FYFD

Category: Research

  • 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.)

  • Flow in the Heart

    Flow in the Heart

    Few flows are more integral to our well-being than blood flow through the heart. Over the course of our lives, our hearts develop from a few cells pushing viscous blood through tiny arteries to the muscular center of a vast circulatory network, capable of powering us through incredible physical feats. What’s most astonishing about all this is that the heart goes through all these changes and adaptations without ever pausing. 

    Peering into the heart to see it in action is difficult, but researchers today are combining imaging techniques like CT and MRI with computational fluid dynamics to build patient-specific heart models. Not only does this help us understand hearts in general; it’s paving the way toward predicting how a specific treatment may affect a patient. Imagine, for example, being able to simulate and compare different models of an artificial heart valve to see which will work best for a particular patient. We’re not to the point of doing so yet, but it’s a very real possibility in the future. 

    To see some examples of predicted and measured heart flows, check out this video by J. Lantz. In the meantime, happy Valentine’s Day! (Image credits: Linköping University Cardiovascular Magnetic Resonance Group, video source; via Another Fine Mesh)

  • 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)

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    Swirling Polygons

    We don’t usually think of fluids forming corners, but they can. Here you see liquid nitrogen in a simple pot. Since the pot is much hotter than the boiling point of the nitrogen, the liquid nitrogen is floating on a layer of its own vapor. This is called the Leidenfrost effect. That nearly frictionless contact with the pot means that stirring the nitrogen conveniently spins it up into these rotating polygons, visible in high-speed footage. The faster you stir the nitrogen, the more points you get. 

    Check out the full video below for instructions on how the researchers constructed their set-up. If you try it, though, remember to have plenty of ventilation. When the nitrogen vaporizes, its volume increases dramatically, and if you’re not careful, it will displace too much oxygen and make it hard to breathe. (Image and video credit: A. Duchesne et al., source)

  • 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: Crowds

    Collective Motion: Crowds

    It’s sometimes taken for granted that, in groups, people can behave a lot like a fluid or a granular material. This allows scientists to adapt models developed for those materials to understand how crowds move. But in doing so, it’s always important to test just how far the comparison holds; in other words, just how much does a crowd of people behave like a fluid or granular material?

    That’s the purpose behind the experiment you see above, where a dense crowd of people shift in response to a “cylindrical intruder”. This is a classic experiment for something like a granular material, and there are clear similarities. Most of the crowd’s shifting comes only a short way from the intruder, and their passage leaves a small, empty wake that slowly fills back up.

    But other aspects of the experiment are very different from the granular equivalent. Instead of moving only when contact forces cause them to, the crowd shifts in anticipation of the intruder’s passage. They also use a more confined motion; crowd members primarily shift to the side to allow the intruder by, whereas grains tend to follow a more circular pattern of motion. Interestingly, if the intruder approaches from behind – and thus crowd members cannot anticipate them – the crowd’s motions will actually better match a granular material. (Image and research credit: A. Nicholas et al., source)

    All this week at FYFD we’re looking at collective motion. Check out our previous posts here and here.