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    The Sharpshooter Insect

    The sharpshooter is a small, sap-sucking insect capable of consuming more than 300 times its body weight in fluid each day. To sustain that level of intake, the insect also has to have a robust mechanism for expelling excess fluid, and that particular talent has earned the insect the nickname of the “pissing fly”. Together a group of sharpshooters can expel enough fluid to imitate rain (top).

    Individually, the insects form a droplet on hydrophobic hairs near their anus. Once the droplet is large enough, those hairs bend like a spring, and the droplet gets catapulted off the insect with an acceleration greater than 20g. That makes it among the fastest reactions in the natural world – more than twenty times the acceleration of a cheetah. Understanding this mechanism is valuable for engineers building robotics as well as for finding ways to counter the agricultural menace the sharpshooters present when it comes to spreading diseases among infected crops. (Image and video credit: E. Challita et al.; via WashPo; submitted by Marc A.)

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    Massive Worthington Jet

    The FloWave facility in Scotland is one of the coolest ocean simulators out there. Equipped with 168 individual wave makers and 28 submerged flow-drive units, it’s capable of recreating almost any ocean conditions imaginable. So naturally the Slow Mo Guys used it to create a giant spike wave.

    Essentially, this is an oversized Worthington jet, the same as the ones you see after a droplet hits the surface. But with several thousand tonnes of crystalline clear water, the effect of that wave focusing is pretty spectacular. When you’re watching the high-speed footage, be sure to pay attention to the details, like the glassy surface of the collapsing jet, or the way holes open and expand as the splash curtain comes down around Dan’s head (above). Longtime readers will recognize many familiar features. (Image and video credit: The Slow Mo Guys)

  • Forming an Oxbow

    Forming an Oxbow

    Without human intervention, meandering rivers become more sinuous over time. This is driven by the flow around a river bend, which tends to push sediment from the outer bank of the curve to the inner, making the bend more pronounced. Eventually, loops in the river can pinch off and form a separate oxbow lake, as seen in the animation above and video below.

    By studying many photo sequences like this one, researchers have concluded that how quickly a river bend meanders depends on its curvature. In general, the higher the curvature, the faster the river bend will migrate. When rivers deviate from this rule of thumb, it’s typically because part of a river bank is tougher to erode than other sections. (Image and video credit: Z. Sylvester/Geolounge; research credit: Z. Sylvester; via Landsat; submitted by Aatish B.)

  • Swallowing Physics

    Swallowing Physics

    Swallowing – whether of food, beverage, or medication – is an important process for humans, but it’s one many struggle with, especially as they age. To help study the physics behind swallowing, one research group has built an artificial mouth and throat model, shown in the bottom row of images. The model uses rollers to imitate the wave-like motion of swallowing. 

    In our mouths, chewed food typically combines with saliva to form a soft ball we can move from our tongue and down our throat with a series of reflex actions. How easily we swallow something depends on its flow properties, our saliva, shape, and more. 

    In their early studies of model swallowing, researchers have focused on what it takes to swallow pills (suspended in liquid). What they found is probably consistent with your own experience: smaller pills are easier to swallow than large ones, and elongated pills are easier to swallow than round ones of the same volume. That seems to be a function of elongated pills’ smaller cross-section when aligned with flow going down the throat. As the research continues, scientists hope to explore what can be done to make food easier to swallow for those who struggle with it. (Image credits: meal – D. Shevtsova; model – M. Marconati; via APS Physics; submitted by Kam-Yung Soh)

  • Powdery Trails

    Powdery Trails

    Because air and water are colorless and transparent, we cannot see most of the flows around us – but they’re always there. In a recent series, photographer Jess Bell has been capturing images of jumping dogs trailing a colorful powder wake. There’s no compositing in the photos. Bell puts powder on the dogs, then photographs them as they jump. The results show the billowing, turbulent wakes left by the dogs. I particularly like how you can see the stream of powder coming from some of the dogs’ ears. For more of Bell’s work, check out her website and Instagram. (Image credit: J. Bell; via PetaPixel and Rakesh R.)

  • Exploding a Drop

    Exploding a Drop

    Leidenfrost drops levitate over a hot substrate on a thin layer of their own vapor, constantly replenished as the drop evaporates. For the most part, previous studies have focused on pure droplets, but a new one looks at what happens when you add surfactants – and the results are, well, explosive.

    Surfactants are a type of chemical that like to gather at the surface of a drop, and, unlike water, they’re nonvolatile – they don’t evaporate easily. So as the Leidenfrost drop evaporates and shrinks, the surface of the drop becomes more and more crowded with surfactant molecules. Eventually, they form an elastic shell around the remaining water, making evaporation more difficult.

    Inside the droplet, the temperature continues to rise, eventually reaching a point where bubbles of vapor can nucleate inside. When that happens, the bubbles expand almost instantaneously and the internal pressure spike bursts the shell, causing the entire droplet to explode. (Image and research credit: F. Moreau et al.)

  • Inside Fondue

    Inside Fondue

    Cheese fondue is a complex – and delicious – Swiss delicacy. The perfect fondue requires the right mix of ingredients and preparation to get the rheology – the flow character – just right. Fondue is a colloid, a fluid containing a mixture of suspended insoluble particles.

    The major components, rheologically speaking, are fat globules and casein proteins from the cheese, ethanol from the wine, and some added starch. Left on their own, the fat and casein tend to separate, something that’s sure to ruin the fondue. Adding the right amount of starch prevents that separation and keeps the fondue together. The viscosity of fondue is very important as well. If it’s too runny or too gummy, the mouthfeel will be wrong and it may not stick to the bread when dipped. Adding wine decreases the viscosity.

    All in all, the quality and perception of a good fondue relies heavily on its rheological character. Without the right proportion of ingredients to set the perfect viscous and chemical character, the dish literally comes apart. (Image credit: Pixabay; research credit and submission: P. Bertsch 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.

     

  • Collective Motion: Waving Bees

    Collective Motion: Waving Bees

    Giant honeybees live in huge open nests. To protect themselves, they’ve developed a mesmerizing wave-like defense known as shimmering. When shimmering, the bees in a hive, beginning from a distinct spot, will flip over to expose their abdomens. Taken together, this creates large-scale patterns like those seen above.

    Scientists have connected the behavior to the presence of wasps that prey on the bees. It seems that shimmering helps to repel the wasps without putting individual bees in danger. If shimmering doesn’t ward off the wasps, the bees can also use their flight muscles to heat the area around the intruder to a wasp-lethal temperature – or, individuals bees can sacrifice themselves by stinging the wasp. (Image credit: Beekeeping International, source; research credit: G. Kastberger et al.; via Gizmodo)

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

  • Collective Motion: Worms

    Collective Motion: Worms

    Although most animals are more solid than fluid, what happens when you put many of them together can be strikingly fluidic. Above you see the black aquatic worm, Lumbriculus variegatus, which must keep moist to stay alive. An individual worm will die within an hour of being removed from the water, but, in a group, the worms can survive far longer. They do so, in part, by acting like a viscoelastic fluid, a material with both solid (elastic) and fluid (viscous) properties.

    In small groups, the worms squirm tightly together to minimize their collective surface area and prevent themselves from drying out. But in larger groups, the worm blobs begin sending out feelers, searching for more advantageous circumstances. In the top image, you can see this causes three of the blobs to ultimately merge into an even bigger one. The worm collective can also “liquify”, allowing the blob to change shape and tackle obstacles like flowing through a pipe. (Image and research credit: Y. Ozkan-Aydin et al.; via Science)

    This is the second post in our series on collective motion. Check out the first post here.

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