FYFD

Tag: surface tension

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    Engineering Droplets

    A jet of falling liquid doesn’t remain a uniform cylinder; instead, it breaks into droplets. In this video, Bill Hammack explores why this is and what engineers have learned to do to control the size of the droplets formed.

    The technical name for this phenomenon is the Plateau-Rayleigh instability. It begins (like many instabilities) with a tiny perturbation, a wobble in the falling jet. This begins a game of tug of war. One of the competitors, surface tension, is trying to minimize the surface area of the liquid, which means breaking it into spherical droplets. But doing so requires forcing some of the the liquid to flow upward, against both gravity and the liquid’s inertia. The battle takes some time, but eventually surface tension wins and the jet breaks up.

    That’s not necessary a bad thing. It’s actually key to many engineering processes, like ink-jet printing and rocket combustion, as Bill explains in the full video. (Video and image credit: B. Hammack; submitted by @eclecticca)

  • Capillary Action and Sand Castles

    Capillary Action and Sand Castles

    Capillary action – or capillarity – is the ability of liquids to flow through narrow constrictions. It results from intermolecular forces between fluids and solids. It’s a combination of surface tension – which creates cohesion within the liquid – and adhesion, which allows the liquid and solid to hold to one another. Together, these forces propel the liquid to flow through narrow gaps.

    In the video below, a saturated mixture of sand and water is poured into a mold on a bed of dry sand. When left to settle, much of the water flows from the mold into the dry sand bed through capillary action. When the mold is removed (top), the sand holds its shape, something it can’t do without a porous bed to soak in the excess liquid. (Image and video credit: amàco et al.)

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    Drinking Coffee in Space

    You probably don’t give much thought to the forces involved in drinking here on Earth. That’s because gravity’s effects dominate over everything else. Our cups are designed to hold a liquid until we use gravity to pour it into our mouths. But that technique doesn’t work in microgravity. There other forces govern how liquids flow: specifically surface tension and capillary action.

    Both of these forces are the result of intermolecular attractions. In the case of surface tension, it’s the attraction that the molecules of a liquid feel for one another that keeps them in a cohesive bunch. Capillary action is similar, but it’s an attraction between the liquid molecules and those of the solid they’re wetting. When you combine them both, you get the ability for liquids to climb up a narrow gap and pull more liquid up behind them. That’s the key science behind every version of the “space cup” developed by astronaut Don Pettit and his collaborators. 

    To hear more about the development and engineering of the cup (and exactly why it makes drinking coffee so much more enjoyable in space than it would be otherwise) check out the full video. And, in case you’re wondering, there’s a special microgravity champagne flute, too! (Image and video credit: It’s Okay to Be Smart)

  • The Bouncing Drop

    The Bouncing Drop

    For a droplet to bounce, we expect it to hit a wall or a sharp interface of some kind. But in a new study, researchers demonstrate a droplet that bounces with neither. Shown above is an oil droplet sinking through a stratified mixture of ethanol (toward the top) and water (toward the bottom). Because the oil is heavier than ethanol, it initially sinks, dragging some of the ethanol with it as it falls. Over time, some of that ethanol rises again, forming what’s known as a buoyant jet.

    Simultaneously, the gradient of ethanol to water between the top and bottom of the drop creates an imbalance in surface tension. The ethanol near the top of the drop has a lower surface tension than the water at the bottom. This creates a downward Marangoni flow along the drop interface.

    The bounce itself happens quickly after a long, slow sinking period. As the drop’s sinking slows, the buoyant jet weakens until it disappears completely. At the same time, the downward Marangoni flow pulls fresh ethanol-rich fluid toward the top of the drop. That increases the surface tension difference and strengthens the Marangoni flow, creating a positive feedback loop. In less than a second, the Marangoni flow increases by two orders of magnitude, pulling so hard that the drop shoots upward.

    That resets the cycle by weakening the Marangoni flow and strengthening the buoyant jet. The droplet can continue bouncing for about 30 minutes until the concentration gradient is so well-mixed that the cycle can’t continue. (Image and research credit: Y. Li et al.; via APS Physics; submitted by Kam-Yung Soh)

  • Coalescence at the Smallest Scales

    Coalescence at the Smallest Scales

    The coalescence of two water droplets happens so quickly, it’s essentially impossible to see, even with high-speed cameras. For this reason, researchers have turned to simulating molecular dynamics – essentially building computer programs that model the actions of all the molecules contained in the water droplets. Viewed this way, the very first contact between drops comes from thermal fluctuations – the random jumping of molecules across the separating gap. Once the bridge starts to form, it continues to grow, driven by thermal forces and opposed by surface tension. Eventually, this thermal regime gives way to the more familiar hydrodynamic one, where the bridge is large enough for flow to drive its growth. (Image credits: experiment – S. Nagel et al.; simulation – S. Perumanath et al.; research credit: S. Perumanath et al.; submitted by Rohit P.)

