Place a drop of liquid on a surface much, much hotter than the liquid’s boiling point, and the portion of the drop that impacts will vaporize immediately. This leaves the droplet hovering on a thin layer of vapor. With a fluid like water, the vapor state is a much more efficient insulator than the liquid state. Thus, the vapor layer actually protects the liquid droplet, enabling it to boil off at a much slower rate than if the drop were touching the heated surface. This is known as the Leidenfrost effect, and it can be used to create self-propelled droplets. (Image credit: R. Thévenin and D. Soto)
Category: Research
Fingering Under Elastic
Take a couple panes of glass and stick a viscous fluid in between them; you’ve now constructed what fluid dynamicists call a Hele-Shaw cell. If you inject a low-viscosity fluid, like air, into the cell, you’ll get a beautiful finger-like pattern like the one shown on the left. If you change one of the walls to an elastic sheet, though, things get a bit different. The flexibility of the wall allows the upper surface to inflate as air gets pushed in. This can suppress the usual viscous fingers, as seen in the center animation. However, if you push the air in quickly, as in the right animation, the sudden inflation can wrinkle the elastic sheet. In this case, the wrinkles are the dominant influence, causing the the fluid to finger – but in an entirely different way than before! (Image credit: D. Pihler-Puzovic et al., sources 1, 2, 3; see also)
A Buoyant Rise
Hold a buoyant sphere like a ping pong ball underwater and let it go, and you’ll find that the ball pops up out of the water. Intuitively, you would think that letting the ball go from a lower depth would make it pop up higher – after all, it has a greater distance to accelerate over, right? But it turns out that the highest jumps comes from balls that rise the shortest distance. When released at greater depths, the buoyant sphere follows a path that swerves from side to side. This oscillating path is the result of vortices being shed off the ball, first on one side and then the other. (Image and research credit: T. Truscott et al.)
Non-Newtonian Splashes
What happens when a stream of liquid falls through a screen? As the above video shows, water creates a beautiful flower-like burst of fluid when it hits a screen. Adding a little polymer to the water makes it non-Newtonian and more viscous. When hitting the screen, this slows it down but doesn’t prevent the fluid from flowing.
Add enough polymer, though, and the fluid becomes what’s known as a yield-stress fluid. These fluids behave much like a solid–they don’t flow–until you apply a certain amount of stress. Then they’ll flow. If you’ve ever tried to get ketchup out of a glass bottle, then you’re familiar with how these yield-stress fluids act. When dropped onto a screen, the yield-stress fluid just forms a pile–unless the impact speed is high enough to create the necessary force to get the fluid to flow! (Video credit: B. Blackwell et al.)
Plesiosaur Swimming
Plesiosaurs are marine reptiles that thrived during the Jurassic period and went extinct some 66 million years ago. Since the first discoveries of plesiosaur fossils centuries ago, scientists have debated how the four-limbed creature would have swam. One approach to answering this question is to examine the efficiency of different strokes. Researchers have done this computationally by building a digital plesiosaur with biologically realistic joint motions. They then couple the model plesiosaur’s body motions with the movement of fluid around the body. With this computational model, they then simulate many different methods for moving the plesiosaur’s limbs and search for the most efficient one.
What they found is that the plesiosaur’s propulsion is dominated by its forelimbs, which likely moved with a flight stroke similar to that of a penguin or sea turtle. Despite their size, the hindlimbs were able to produce very little thrust, suggesting that they were primarily used for stability and maneuverability. (Image credits: S. Liu et al., GIF source)
Clogging, In Hourglasses and Crowds
Hourglasses are pretty common, but you’ve probably never given much thought to the way they flow. An hourglass designer has to carefully select the sizing of the neck and the grains. Choosing a neck that’s too small relative to the grain size will result in frequent clogs but choosing too large a neck will make setting the timing difficult. Interestingly, it doesn’t matter whether the hourglass is filled with air or with water–the same principle holds.
Where this knowledge becomes especially useful, though, is when dealing with crowds. We’ve all experienced the frustration of being in a large crowd trying to fit through a small exit. Paradoxically, the fastest way to get a large number of particles (or sheep or people) through a narrow opening is to slow each individual down. This can either be done by instructing everyone to slow down or by forcing that same result by placing an obstacle immediately before the exit. The reduction in speed reduces clogging, which means everyone gets through faster! (Video credit: A. Marin et al.)
Frost Spreading
Frost typically forms when supercooled droplets of water scattered across a surface freeze together. The freezing spreads via tiny ice bridges that link droplets together into a frozen network. The animation above shows this process in action. Freezing starts in a droplet off-screen on the right and quickly spreads. Watch carefully, and you can see the ice bridges growing toward the unfrozen droplets. This is because the ice bridges are fed by water vapor evaporating from the droplets. If one can spread the droplets far enough from one another, it’s possible for a droplet to evaporate completely before the ice bridge reaches it, thereby disrupting the spread of frost. (Video credit: J. Boreyko et al.; research paper)
Researching Wind Turbines
Two of the most awesome things (in my admittedly biased opinion) about fluid dynamics are the amazing facilities we build for experiments and the tests they allow us to do. In this video, you get a behind-the-scenes look at one such facility, used for wind turbine research at Princeton.
One challenge of wind turbine research is accurately capturing the aerodynamic effects of full-scale wind turbines in the controlled-environment of a laboratory. At Princeton, they match conditions between their model turbines and the real ones by drastically raising the density in their wind tunnel. This means that running the tunnel requires a series of compressors and storage tanks full of compressed air, and it also means that the wind tunnel itself has to be quite hefty to handle the pressure difference inside and out. Definitely check out the full video for more on their wind tunnel and what it can help them learn about wind turbines. (Video credit: M. Miller and J. Keifer; submitted by M. Miller)
Spore Squirting
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.)
Hummingbird Drinking
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)
