FYFD

Tag: surface tension

  • The Colorful Dissolution of Candies

    The Colorful Dissolution of Candies

    Many solids can dissolve in liquids like water, and while this is often treated as a matter of chemistry, fluid dynamics can play a role as well. As seen in this video by Beauty of Science, the dissolving candy coating of an M&M spreads outward from the candy. This is likely surface-tension-driven; as the coating dissolves, it changes the surface tension near the candy and flow starts moving away thanks to the Marangoni effect. With multiple candies dissolving near one another, these outward flows interfere and create more complex flow patterns. 

    These flows directly affect the dissolving process by altering flow near the candy surface, which may increase the rate of dissolution by scouring away loose coating. They can also change the concentration of dissolved coating in different areas, which then feeds back to the flow by changing the surface tension gradient. (Video and image credit: Beauty of Science)

  • Cavity Collapse

    Cavity Collapse

    One of the most iconic images in fluid dynamics is that of a drop impacting a liquid. When a drop hits a pool, it creates a crater, or cavity. That cavity expands and then collapses to form a jet that rebounds above the pool’s surface. If the jet is fast enough, it will eject one or more droplets before it falls back into the pool. Faster droplets, like the one that formed the cavity and jet shown above, actually create slower and fatter jets. In this regime, the complicated interplay of surface tension and gravity effects results in a jet velocity that is independent of impact speed and the liquid’s viscosity. Understanding this jet and splash dynamics is important for many industrial applications, including ink-jet printing. (Image credit: G. Michon et al.)

  • Water Skiing Beetles

    Water Skiing Beetles

    Waterlily beetles employ an unusual method of getting around: they skim across the water surface. The beetles are mostly covered in tiny hairs that help make their body hydrophobic (water-repellent) – a common adaptation for insects that spend their time sitting on the water’s surface – but the beetles also have hydrophilic claws on their legs that help anchor them to the water’s surface. When they need to move quickly, the beetles lean upright and start flapping their wings, creating thrust that helps push them along the interface. Between water’s viscosity and drag from the waves the insect generates, it has to expend a lot of energy for this method of travel – more than these insects do flying in air – but researchers suspect that staying at the surface could remain beneficial for the beetles because it’s easier to locate their floating food sources this way. (Image credit: H. Mukundarajan et al., source; via New Scientist)

  • Spreading Bubbles Help Nature’s Scuba Divers

    Spreading Bubbles Help Nature’s Scuba Divers

    How liquid droplets spread on solid surfaces is pretty well understood, but researchers have looked less at the related problem of how a gas spreads. In a recent paper, scientists have examined the spreading dynamics of bubbles impacting an immersed solid. As the bubble contacts the surface, it quickly squeezes out water trapped between the bubble and the gas layer trapped at the solid surface. The bubble squishes as surface tension tries to flatten the liquid-gas interface. Buoyancy also helps flatten the bubble. The spreading is remarkably fast, taking only about 10 milliseconds. That’s good news for the many insects who use trapped air bubbles like these to breathe underwater. Check out the video below to learn about some of these natural scuba divers.  (Image credit: H. de Maleprade et al., source; video credit: Deep Look)

  • Self-Propelling Drops

    Self-Propelling Drops

    Droplets of acetone deposited on a bath of warm water can float along on a Leidenfrost-like vapor layer. The droplets are self-propelling, too, thanks to interactions between the acetone and water. Acetone can dissolve in water, and when acetone vapor beneath the drop gets absorbed into the water bath, it lowers the local surface tension. That drop in surface tension creates a pull in the direction of a higher surface tension; this is what is known as the Marangoni effect. Because of that flow in the direction of higher surface tension, the acetone drop accelerates away. (Image credit: S. Janssens et al., source)

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    Hot Versus Cold

    Did you know that you can hear the difference between hot and cold water when they’re poured? Go ahead and give the video above a listen to try it out. I’ll wait.

