FYFD

Tag: freezing

  • Reader Question: Snow from Boiling Water?

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    Reader kylewpppd asks:

    Have you seen the post of a man in Siberia throwing boiling water off of his balcony? Can you provide a better explanation of what’s going on?

    As you can see in the video (and in many similar examples on YouTube), tossing near boiling water into extremely cold air results in an instant snowstorm. Several effects are going on here. The first thing to understand is how heat is transferred between objects or fluids of differing temperatures. The rate at which heat is transferred depends on the temperature difference between the air and the water; the larger that temperature difference is the faster heat is transferred. However, as that temperature difference decreases, so does the rate of heat transfer. So even though hot water will initially lose heat very quickly to its surroundings, water that is initially cold will still reach equilibrium with the cold air faster. Therefore, all things being equal, hot water does not freeze faster than cold water, as one might suspect from the video.

    The key to the hot water’s fast-freeze here is not just the large temperature difference, though. It’s the fact that the water is being tossed. When the water leaves the pot, it tends to break up into droplets, which quickly increases the surface area exposed to the cold air, and the rate of heat transfer depends on surface area as well! A smaller droplet will also freeze much more quickly than a larger droplet.

    What would happen if room temperature water were used instead of boiling water? In all likelihood, a big cold bunch of water would hit the ground. Why? It turns out that both the viscosity and the surface tension of water decrease with increasing temperature. This means that a pot of hot water will tend to break into smaller droplets when tossed than the cold water would. Smaller droplets means less mass to freeze per droplet and a larger surface area (adding up all the surface area of all the droplets) exposed. Hence, faster freezing!

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    Freezing Bubbles

    If you find yourself some place really cold this holiday season, may I suggest stepping outside and having some fun freezing soap bubbles? The crystal growth is quite lovely, as seen in this photograph. If you live in warmer climes, fear not, you can always experiment in your freezer. It would be particularly fun, I think, to see how a half-bubble sitting on a cold plate freezes in comparison to a droplet like this one. (Video credit: Mount Washington Observatory)

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    Rapid Freezing

    Thermodynamics can play strange games with liquids.  Here a bottle of chilled soda water is used to demonstrate a method of rapid freezing.  Because the water is at a higher pressure than atmospheric, its temperature can be lower than the normal freezing point in a standard atmosphere.  This is why the soda water remains a liquid in the bottle.  However, when the bottle is opened, the pressure drops and the water’s temperature is too low to remain a liquid, so it rapidly freezes in the bottle. A similar mechanism may be at work below Antarctic glaciers. As the internal flow beneath the ice sheet forces water up submerged mountainsides, the pressure drops, causing the water to freeze into new ice at the bottom of the glacier. 

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    Freezing in a Microchannel

    Fluid mechanics at the microscale can behave quite differently than in our everyday experience. Microfluidic devices–sometimes known as labs on a chip–are becoming increasingly important in research and daily life. For example, the test strips used by diabetics to check their blood sugar levels are microfluidic devices.  In this video, researchers use a microfluidic channel to observe the freezing of supercooled water droplets. As the droplet first passes into the cold zone of the channel, it flash freezes, filling from the inside out with ice crystals. As it continues through the cold zone, the drop freezes fully, beginning at the outside surface and working inward. As it does so, the ice droplet fractures due to stresses. (Video credit: Stan et al)

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    Brinicles

    In the frozen reaches of our planet, the atmosphere and ocean can interact in bizarre ways.  Under calm ocean conditions when the air at sea level is much colder than the water temperature brinicles–the underwater equivalent to an icicle–can form. The cold air above rapidly freezes ocean water at the surface, concentrating water’s salt content into a very cold brine which sinks rapidly. As this brine descends, it freezes the water around it into an ice sheath. As the brinicle grows and eventually reaches the sea floor, its cold temperatures can wreak havoc on the creatures living there.

  • Freezing Drops

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    The physics of droplets freezing is important for understanding applications like ice formation on airplane wings. Here we see how a warm droplet deposited on a cold plate freezes. A freezing front advances through the drop, which expands vertically as it freezes. Ultimately, the expansion of the ice and the surface tension of the water create a pointed singular tip.

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    Freezing Soap Bubbles

    This is what it looks like when a soap bubble freezes. Perhaps not strictly fluid mechanical in nature, but it’s a nice thermodynamics demonstration.