Explaining the Roaming Rocks

For nearly a century, the long meandering tracks etched into Death Valley's Racetrack Playa remained a mystery. Clearly, some force was pushing the heavy rocks there and leaving behind these grooves.  (Image and video credit: Physics Girl; for related research see here) Read the full article

Dispelling Ice

In winter weather, delays pile up at airports when planes need de-icing. Our current process involves spraying thousands of gallons of chemicals on planes, but these chemicals are easily removed by shear stress and dissolution, meaning that by the time a plane takes off, there is little to no de-icing agent remaining on the plane. Instead, those chemicals become run-off. (Image, video, research, and submission credit: R. Chatterjee et al.; see also) Read the full article

Erie Ice

Lake Erie, the shallowest of the Great Lakes, sees large swings in ice cover over the winter. In late January 2022, the lake was nearly completely frozen over, with 94 percent of its area covered in ice. By February 3rd, ice cover had dropped to 62 percent before rising again to 90 percent by the 5th.  (Image credit: J. Stevens/USGS; via NASA Earth Observatory) Read the full article

Liquid water is easily electrically charged, due to its polar nature. That's why rubbing a comb is enough to deflect a stream of water. Ice is harder to charge, but it can happen, especially when there are temperature gradients across the ice. (Image credit: frost - Miriams-Fotos, figure - R. Mukherjee et al.; research credit: R. Mukherjee et al.; via ChemBites; submitted by Kam-Yung Soh)

Defrosting and deicing surfaces is an energy-intensive affair, with lots of heat lost to warming up system components rather than the ice itself. In a new study, researchers explore a faster and more efficient method that focuses on heating just the interface. They coated their working surface in a thin layer of iridium tin oxide, a conductive film used in defrosting. Then, once the surface was iced over, they applied a 100 ms pulse of heating to the film. That localized heat melted the interface, and gravity pulled away the detached ice. Compared to conventional defrosting methods, this technique requires only 1% of the energy and 0.01% of the time. If the method scales reliably to applications like airplane deicing, it would provide enormous savings in time and energy. (Image and research credit: S. Chavan et al.)

Cold rain splashing on airplane wings can freeze in instants. To prevent that, researchers look for ways to minimize the time and area of contact a drop has. Hydrophobic coatings and textures can do some of the work, but they are easily damaged and don’t always work well when it comes to freezing.

The new technique shown here uses ring-shaped “waterbowls” to help deflect drops. As the drop impacts and spreads, the walls of the ring texture force the lamella up and off the surface. This reduces both the time and area of contact and, under the right circumstances, cuts the heat transfer between the fluid and surface in half. The technique is useful for more than just preventing freezing, though; it would also be helpful for waterproofing breathable fabrics, where shedding moisture quickly without clogging pores is key to keeping the wearer dry. (Image and research credit: H. Girard et al.; via MIT News and Gizmodo)

Preventing ice and frost from forming on surfaces -- especially airplane wings -- is a major engineering concern. The chemical de-icing cocktails currently used in aviation are a short-lived solution, and while superhydrophobic surfaces can be helpful, they tend to be easily damaged and therefore impractical. Another possible solution, shown here, are so-called phase-switching liquids -- substances like cyclohexane that have freezing points higher than that of water. This means that they form a solid coating near the freezing temperature of water.

Water droplets on these coatings move in a random stick-slip walk (above) but they tend not to freeze. This is because freezing requires the droplets to release heat, which melts part of the phase-switching liquid. Now, instead of solidifying to the surface, the droplet moves on a film of the phase-switching liquid. Re-freezing that liquid is tough because it’s thermodynamically unfavorable, and the smoothness of the liquid layer makes it harder for ice to find a nucleation point. In lab tests, the phase-switching liquid surfaces resisted ice and frost more than an order of magnitude longer than conventional materials. (Image and research credit: R. Chatterjee et al.; video credit: Univ. of Illinois at Chicago; submitted by Night King)

At the altitudes where aircraft fly, it’s often cold enough for water drops to freeze in seconds or less. Once attached to a wing, such frozen drops disrupt the flow, reducing lift and increasing drag. To help understand how such droplets freeze, scientists study droplet impact on cold surfaces. Starting at room temperature (counter-clockwise from upper left), a drop will spread on the surface, then retract. When the temperature is colder, parts of the droplet freeze before retraction completes, leaving a thin sheet with a thicker center. At even colder temperatures, the droplet’s rim destabilizes and freezing occurs before the droplet has time to retract fully. And at the coldest temperatures, the droplet breaks apart into a frozen splash. (Image and video credits: V. Thievenaz et al.)

Keeping ice from forming on a surface is a major engineering challenge. Typically, there’s no controlling certain factors -- like the size and impact speed of droplets -- so engineers try to tame ice by changing the surface. This can be through chemicals -- as with deicing fluids used on aircraft -- or by tuning the surface itself. 