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    Agnes Pockels: Surface Science Pioneer

    Today’s FYFD video tells a story I’ve wanted to share for a couple of years now. It’s about the life and work of Agnes Pockels, a woman born in the mid-nineteenth century who, despite a lack of formal scientific training, made major contributions to the understanding of surface tension and to the experimental apparatuses and methodologies used in surface chemistry in general. She accomplished all of this not in a scientific lab, but from her kitchen.

    Pockels’ story is one of curiosity, determination, and meticulous scientific inquiry. Chances are that you’ve never heard of her, but you really should. Check out the full video below to learn more! (Image and video credit: N. Sharp)

  • Magma Mixing

    Magma Mixing

    Magmas typically consist of a mixture of molten liquid, bubbles, and solid crystals. As they mix, those crystals can sink from one viscous layer into another. To investigate this sort of process, researchers studied solid particles sinking across an interface between two viscous liquids. This is what we see above. One fluid is clear; the other is dyed red, and gravity points toward the left so the particles fall from right to left.

    What happens when the particle reaches the interface between fluids depends on three main factors: the gravitational force acting on the particle, the surface tension at the interface, and the ratio of the viscosities of the two fluids. The researchers observed two main outcomes. In one (top), the particle slows at the interface and breaks through slowly, its surface wetted by the second fluid so that it drags little to none of the previous fluid with it. The researchers named this the film drainage mode. It tends to occur when the viscosity ratio between fluids is large.

    The second method, shown in the bottom image, is the tailing mode. As the particle approaches, the interface deforms. A thick layer of the first fluid coats the particle even as it pass through, forming a tail that destabilizes behind the falling particle. This mode occurs when the viscosity ratio is small or the gravitational force is large compared to the surface tension. (Image and research credit: P. Jarvis et al.)

  • Keeping Bubbles Around

    Keeping Bubbles Around

    Bubbles don’t stick around in pure water. Surfactants are needed to stabilize the thin liquid film for longer than the blink of an eye. But that’s not necessarily the case for other liquids. As the video below shows, a bubble in isopropyl alcohol is quite stable. This is because of the alcohol’s volatility – its ability to evaporate easily.

    As the alcohol in the bubble film evaporates, it cools the film, creating a difference in surface tension that pulls fresh alcohol up into the bubble film. It’s so efficient at pulling alcohol up that the alcohol can’t evaporate fast enough to use it all. Once the excess alcohol is heavy enough, it slides back down the side of the bubble. Overall, though, the process is enough to keep a bubble in pure isopropyl alcohol from rupturing for minutes to hours at a time. (Image and video credit: M. Menesses et al.)

  • Collective Motion: Nematodes

    Collective Motion: Nematodes

    We often imagine that collective motion creates an advantage – that the schooling fish and flocks of birds gain something from this behavior – but that’s not always the case. Above, you see nematodes moving through a thin liquid layer. Random collisions occasionally bring the nematodes into contact, and once that happens, surface tension holds them together with a force that exceeds what their muscles can supply. Essentially, they move together for the same reason that Cheerios clump together in your cereal bowl. But despite being stuck alongside one another, there’s no change in how the nematode moves. It sees neither an advantage nor a disadvantage from being attached to its neighbor. (Image and research credit: S. Gart et al., source)

    This post completes our series on collective motion. Check out the previous posts about honeybee waves, how crowds are like sand, the fluid properties of worms, and why a lack of randomness makes predicting group behaviors hard.

     

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    “The Empire of C”

    Filmmaker Thomas Blanchard has once again released a beautiful, fluid-filled short to captivate us. Built from paint, oil, and liquid soap, “The Empire of C” feels like it gives viewers a birds-eye perspective over a fantastical land. I was particularly drawn to two fluid dynamical aspects of the film. The first were the dendritic sequences in the opening, which feel a bit like watching river deltas form in real time. Despite their resemblance to the Saffman-Taylor instability, I think these fingers are interfacially driven – meaning that they result from differences in surface tension between the different liquids Blanchard is using. 

    The second thing that caught my eye and made me rewind the video over and over were the glittery droplets. The glitter acts like tracer particles, allowing you to see the flow inside the droplets. Check out that counter-circulation compared to the paint flowing by outside! It’s a reminder that even inside a seemingly still droplet, there’s lots going on. (Video and image credit: T. Blanchard)