New FYFD video! Learn all about salps, vortex rings, and underwater robots. Thanasi Athanassiadis takes me inside his lab and his newly published research into how proximity affects the thrust two vortex rings can produce. 

There are a ton of little things I love about how this video came out, especially the chalkboard animations. Check it the full video below and click through to the video description for lots more information about salps and vortex rings.

(Image and video credits: N. Sharp and A. Athanassiadis; Original salp images: A. Migotto and D. Altherr)


It’s only fitting to take a moment to look back at 2014 as we step into the New Year. It was a big year in many respects - we hit 1000 posts and broke 200,000 followers; I started producing FYFD videos on our YouTube channel; and, on a personal note, I finished up my PhD. But since we’re all about the science around here, I will give you, without further ado, the top 10 FYFD posts of 2014:

1. Bioluminescent crustaceans use light for defense
2. What happens when you step on lava
3. Flapping flight deconstructed
4. Wingtip vortices demonstrated
5. Saturn’s auroras
6. Raindrops’ impact on sand
7. Water spheres in microgravity
8. The surreal undulatus asperatus cloud
9. Inside a plunging breaker
10. A simply DIY Marangoni effect demo

I can’t help but notice that 9 out of the 10 posts feature animated GIFs. Oh, Tumblr, you rascals. Happy New Year! (Image credits: BBC; A. Rivest; E. Lutz; Nat. Geo/BBC2; ESA/Hubble; R. Zhao et al.; D. Petit; A. Schueth; B. Kueny and J. Florence; Flow Visualization at UC Boulder)

When it’s really cold outside–to the tune of -40 degrees (Fahrenheit or Celsius)–physics can get a little crazy. In this photo, boiling-hot water from a thermos turns into an instant snowstorm when tossed. How is this possible? It turns out there are a combination of factors that affect this. Firstly, the rate of heat transfer between two objects depends on the magnitude of the temperature difference between them. The bigger the difference in temperature, the faster the hot object cools. Of course, as the hot object cools down, the temperature difference between it and its surroundings is smaller and the rate of heat transfer decreases.

The second important factor here is that the water is being tossed. When you throw water, it breaks into droplets, and droplets have a large surface area compared to their volume. As it turns out, the rate of heat transfer also depends on surface area. By breaking the hot water into smaller droplets, you increase the surface area exposed to the cold air, allowing the hot water to freeze faster. (Image credit: M. Davies et al.; via Gizmodo)

Also: Since there are a few events scheduled around the country over the next couple months, I’ve added an events page where you can find details for those appearances. And as always, if you’re interested in scheduling a talk or event, feel free to contact me directly.


New FYFD video! In which Dianna Cowern (Physics Girl) joins me to explore boundary layer transition and how a couple of small bits of roughness could be a huge problem for the Space Shuttle during re-entry. A lot of people have asked me what I did for my PhD research, and the truth is, I’ve never really discussed my own work here on FYFD. This video is probably the closest I’ve come. The story I tell about STS-114 is one that appears in the first chapter of my dissertation, and it did, in many respects, motivate my work exploring roughness effects on transition in Mach 6 boundary layers. I hope you enjoy my video, and don’t forget to check out Dianna’s video, too! (Video credit: N. Sharp/FYFD)


For the next week, FYFD is going to be exploring the physics of walking on water. Birds, bugs, and balls can all do it - we’ll look at how! To top off the week, I’ll be holding my first-ever FYFD live webcast on Saturday, March 5th at 1 pm EST (10 am PST; 6 pm GMT). My guests are Professor Tadd Truscott and PhD student Randy Hurd of the Splash Lab! Tadd, Randy, and their Splash Lab compatriots have been responsible for some of my favorite FYFD topics over the past five years and I’m super excited to have them on the webcast. 

Normally, my webcasts will be reserved for FYFD’s $5+ Patreon patrons, but since this is a special occasion, we’re going to make the Hangout on Air link live to any FYFD patron on Patreon. Not a patron yet? What are you waiting for? Go sign up! You don’t want to miss this. 

