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A Particle-Filled Splash
A drop of water that impacts a flat post will form a liquid sheet that eventually breaks apart into droplets when surface tension can no longer hold the water together against the power of momentum flinging the water outward. But what happens if that initial drop of water is filled with particles? Initially, the particle-laden drop’s impact is similar to the water’s – it strikes the post and expands radially in a sheet that is uniformly filled with particles. But then the particles begin to cluster due to capillary attraction, which causes particles at a fluid interface to clump up. You’ve seen the same effect in a bowl of Cheerios, when the floating O’s start to group up in little rafts. The clumping creates holes in the sheet which rapidly expand until the liquid breaks apart into many particle-filled droplets. To see more great high-speed footage and comparisons, check out the full video. (Image credit and submission: A. Sauret et al., source)
November 16, 2016
Starting a Lighter
Lots of fluids are transparent, which makes it hard for us to appreciate their motion. One technique for making these invisible motions visible is schlieren photography, which makes differences in density visible. Here it’s combined with high-speed video to show what happens when you use a lighter (minus the spark!). When the fuel starts flowing, it’s unstable and turbulent, but after that initial start-up, you can see the jet settle into a smooth and laminar flow. Wisps of fuel diffuse away from the jet as the fluid disperses. As the valve shuts off, the flow becomes unstable again, and the remains of the lighter fluid diffuse away. (Video credit: The Missing Detail)
August 2, 2016
Martian Ripples
Earth and Mars both feature fields of giant sand dunes. The huge dunes are shaped by the wind and miniature avalanches of sand, and their surface is marked by small ripples less than 30 centimeters apart. These little ripples are formed when sand carried by the wind impacts the dunes. But Martian dunes have a second, larger kind of ripple, too. These sinuous, curvy ripples lie about 3 meters apart and cast the dark shadows seen in the images above. On Earth we see ripples like these underwater, where water drags sand along the surface. On Mars, the same process is thought to play out with the wind, and so scientists have named these wind-drag ripples. (Image credit: NASA/JPL/MSSS; via APOD, full-res; submitted by jshoer)
July 25, 2016
Flying in Cramped Quarters
A new study has found that budgerigars (also commonly known as parakeets or budgies) fly at only two distinct speeds. The researchers flew the birds in a tapered tunnel to see how they navigated in response to widening or narrowing paths. What they found, regardless of the flight direction in the tunnel, is that the birds fly at approximately 9.5 m/s in areas wider than 2.5 times their wingspan and drop suddenly to a speed about half that when in narrower areas. The higher speed falls within the bird’s most energy-efficient range, suggesting that the birds may prefer flying at this condition. Insects like bumblebees also change speeds when entering cluttered environments, but the insects do so gradually, not suddenly like the budgerigars. The reason for this difference is not yet known, but it could relate to how the animals sense their environment or to differences in their flight efficiency when varying speed. (Image credit: J. Bendon; research credit: I. Schiffner and M. Srinivasan; submitted by Marc A.; h/t to Irmgard B.)
July 6, 2016
Daily Fluids, Part 4
Inside or outside, we encounter a lot of fluid dynamics every day. Here are some examples you might have noticed, especially on a rainy day:
Worthington Jets
After a drop falls into a pool, there’s a column-like jet that pops up after it and sometimes ejects another small drop. This is known to fluid dynamicists as a Worthington jet, but really it’s something we all see regularly, especially if you watch rain falling onto puddles or look really closely at your carbonated drink.
Crown Splash
Like the Worthington jet, crown splashes often follow a drop’s impact into another liquid. But they can also show up when slicing or stomping through puddles!
Free Surface Dynamics
Anytime you have a body of water in contact with a body of air, fluid dynamicists call that a free surface. How the interface between the two fluids shifts and transforms is fascinating and complicated. Waterfalls are a great example of this, but so are ocean waves or even the ripples from tossing a rock into a pond.
Hydrophobic Surfaces
Water-repellent surfaces are called hydrophobic. Water will bead up on the surface and roll off easily. While many manmade surfaces are hydrophobic, like the teflon in your skillet, so are many natural surfaces. Many leaves are hydrophobic because plants want that water to fall to the ground where their roots can soak it up. Keep an eye out as you wash different vegetables and fruits and see which ones are hydrophobic!
Check out all of this week’s posts more examples of fluid dynamics in daily life. (Image credit: S. Reckinger et al., source)
June 24, 2016
Daily Fluids, Part 3
A lot of the fluid dynamics in our daily lives centers around the preparation and consumption of food. (And in its digestion afterward, but that’s another story!) Here are a few examples of fluid dynamics you might not have realized you’re an expert on:
Low Reynolds Number Flows
This is a fancy way of discussing the motion of syrup, honey, and other thick and viscous fluids we interact with in our lives. These flows are typically slow moving and exhibit some neat properties like coiling or being possible to unstir.
