FYFD

Tag: double diffusive convection

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    Salt Fingers

    Any time a fluid under gravity has areas of differing density, it convects. We’re used to thinking of this in terms of temperature — “hot air rises” — but temperature isn’t the only source of convection. Differences in concentration — like salinity in water — cause convection, too. This video shows a special, more complex case: what happens when there are two sources of density gradient, each of which diffuses at a different rate.

    The classic example of this occurs in the ocean, where colder fresher water meets warmer, saltier water (and vice versa). Cold water tends to sink. So does saltier water. But since temperature and salinity move at different speeds, their competing convection takes on a shape that resembles dancing, finger-like plumes as seen here. (Video and image credit: M. Mohaghar et al.)

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  • The Snowy Salt of the Dead Sea

    The Snowy Salt of the Dead Sea

    At nearly 10 times saltier than the ocean, the Dead Sea is one of the saltiest places on Earth, and since 1979, scientists have observed it growing even saltier as snow-like salt precipitates to the bottom of the lake. Numerical simulations have now confirmed that this salt-fall is the result of double-diffusive salt fingers.

    Here’s how the mechanism works: the upper layer of the lake is made up of warmer, saltier water covering deeper, colder waters. As the sun evaporates water near the surface, what’s left behind becomes saltier and heavier. Tiny pockets of this warm, salty water sink into colder regions and rapidly cool. The heat can move a lot more quickly than the salt, though, and since cold water cannot hold as much salt as warmer water, some of the salt precipitates out. That forms the falling crystals scientists observe sinking to the bottom of the lake. (Image and research credit: R. Ouillon et al., source; via Physics World; submitted by Kam-Yung Soh)

  • Layered Latte Physics

    Layered Latte Physics

    Latte lovers may be familiar with the layered latte, a beverage with distinctive horizontal layers mixing espresso and milk, but you may not have taken the time to wonder how these layers form. Like many layering phenomena in our oceans, the layered latte is the result of double-diffusive convection. This means that there are two variables that both affect density in the fluid mixture and that they act at different rates.

    In the latte, those factors are 1) the different densities of the milk and espresso and 2) density changes caused as the latte cools to room temperature. A layered latte forms when the lighter espresso is poured into denser milk. If it’s poured quickly enough, the momentum of the pour forces some of the espresso down into the milk, despite the buoyant force that tries to keep the espresso on top. So that initial pour sets up a density gradient that runs from pure espresso at the top to pure milk on the bottom, with varying mixtures of the two in between.

    The distinct layers won’t form until the latte begins cooling off. Along the walls of the container, heat is lost more quickly, causing fluid to cool and start sinking. But a specific bit of fluid can only sink until the fluid surrounding it is the same density. That can carry a cooler bit of latte to the bottom of a layer, but not into the denser layer below. At this point, our bit of latte moves inward, starts to warm up, and circulates up through the center of its layer. As when it sank, the fluid can only move up until it encounters a layer with equal or lesser density, at which point it must move horizontally instead. This thermal convection, combined with the density gradient formed by the initial pour, sets up the distinctive layers of the latte. The layers are quite stable – neither gentle stirring nor taking a sip will disrupt them for long – provided the drink remains warmer than the surrounding air. (Image credits: kopeattugu/Instagram, N. Xue et al.; research credit: N. Xue et al.; via NYTimes; submitted by Kam-Yung Soh)