As a bonus, here’s Randy demonstrating his research:

A video posted by Splash Lab (@thesplashlab) on Feb 10, 2016 at 1:47pm PST

(Original grebe image: W. Watson/USFWS; all other photos: The Splash Lab)


This is FYFD’s 1500th post! Can you believe it? Fifteen hundred posts is a heck of a lot of fluid dynamics. I’ve covered everything from the teeny tiniest scales to the astronomically huge, from events that happen in the blink of an eye to ones that require decades of patience. Today I encourage you to check out the archives whether by scrolling the visual archive, digging in by keyword, or by clicking here for something random.

Whether you’ve been here for 1 post or for all 1500, thank you! And special thanks, of course, to my Patreon patrons. If you’re a fan and want to help FYFD keep flowing and growing, please consider becoming a patron, too. (There’s cool perks available.) Here’s to the next 1500 posts!

P.S. Big thanks also to Randy Ewoldt and his lab for their fantastic viscoelastic FYFD timelapse. Isn’t it awesome?! (Image credits: N. Sharp - top image, Ewoldt Research Group - bottom image)


Watching rain drops hit a puddle or lake is remarkably fascinating. Each drop creates a little cavity in the water surface when it impacts. Large, energetic drops will create a crown-shaped splash, like the ones in the upper animation. When the cavity below the surface collapses, the water rebounds into a pillar known as a Worthington jet. Look carefully and you’ll see some of those jets are energetic enough to produce a little satellite droplet that falls back and coalesces. Altogether it’s a beautifully complex process to watch happen over and over again. (Image credit: K. Weiner, source)


Help us do some science! I’ve teamed up with researcher Paige Brown Jarreau to create a survey of FYFD readers. By participating, you’ll be helping me improve FYFD and contributing to novel academic research on the readers of science blogs. It should only take 10-15 minutes to complete. You can find the survey here.

This dramatic example of Kelvin-Helmholtz clouds was taken near the Galapagos Islands last week. The shark-fin-like clouds are the result of two air layers moving past one another. The velocity difference at their interface creates an unstable shear layer that quickly breaks down. The resemblance of the clouds to breaking ocean waves is no coincidence – the wind moving over the ocean’s surface generates waves via the same Kelvin-Helmholtz instability. In the case of the clouds above, the lower layer of air was moist enough to condense, which is why the pattern is visible. Clouds like these don’t tend to last for long because the disturbances that drive the instability grow exponentially quickly, leading to turbulence. (Image credit: C. Miller; via Washington Post; submitted by @jmlinhart)


Help us do some science! I’ve teamed up with researcher Paige Brown Jarreau to create a survey of FYFD readers. By participating, you’ll be helping me improve FYFD and contributing to novel academic research on the readers of science blogs. It should only take 10-15 minutes to complete. You can find the survey here.

FYFD is 5 years old! Hard to believe it’s been five whole years. Thank you to everyone who has helped along the way, especially those of you who produce, submit, and share such beautiful fluid dynamics.

Thanks also to everyone who is participating in our reader survey. We’re getting a lot of great feedback. If you haven’t taken it yet, there’s still time!

And, finally, in honor of five years of FYFD, I present you with the five most popular FYFD posts of all time:

1. Swimming through surface tension - Originally posted 7 Feb 2013
2. Bioluminescence as a defense mechanism - Originally posted 4 Sep 2014
3. Liquid mushroom - Originally posted 19 Feb 2013
4. Dancing droplets - Originally posted 30 Mar 2015
5. Stepping on lava - Originally posted 19 Dec 2014

Today marks the third birthday of FYFD, and it’s been a pretty crazy ride so far. Three years ago, I would have never predicted a blog about fluid dynamics could gain over 170,000 followers. (Thanks for proving me wrong!) As part of my efforts to continue FYFD’s science outreach, I am conducting a reader survey. My goals are to learn about FYFD’s reader demographics and to solicit advice for future improvements to the site. Please take a few moments to participate!

