A water droplet deposited on a cold surface freezes from the bottom up. As anyone who has made ice cubes knows, water expands when it freezes. But watch the outline of the drop carefully. The drop isn’t expanding radially outward while it freezes. Instead the remaining liquid part of the drop forms what’s known as a spherical cap, a shape like the sliced-off top of a sphere. Surface tension creates that spherical shape, but the water still has to expand when it freezes. The result? The last bit of the drop freezes into a point! This means that surface tension maintains the drop’s spherical shape, for the most part, and all the expansion the water does takes place vertically. (Video credit: D. Lohse et al.)
Tag: freezing
Frost Spreading
Frost typically forms when supercooled droplets of water scattered across a surface freeze together. The freezing spreads via tiny ice bridges that link droplets together into a frozen network. The animation above shows this process in action. Freezing starts in a droplet off-screen on the right and quickly spreads. Watch carefully, and you can see the ice bridges growing toward the unfrozen droplets. This is because the ice bridges are fed by water vapor evaporating from the droplets. If one can spread the droplets far enough from one another, it’s possible for a droplet to evaporate completely before the ice bridge reaches it, thereby disrupting the spread of frost. (Video credit: J. Boreyko et al.; research paper)
Freezing Soap Bubbles
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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Pancake Ice in the Sea
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!
Fluids Round-Up
New year, new (or renewed) experiments. This is the fluids round-up, where I collect cool fluids-related links, articles, etc. that deserve a look. Without further ado:
- Above is a new music video from the Julia Set Collection, featuring all non-CGI, fluids-based visuals. I spy soap films, vibrating liquids, and lots of cool effects with reflection and refraction. We featured some of their previous work, too.
- The Atlantic has a great piece about jellyfish and how they might just change our understanding of efficient swimming.
- Check out the wild shape-shifting of these drops of oil during freezing and learn about the plastic crystal phase some matter experiences.
- Nature has an interesting article on active matter, an intersection of physics and biology exploring how matter self-organizes, whether at the level of cells or the flocking of birds. (submitted by 1307phaezr)
- Ever wonder what the human face looks like in 457 mph winds? Wonder no more.
- Gizmodo has a beautiful set of macro photos of snowflakes. Interested in how snowflakes form and why there are so many different shapes? We’ve got you covered.
- Wired takes a look at the surf forecaster who predicts the waves for the Mavericks big-wave competition.
- Robert Krulwich (and friends) took a closer look at our fish in microgravity. Here’s what they learned!
(Video credit and submission: Julia Set Collection/S. Bocci; image credit: IRPI LLC, source)
Boiling Water to Snow
When it’s really cold outside–to the tune of -40 degrees (Fahrenheit or Celsius)–physics can get a little crazy. In this photo, boiling-hot water from a thermos turns into an instant snowstorm when tossed. How is this possible? It turns out there are a combination of factors that affect this. Firstly, the rate of heat transfer between two objects depends on the magnitude of the temperature difference between them. The bigger the difference in temperature, the faster the hot object cools. Of course, as the hot object cools down, the temperature difference between it and its surroundings is smaller and the rate of heat transfer decreases.
The second important factor here is that the water is being tossed. When you throw water, it breaks into droplets, and droplets have a large surface area compared to their volume. As it turns out, the rate of heat transfer also depends on surface area. By breaking the hot water into smaller droplets, you increase the surface area exposed to the cold air, allowing the hot water to freeze faster. (Image credit: M. Davies et al.; via Gizmodo)
Also: Since there are a few events scheduled around the country over the next couple months, I’ve added an events page where you can find details for those appearances. And as always, if you’re interested in scheduling a talk or event, feel free to contact me directly.
Freezing From Below
Watch closely as a droplet freezes on a cold surface, and you’ll observe something surprising. First, a freeze front will appear, traveling upward from the substrate. It curves slightly near the edges, leaving a liquid cap atop the frozen drop. But, as we’ve all discovered, water expands as it freezes. We can watch the drop freezing and see that the water isn’t expanding radially. Instead, the water expands vertically, forming a sharp tip or cusp just as the drop freezes completely. Remarkably, the geometry of the final tip doesn’t depend on the temperature of the substrate or on the wetting contact angle. (Video credit: L. Posada)
Growing Icicles
For those from colder climates, icicles are a familiar part of winter. They come in a multitude of shapes and sizes, many of which have been captured and cataloged in the Icicle Atlas project. The site contains images, videos, and measurements of more than 230 icicles grown in the lab over the course of four years of research into how and why icicles form the way they do. One interesting result of the work was the discovery that the ripples commonly found on icicles are directly related to impurities. Icicles grown with pure water remain smooth, and only those with ionic impurities, like salt, develop ripples. Check out more images and icicle research at the Atlas. (Video credit: A. Chen and S. Morris/The Icicle Atlas; via Discover)
Grow Your Own Snowflakes
If your Christmas holiday was a little too green (like mine was), Science Friday has just the activity for you – grow your own snowflakes! With a few materials you probably already have and some dry ice from the store, you can grow and observe ice crystals at home. Although these crystals form from water vapor instead of water droplets like proper snowflakes, they do exhibit different structures depending on temperature and humidity, just the way natural snowflakes do. (Video credit: Science Friday/F. Lichtman)
Growing Snowflakes
It’s easy to miss the beauty of a snowflake if you don’t take a close look. These tiny crystals form when water freezes onto a dust particle or other nucleation site, and they grow as water vapor freezes on to the nucleus. The structured appearance of a snowflake comes from the bonds formed between water molecules, but the exact type and shape of crystal formed–not all snowflakes are six-sided!–depends on the local temperature and humidity during freezing. This microscopic timelapse video by Vyacheslav Ivanov lets you watch the process in action. (Video credit: V. Ivanov; via io9)
