Could this proof-of-concept experiment actually translate into anti-icing windshields, airplane wings, or HVAC components?

According to the researchers, the technology should work on any surface that allows you to pattern water/ice. That’s promising. On the other hand, there’s a lot of work involved in taking something from the lab to the real world. For example, we’d need a method for patterning ice stripes on a large object. We’d also need to make sure that those patterns don’t interfere with the other functions of the surface or disrupt the airflow, in the case of an airplane wing.

As someone who lives where the winters are long and cold, I look forward to following the progress of this effort. In the meantime, my ice scraper is already in the car. You can never be too prepared for that first icy frost.

—Kendra Redmond
Jack Frost

Jack Frost is never heard or seen,
But you can tell just where he’s been,
For while you’re fast asleep at night,
He paints the world a sparkling white.

His freezing fingers, in a trice
Turn every puddle into ice.
On window panes those fingers trace
Fine twinkling patterns, just like lace.

On silken webs some spider’s spun
Jack Frost hangs crystals just for fun.
You can’t see Jack Frost, it’s true,
But he can still reach out at you.

So, wrap up warm, please, take my tip,
Or you’ll feel Jack Frost’s icy grip.

—Jillian Harker

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