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  • The Coalescence Cascade and Surfactants

    The Coalescence Cascade and Surfactants

    Drops of a liquid can often join a pool gradually through a process known as the coalescence cascade (top left). In this process, a drop sits atop a pool, separated by a thin air layer. Once that air drains out, contact is made and part of the drop coalesces. Then a smaller daughter droplet rebounds and the process repeats.

    A recent study describes a related phenomenon (top right) in which the coalescence cascade is drastically sped up through the use of surfactants. The normal cascade depends strongly on the amount of time it takes for the air layer between the drop and pool to drain. By making the pool a liquid with a much greater surface tension value than the drop, the researchers sped up the air layer’s drainage. The mismatch in surface tension between the drop and pool creates an outward flow on the surface (below) due to the Marangoni effect. As the pool’s liquid moves outward, it drags air with it, thereby draining the separating layer more quickly. The result is still a coalescence cascade but one in which the later stages have no rebound and coalesce quickly. (Image and research credit: S. Shim and H. Stone, source)

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  • When Chaos is Not So Chaotic

    When Chaos is Not So Chaotic

    In industry, tanks are often agitated or stirred to mix different elements. The goal is to create a laminar but chaotic flow field throughout the mixture. Introducing particles to such a system reveals that things are not quite as chaotic as they might seem. The photographs above show the pathlines of various large, glowing particles initially poured into the tank from above. Over time, the particles scatter off of structures in the mixed sections of the tank and end up trapped in vortex tubes that form above and below the agitator. Once trapped in the vortex tube, the particles follow helical paths inside the tube, creating patterns like those seen in the lower two photos. (Image and research credit: S. Wang et al., 1, 2, 3)

  • Putting Out Fires

    Putting Out Fires

    Fires in large, open spaces like aircraft hangers can be difficult to fight with conventional methods, so many industrial spaces use foam-based fire suppression systems. These animations show such a system being tested at NASA Armstrong Research Center. When jet fuel ignites, foam and water are pumped in from above, quickly generating a spreading foam that floats on the liquid fuel and separates it from the flames. Since the foam-covered liquid fuel cannot evaporate to generate flammable vapors, this puts out the fire. 

    The shape of the falling foam is pretty fascinating, too. Notice the increasing waviness along the foam jet as it falls. Like water from your faucet, the foam jet is starting to break up as disturbances in its shape grow larger and larger. For the most part, though, the flow rate is high enough that the jet reaches the floor before it completely breaks up. (Image credit: NASA Armstrong, source)

  • Escaping Quicksand

    Escaping Quicksand

    Quicksand is complicated stuff. It’s typically a mixture made up of sand, clay, and water. To get those ingredients into a proper quicksand mixture, you have to liquefy the particles by saturating the spaces between them with water, as the jumping tourists in the top animation are doing. (That’s not to say that you can’t just find a patch of quicksand – just that something has to have pumped that area full of water first.)

    If you end up in quicksand, don’t panic. Quicksand is denser than a human, which means that, at the worst, you won’t sink in much further than your waist (middle image). It’s tough to move once you sink because your weight has squeezed a lot of the water out from between the sand and clay particles, thereby drastically increasing the viscosity. To get out, try putting weight on one leg and wiggling the other back and forth (bottom image). This lets water back in the mixture and hopefully lets you free that leg. Once one leg is free, try to kneel on it and work the other leg out. (Image credits: making quicksand – T. L. Nguyen, source; stuck – National Geographic, source; escape – Tech Insider, source; research credit: G. Evans et al., A. Khaldoun et al.)

  • 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)

  • Flow Above the Treetops

    Flow Above the Treetops

    As this smoke visualization shows, trees have a significant impact on airflow around them. Flow in the image is from left to right. On the left, the upstream air is traveling in smooth, laminar lines that are quickly disrupted as the flow moves into the trees. After the first shorter trees, flow inside the wooded area has been broken up and slowed. Above the canopy, the smoke streaklines have also slowed and become more turbulent. Understanding how wind and trees interact is important in a variety of applications, including when adding renewable energy options to buildings and when predicting the spread of forest fires. (Image credit: W. Frank et al.)

  • Sniffing Underwater

    Sniffing Underwater

    You’d be forgiven for thinking that the star-nosed mole looks funny. Its distinctive star-shaped nose is a highly-sensitive organ, but the mole doesn’t just use it for finding its way through the underground tunnels it lives in. These moles can actually sniff underwater. By exhaling a bubble and then re-inspiring it, the moles collect scent particles that they can use to locate food. In experiments, both star-nosed moles and water shrews could use this technique to successfully follow a scent trail, demonstrating exploring and pausing behaviors similar to terrestrial sniffing as they did. To learn more about this impressive mammal, listen to the latest episode of Science Friday, where research Ken Catania describes his work with them. (Image credits: K. Catania; via Science Friday)

  • Watching a Model Rocket Burn

    Watching a Model Rocket Burn

    Rockets operate on a pretty simple principle: if you throw something out the back really fast, the rocket goes forward. Practically speaking, we accomplish this with a combination of chemistry and physics, by burning fuel and oxidizer together and accelerating the exhaust out a nozzle. Solid rocket propellant, like that found in the model rockets shown here, is a combination of fuel and oxidizer that don’t react until they’re ignited. You don’t want your rocket to just explode as soon as it’s lit, though, so solid rocket motors are carefully designed to burn in a particular way. By packing the propellant into different shapes – and even including patterns of propellants with different burn rates – engineers can create a rocket that burns with the thrust pattern they want.

    In the case of this model rocket motor, what we observe is not really how it is intended to burn; you can see how some of the combustion products are working their way out of cracks that wouldn’t normally exist. But the video and animation do show how the burn front moves gradually through the engine, allowing it to produce a relatively steady amount of thrust for a longer period before reaching the darker burning propellant on the left, which would normally launch the model rocket’s parachute. (Image and video credit: Warped Perception; via Gizmodo)

  • Growing Droplets on a Trampoline

    Growing Droplets on a Trampoline

    Droplets on a liquid surface will typically coalesce, thanks to gravity and the low viscosity of the air layer between them and the pool. In certain cases, droplets will partially coalesce, producing smaller and smaller droplets until they finally coalesce completely. Vibrating the liquid surface can help prevent this coalescence but only when droplets are small.

    In fact, if the pool is more viscous than the droplets, bouncing can be used to produce droplets of a desired size, as shown above. Because the droplets are less viscous, they deform more than the pool does – behaving somewhat like a bouncy ball hitting a rigid wall. In this system, large droplets are unstable and will undergo partial coalescence until they are small enough to bounce stably. The size of stable drops is determined by the frequency and acceleration of the bouncing bath; by tuning these parameters, researchers can select what size droplets they want to end up with. (Research credit: T. Gilet et al.; images and submission by N. Vandewalle)

  • When Vortices Collide

    When Vortices Collide

    In a new ad campaign for paint manufacturer Sherwin-Williams, the production team at Psyop show off some awesome fluid dynamics by swirling and injecting paint underwater. You can see one sequence above, where red and blue paint vortex rings collide head-on before breaking down into a purple turbulent cloud. (What a great way to demonstrate the mixing power of turbulence, right?) Here’s the full 30-second ad clip. Impressively, everything in the video is a practical effect, even the segment that flies past multicolored turbulent plumes. You can see how they filmed everything in their behind-the-scenes featurette below. In the meantime, enjoy the mesmerizing beauty of real-world physics and check out FYFD’s “fluids as art” tag for more examples. (Image and video credit: Psyop for Sherwin-Williams; submitted by Alan B.)

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