It’s mid-October, and even if you live in one of the parts of the world that experiences wintertime cold, you probably haven’t had to scrape ice off your windshield yet. But imagine if you never had to do that again.
That’s the potential upshot of a new study from the University of Nebraska-Lincoln (UNL) and several scientific institutions in China. Experiments and simulations showed that, just like liquid droplets bead up on water-repellent surfaces rather than spread out, at colder temperatures those same droplets will “freeze upward.” Microscopically, they resemble a six-armed snowflake, with only a small portion of the base adhering to the surface. And with a gust of air, ice formed on water-repellent surfaces can be blown away.
Droplets on absorbent surfaces, by contrast, grow along the surface; molecular-level simulations have showed they almost immediately begin forming two stacked layers of ultra-thin, hexagonal ice – a form that has been dubbed “Nebraska ice” by study co-author Xiao Cheng Zeng. This ice encourages water molecules to essentially skate across it, and colonize other areas of the surface.
Zeng, who is a chemistry professor at UNL, explains it like this: "If the water and the surface don't have much chemistry in the beginning – they don't like each other – it's kind of like a divorce or separation. But if they like each other, they marry and stay together for a long time.”
The study findings suggest that applying water-repellent coatings or engineering surfaces that inherently repel water could enable windshield ice to be removed by a strong breeze.
Interestingly, the researchers found that the way water freezes on solid surfaces has much more to do with the surfaces themselves than temperature or pressure. The key is the angle formed where the droplets meet the surface, also known as the contact angle. A hydrophilic material allows droplets to spread across it at a small contact angle, whereas a hydrophobic surface forces them to bead up and form a larger angle. On a lab-fabricated or computer-simulated, defect-free surface, ice transitions from along-surface to off-surface growth at a contact angle between 30 to 45 degrees.
Increasing surface roughness, which the researchers did by enlarging its nanoscopic pores, also decreases the angular threshold – making ice easier to remove.
"People have been studying how water interacts with surfaces for a long, long time," added Zeng. "But this phenomenon was off the radar until now."