    As explained in the video, the viscosity of water changes with temperature – the higher the temperature, the lower the viscosity. In fact, the viscosity of water at 10 degrees Celsius is more than 4 times higher than the viscosity at 100 degrees Celsius! That’s pretty significant, and it’s a big enough difference that we can hear it in the splash, even if we don’t see the difference when pouring. 

    Surface tension also decreases with temperature but not nearly as strongly. That 100 degrees Celsius water has 25% less surface tension than the 10 degrees Celsius water. But the combination of this change in viscosity and change in surface tension is why your cold water is more likely to dribble down the spout of your coffee pot when you’re filling the coffee machine than when you’re pouring coffee from the same pot. (Video credit: Steve Mould and Tom Scott; submitted by entropy-perturbation)

  • Surface Tension’s Pop

    Surface Tension’s Pop

    Surface tension in a liquid arises from molecular forces. Within a liquid like water, a molecule inside the fluid experiences equal tugs from similar molecules in every direction. A molecule at the surface, on the other hand, experiences the pull of similar molecules only on some sides. The net effect of this imbalance is a tensile force along the liquid surface that acts kind of like a sheet of elastic rubber – this is the effect we call surface tension. If you break the surface tension in a soap film like the one shown above, any tear will expand rapidly as the intact surface tension at the edges pulls the interior fluid away from the tear. (Image credit: C. Kalelkar and A. Sahni, source)

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    Sloshing in Space

    Last month, French astronaut Thomas Pesquet posted a video of some experiments he did on the International Space Station exploring the movement of fluids in microgravity. He filmed the experiments as part of the SPHERES Slosh project. Sloshing is the technical term for how liquids respond to the motion of their container, and it’s a tough problem whether you’re carrying a full coffee mug on Earth or dealing with a partially-emptied fuel canister in orbit.

    Here on Earth, gravitational forces dominate how fluids respond, but in microgravity, surface tension is a more powerful player. Pesquet’s demonstrations help scientists here on Earth better understand and model how liquids respond to movement in space. One major application for this is in spacecraft fuel tanks, which engineers must be able to design so that they empty themselves consistently with or without the added complications of spinning, maneuvering, or impulsive kicks of acceleration. (Video and image credit: ESA; submitted by gdurey)

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    Bursting Droplets

    Mixing multiple fluids can often lead to surprising and mesmerizing effects, whether it’s droplets that dance or tears along the walls of a wine glass. A recent paper highlights another such mixture-driven instability – the bursting of a water-alcohol droplet deposited on an oil bath. The Lutetium Project tackles the physics behind this colorful burst in the short video above. The behavior is driven by the quick evaporation rate of alcohol in the droplet and the way this changing chemical concentration affects surface tension in the droplet. Alcohol evaporates more quickly from the edges of the drop, creating a region of higher surface tension around the edge. This pulls fluid to the rim of the drop, where it breaks up into droplets that get pulled outward as the inner drop shrinks.

    The oil bath plays an important role in the instability, too. Without it, friction between the drop and a wall is too high for the droplet to “burst”. A thick layer of oil acts as a lubricant, allowing the escaping satellite drops to speed away. (Video and image credit: The Lutetium Project; research credit: L. Keiser et al.; submitted by G. Durey)

  • Supporting Bubbles

    Supporting Bubbles

    Surface tension holds small droplets in a partial sphere known as a spherical cap. But when droplets become larger, they flatten out into puddles due to the influence of gravity. In contrast, soap bubbles remain spherical to much larger sizes. The bubble pictured above, for example, is more than 1 meter in radius and nearly 1 meter in height.

    There is a maximum height for a soap bubble, though, and it’s set by the physical chemistry of the surfactants used in the soap. To support itself, the bubble requires a difference in surface tension between the top and bottom of the bubble. A higher surface tension is necessary at the top of the bubble to help prevent fluid from draining away. The difference in surface tension between the top and bottom of the bubble can never be greater than the difference in surface tension between pure water and the soap mixture – thus those values set a maximum height for a bubble. The researchers found their bubbles maxed out at a height of about 2 meters, consistent with their theoretical predictions. (Image credit: C. Cohen et al.; via freshphotons)