Fuck Yeah Fluid Dynamics

Celebrating the physics of all that flows. Ask a question, submit a post idea or send an email. You can also follow FYFD on Twitter and YouTube. FYFD is written by Nicole Sharp, PhD.

Recent Tweets @fyfluiddynamics

The fungus Pilobolus spreads its spores with a squirt cannon. Each spore sits on the end of a round fluid-filled pod. Like many plants, the fungus uses a process called osmosis to pump water into the pod. Through osmosis, the fungus increases the concentration of certain molecules inside the pod, which draws water into the pod and increases its pressure. Eventually, the pod ruptures, sending the spore aloft on a jet of fluid that accelerates it at 20,000+g! (Image credit: BBC Earth Unplugged, source; research credit: L. Yafetto et al.)

Hey guys,

Tomorrow (October 14), I’m heading off on vacation for a couple weeks out of range of the Internet. I’ve queued up entries for while I’m gone and my friend Claire from Brilliant Botany (check it out!) has kindly agreed to watch over the Tumblr queue and make sure it posts like it’s supposed to. So you should hopefully experience no interruptions to regular posts. But I won’t be responding to asks, submissions, emails, etc. until after I return at the end of the month.

Have a lovely October, readers! I’m off in search of penguins and iguanas.

Nicole

The Slow Mo Guys bring their high-speed skills to underwater explosions in this new video. The physics of such explosions is very neat (but also incredibly destructive). When the fuse ignites, a blast wave travels outward in a sphere, creating a bubble filled with gas. Eventually, the pressure of the surrounding water is too great for the bubble to expand against. When its expansion slows, that much larger pressure from the surrounding water starts to crush the bubble back down. Decreasing the volume of the bubble raises its pressure and its temperature again, and this often reignites any leftover fuel and oxidizer left in the bubble. The secondary shock bubble will re-expand, kicking off another round of expansion and collapse. (Video credit: The Slow Mo Guys; submitted by potato-with-a-moustache)

Hummingbirds are master acrobats, able to hover and drink simultaneously before flitting off to the next flower. At first glance, you might expect that their tongues are simply tiny straws that use surface tension and capillary action to draw up nectar. But it turns out that process is just too slow for the fast-paced birds.

Instead, hummingbirds use a forked tongue with a long groove on either half. When the hummingbird extends its tongue, its beak compresses the grooves and squeezes them together. Once the tongue reaches nectar, the grooves expand, which draws nectar up along the full length of the tongue grooves. This allows the bird to fill its tongue much faster than it could otherwise, enabling the hummingbird to lick up nectar more than 10 times a second. 

There’s a neat excerpt from a documentary including this research over here (Tumblr won’t allow the embedded version); the full documentary premieres today on PBS. (Image credits: A. Rico-Guevara et al., sources 1,2; submitted by mypronounsareherrchancellor)

A thin layer of hydrophobic particles dispersed at an oil-water interface is strong enough to prevent a water droplet from coalescing. The researchers refer to this set-up as their granular raft. As the red-dyed water droplet gets larger (top row), it deforms the raft more and more, but the grains continue to keep the drop separate from the fluid beneath (middle row). When water is removed from the droplet, wrinkles form on the raft as the drop’s volume shrinks. This is because the contact line – where the droplet, grains, and air meet – is pinned. The grains already touching the drop are held there by adhesion. But since the drop is shrinking, the area on the raft has to shrink, too – thus wrinkles! (Photo credits: E. Jambon-Puillet and S. Protiere, original)

Peering into a vortex feels like staring into an abyss in the Julia Set Collective’s “Gargantua”. Like their previously featured works, this video uses a macro perspective on fluid phenomenon to create an alternate sense of scale. Instead of a whirlpool, we could be observing a wormhole. Part of this is a matter of fooling our brains with perspective, but it also works because, on some level, we recognize that these same fluid patterns occur at very different lengthscales and so it is believable that what we see is much bigger than in reality. (Video credit and submission: S. Bocci/Julia Set Collective)

All matter is made up of molecules. But most of the time we treat fluids as materials with given properties – like density, viscosity, and surface tension – without worrying about the individual molecules responsible for those material characteristics. Now that we have much more powerful computers, though, we can begin to simulate fluid behavior in terms of molecules. 

The animations above show some examples of this. In the top animation, we see a gas condensing into a liquid. As the temperature decreases, molecules start clumping together, and eventually settle into a droplet on the solid surface. The lower animation shows the opposite situation – boiling – in which bubbles of vapor nucleate next to the solid surface and grow as more liquid changes phase. To see more examples, including droplets pinching off, check out the full video.   (Image credit: E. Smith et al., source; submitted by O. Matar)

Big animals like whales and sea lions stay warm in cold waters by having thick layers of insulating blubber. But smaller mammals, like beavers and sea otters, have a different mechanism for staying warm – their thick fur traps air near their skin, keeping the cold water at bay. Researchers used flexible, 3D-printed “hairy” surfaces to see how hair density, diving speed, and fluid viscosity affected the amount of air trapped between hairs. This enabled them to build a mathematical model describing the physics, which can now be used to predict, for example, the characteristics needed for a hairy wetsuit that could keep surfers warm in and out of the water. For more on this research check out MIT News’ video, and for a closer look at sea otter fur – not to mention a healthy overdose of pure adorable – check out the video below.  (Photo credit: F. Frankel; video credit: Deep Look; research credit: A. Nasto et al.)

These satellite images show Rupert Bay in northern Quebec. Sediment and tannins have stained the bay’s waters various shades of brown, which helps show the dynamic flows of the area. Rivers empty into the bay, but the tide appears to be coming in from the northwest as well. The flow is just right to create a wake of alternating vortices off a tiny island near the center of the bay. This pattern is known as a von Karman vortex street and often appears in the wake of spheres, cylinders, and, yes, islands. (Image credit: NASA Earth Observatory; submitted by Adam V.)

Oobleck is probably the Internet’s favorite non-Newtonian fluid. People vibrate it, run across it, shoot it, drop it, and even use it to fix potholes. But how does oobleck hold up to a hydraulic press? Fortunately, that’s been covered, too. Oobleck is a mixture of cornstarch and water, and it’s a bit unusual in that it is a shear-thickening material. That means that the faster you try to deform it, the more it will resist that deformation. Knowing this makes the above video’s results make more sense. When they try to crush the balloon full of oobleck, the deformation happens pretty slowly, so the fluid just flows away.

The same thing happens initially with the pot full of oobleck; it overflows much like any other liquid. But as the press pushes deeper, the oobleck gets confined by the pot’s walls and things change. Research has shown that the shear-thickening of oobleck comes from cornstarch particles jamming up in the fluid. By confining the oobleck, the pot and hydraulic press magnify this jamming effect, causing a spurt of semi-solid cornstarch fingers and leaving the press tool thoroughly trapped by the jammed particles. (Video credit: Hydraulic Press Channel)