One way to do this is by making the surface superhydrophobic -- or extremely water repellent. These surfaces are rough on a nanoscale level, but they’re delicate, and once ice gets a grip on them, it’s even harder to remove. In a recent study, however, researchers used particles with both hydrophobic and hydrophilic -- water-attracting -- properties to create a superior ice-resistant surface. The combination of hydrophobic and hydrophilic aspects to the particles made supercooled droplets break up on contact with the surface. This made the drops smaller and decreased their contact time, making it harder for them to stick and freeze. (Image credit: Pixabay; research credit: M. Schwarzer et al.; via Chembites; submitted by Kam-Yung Soh)

Ice can be a terrible pest, freezing to surfaces like roads and airplane wings and causing all sorts of havoc. Some surfaces, though, can actually prompt a freezing drop to scrape itself off. There are a couple key effects in play here. The first is that the surface is nanotextured -- in other words, it has extremely small structures on its surface. This makes it hydrophobic, or water-repellent. The second key ingredient is that the drop is cooling evaporatively; that means heat is escaping along the air-water interface instead of conducting through the solid surface. As a result, the freezing front forms at the interface and pushes inward. Water expands as it freezes, which tries to force the interior liquid out, toward the bottom of the drop. On a normal surface, this would force the contact line -- where air, water, and surface meet -- to push outward. But the nanotexture of the hydrophobic surface pins that line in place. So the expanding ice pushes the frozen drop upward, scraping it off the surface! (Video and image credit: G. Graeber et al., source)

During winter, Canada’s Arctic Archipelago, home of the Northwest Passage, generally fills with sea ice. These ice bridges form in the long and narrow straits between islands. A new paper models ice bridge formation and break-up, showing that ice bridges can only form when ice floating in the strait is sufficiently thick and compact. To form a bridge, wind must first push the ice together and then frictional forces between individual pieces of ice must be large enough to resist wind or water driving them apart. As temperatures drop, the individual ice chunks can then freeze together into solid sheets until summer returns. 

The existence of a critical thickness and density of the ice field for ice bridge formation has important implications for climate change. As Arctic temperatures warm for longer periods, these waters may no longer generate ice of sufficient thickness and quantity for ice bridges to form. Since ice bridges serve as important oases for marine mammals and sea birds and help isolate Arctic sea ice from warmer waters, their loss will have a profound impact on both Arctic ecology and global climate. (Image credit: NASA Earth Observatory; research credit: B. Rallabandi et al.; via Physics Buzz)

Water is one of those strange materials that expands when it freezes, which raises an interesting question: what happens to a water drop that freezes from the outside in? A freezing water droplet quickly forms an ice shell (top image) that expands inward, squeezing the water inside. As the pressure rises, the droplet develops a spicule -- a lance-like projection that helps relieve some of the pressure. 

Eventually the spicule stops growing and pressure rises inside the freezing drop. Cracks split the shell, and, as they pull open, the cracks cause a sudden drop in pressure for the water inside (middle image). If the droplet is large enough, the pressure drop is enough for cavitation bubbles to form. You can see them in the middle image just as the cracks appear. 

After an extended cycle of cracking and healing, the elastic energy released from a crack can finally overcome surface energy’s ability to hold the drop together and it will explode spectacularly (bottom image). This only happens for drops larger than a millimeter, though. Smaller drops -- like those found in clouds -- won’t explode thanks to the added effects of surface tension. (Image credit: S. Wildeman et al., source)

ETA: A previous version of this post erroneously said this was freezing from the “inside out” instead of “outside in”.

Watching a snowflake grow seems almost magical--the six-sided shape, the symmetry, the way every arm of it grows simultaneously. But it’s science that guides the snowflake, not magic. Snowflakes are ice crystals; their six-sided shape comes from how water molecules fit together. The elaborate structures and branches in a snowflake are the result of the exact temperature and humidity conditions when that part of the snowflake formed. The crystals look symmetric and seem to grow identical arms simultaneously because the temperature and humidity conditions are the same around the tiny forming crystals. And the old adage that no two snowflakes are alike doesn’t hold either. If you can control the conditions well enough, you can grow identical-twin snowflakes! (Video credit: K. Libbrecht)

Inside Science has a new documentary all about fluid dynamics! It features interviews with five researchers about current work ranging from the physics of surfing to the spreading of diseases. Penguins, sharks, archer fish, 3D printing, and influenza all make an appearance (seriously, fluid dynamics has everything, guys). If you’d like to learn more about some of these topics, I’ve touched on several of them before, including icing, penguin physics, shark skin, archer fish, and disease transmission via droplets.  (Video credit: Inside Science/AIP)

I’m not a winter person, but there’s something almost magical about the way water freezes. From instant snow to snow rollers and weird ice formations to slushy waves, winter brings all kinds of bizarre and unexpected sights. The video above is an artistic look at one of my favorites -- freezing soap bubbles. Normally, the thin film of a soap bubble is in wild motion, convecting due to gravity, surface tension differences, and the surrounding air. Such a thin layer of liquid loses its heat quickly, though, and, as ice crystals form, the bubble’s convection and rotation slow dramatically, often breaking the thin membrane. Happily photographer Paweł Załuska had the patience to capture the beautiful ones that didn’t break!  (Video credit: P. Załuska; via Gizmodo)

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Sea ice forms in patterns that depend on local ocean conditions. Pancake ice, like that shown in the above photo from the Antarctic Ross Sea, is formed in rough ocean conditions. Each individual pancake has a raised ridge along its edge, due to wave-induced collisions with other pieces of ice. Over time the smaller pieces of ice will merge together, forming large sheets. Evidence of its turbulent formation will persist, however, in the rough surface of the ice’s underside. For more, check out the National Snow and Ice Data Center. (Image credit: S. Edmonds; via Flow Visualization)

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Reminder: If you’re at the University of Illinois at Urbana-Champaign, I’m giving a seminar this afternoon. Not in Illinois? I’ve got other events coming up, too!