Immiscible Fluids
Oil and water don’t mix, a fact anyone familiar with salad dressings or marinades is well aware of. The way around this is to shake them up! This disperses droplets of the oil within the water (or vinegar or whatever) to create an emulsion. While not truly mixed, it does make for more pleasant eating.
Multiphase Flows
Multiphase flows are ones containing both liquid and gaseous states. Boiling is an example we often see in our daily lives, though carbonated beverages, water sprayers, and sneezes are other common ones.
Leidenfrost Effect
The Leidenfrost effect occurs when liquid is introduced to a surface that is much, much hotter than its boiling point. Part of the liquid instantly vaporizes, leaving droplets to skitter around on a thin vapor layer. This is most often seen around the stove and in skillets. (And, yes, it does qualify as a multiphase flow!)
Tune in all week for more examples of fluid dynamics in daily life. (Image credit: S. Reckinger et al., source)
P.S. – I’m at VidCon (@vidconblr) this year! If you are, too, come say hi and get an FYFD sticker 😀
June 23, 2016
Daily Fluids, Part 2
We play with fluid dynamics all the time, though we don’t always think of it as such. Here are a few ways it shows up in the ways we play:
Aerodynamics
This is the study of air moving past an object. Whether you’re throwing a paper plane, flying a kite, or riding a bike, aerodynamics has an impact on what you’re doing.
Lift
Skipping a rock won’t work unless its impact generates some lift, but we see lift in lots of other places, too, from birds and planes to racecars and sailboats.
Magnus Effect
The Magnus effect relates to lift forces on a spinning object. It can affect the way a frisbee flies, but we see it a lot in ball-related sports, too. The flight of golf balls, volleyballs, baseballs, and soccer balls can all be significantly impacted by the Magnus effect. Check out these videos for a primer on the Magnus effect and the reverse Magnus effect.
Bubbles
Everybody loves playing with bubbles. But they may have more of a impact than you realize, whether it’s in making the foam on your latte, enhancing the aroma of your champagne, or making your joints pop.
Tune in all week for more examples of fluid dynamics in daily life. (Image credit: S. Reckinger et al., source)
June 22, 2016
Daily Fluids, Part 1
Just getting cleaned up and ready for the day involves a lot of fluid physics. Here are a few of the phenomena you may see daily without realizing:
Plateau-Rayleigh Instability
This behavior is responsible for the dripping of your faucet. More specifically, it’s the reason that a falling jet breaks up into droplets. It works on rain, too!
Forced Convection
Everyone is familiar with a winter wind making them colder or hot air from a dryer getting the moisture off their hands. These are examples of forced convection – heat transfer by driving a fluid past a solid. Another common example? The fans in your computer!
Liquid Atomization
This is the process of breaking a liquid into lots of tiny droplets. Aside from any aerosol can ever, this phenomenon is also key to your daily shower and internal combustion in your car.
Archimedes Principle
This might be one of my favorite bits of the whole video because it hearkens back to some of my own earliest fluid dynamics exposure. Archimedes Principle says that buoyancy is equal to the weight of the fluid a body displaces. My mom (a science teacher) taught me about this one in the bathtub! It’s key to everything that ever floated, including us!
Tune in all week for more examples of fluid dynamics in daily life. (Image credit: S. Reckinger et al., source)
June 21, 2016
Martian Viscous Flow
These images from the Mars Reconnaissance Orbiter show what are called viscous flow features. They are the Martian equivalent of glacial flow. Such features are typically found in Mars’ mid-latitudes.
Ground-penetrating radar studies of Mars have shown that some of these features contain water ice covered in a protective layer of rock and dust, making them true glaciers. Another study of similar Martian surface features found that their slope was consistent with what could be produced by a ~10 m thick layer of ice and dust flowing superplastically over a timescale equal to the estimated age of the surface features. Superplastic flow occurs when solid matter is deformed well beyond its usual breaking point and is one of the common regimes for glacial ice flow on Earth. (Image credit: NASA/JPL/U. of Arizona; via beautifulmars)
March 25, 2016
Earth’s Aerosols
The motions of Earth’s atmosphere move more than just air and moisture. As seen in this animation built from NASA satellite data, the atmosphere also transports large amounts of small solid particles, or aerosols, such as dust. Each year the wind carries millions of tons of Saharan dust across the Atlantic, depositing much of it in the Amazon basin. This provides much needed nutrients like phosphorus to plants and animals in the Amazon; check out this video from the Brain Scoop to see what happens in areas that don’t receive these nutrients. Dust is only one of many sources for atmospheric aerosols, though. Sea salt, volcanic eruptions, and pollution are others. All of these aerosols serve as potential nucleation sites for raindrops or snowflakes, and their transport all around the globe by atmospheric winds means that seemingly local effects–like a regional drought or increased pollution in developing countries–can have global effects. (Video credit: NASA Goddard; submitted by entropy-perturbation)
March 13, 2015

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