The FYFD archives contain more than 800 posts. As part of our birthday celebration this week, we’ll take a trip back through the archives to revisit some of my favorites. Stay tuned, and don’t forget to fill out the survey! Thanks for helping make FYFD a success. (Photo credit: Unknown photographer/The Paper Wall)


FYFD features lots of science, but this new video gives you a chance to see the scientists, too! It’s a behind-the-scenes look at the American Physical Society Division of Fluid Dynamics meeting that took place in San Francisco recently. You may recognize some of the stories, but I guarantee there’s new stuff, even if you were there! Special thanks to everyone who helped me make the video; I had a blast doing this. (Video credit: N. Sharp)


Skydivers and freefall acrobats utilize vertical wind tunnels as ground training facilities. Low-speed acrobatics, like gymnastics, relies on inertial forces and angular momentum for flips and attitude changes. But at freefall speeds, aerodynamic forces are much larger, and an acrobat’s orientation relative to the flow has a big effect on his stability and maneuverability. Simple movements of an arm or leg can significantly alter one’s aerodynamics, allowing the acrobats to choreograph controlled and synchronized motion. (Video credit: Red Bull)

Author’s note - After much consideration, I’ve decided to move FYFD to a MWF posting schedule for the time being. Working full-time has its limitations, and I believe the less frequent posting schedule will allow me to dedicate more time to generating new content like FYFD videos. This was a tough decision, but I hope it will help FYFD grow in the long-term. - Nicole


Originally posted: 22 July 2010 This video of the NASA Solar Dynamics Observatory’s launch is such a favorite of mine that it was part of the original inspiration for FYFD and was the very first video I posted. Watch closely as the Atlas V rocket climbs. At 1:51 you’ll see a rainbow-like cloud in upper right corner of the screen. This effect is created by sunlight shining through ice crystals of the cloud. A couple seconds later you see pressure waves from the rocket propagate outward and destroy the rainbow effect by re-aligning the ice crystals. Just after that comes the announcement that the vehicle has gone supersonic. The atmospheric conditions of the launch happened to be just right to make those pressure waves coming off the rocket visible just before they coalesced into a leading shockwave. (Video credit: B. Tomlinson)

Reminder: If you haven’t already, please fill out our reader survey and help us improve FYFD! 


What do shark scales, underwater robots, blood flow, and art have in common? They’re all a part of the latest FYFD video! Check out my behind-the-scenes look at the latest American Physical Society Division of Fluid Dynamics meeting. Meet the researchers and find out about the science everyone was talking about! (Image/video credit: N. Sharp)


What happens when you pour molten aluminum on dry ice? As the Backyard Scientist shows, you get what looks like slippery, sliding, boiling metal. In fact, what you see may remind you of the Leidenfrost effect, where a liquid can slide around over an extremely hot surface on a thin film of its own vapor. Despite the opposite temperature extremes–this is a very cold surface rather than a very hot one–a very similar thing is happening here. The molten aluminum is so much hotter than the dry ice that it causes the dry ice to sublimate, releasing gaseous carbon dioxide that the aluminum slides around on. For the same reason, the aluminum appears to boil in the bottom animation. What we’re really seeing is carbon dioxide gas rising and escaping the aluminum so violently that it carries some of the metal with it. Be sure to check out the full video for more awesome physics!  (Image credit: The Backyard Scientist, source; via Gizmodo)

Do you enjoy FYFD and want to help support it? Then please consider becoming a patron!

Sea ice forms in patterns that depend on local ocean conditions. Pancake ice, like that shown in the above photo from the Antarctic Ross Sea, is formed in rough ocean conditions. Each individual pancake has a raised ridge along its edge, due to wave-induced collisions with other pieces of ice. Over time the smaller pieces of ice will merge together, forming large sheets. Evidence of its turbulent formation will persist, however, in the rough surface of the ice’s underside. For more, check out the National Snow and Ice Data Center. (Image credit: S. Edmonds; via Flow Visualization)

Do you enjoy FYFD and want to help support it? Then please consider becoming a patron!

Reminder: If you’re at the University of Illinois at Urbana-Champaign, I’m giving a seminar this afternoon. Not in Illinois? I’ve got other events coming up, too!


Ethereal forms shift and swirl in photographer Thomas Herbich’s series “Smoke”. The cigarette smoke in the images is a buoyant plume. As it rises, the smoke is sheared and shaped by its passage through the ambient air. What begins as a laminar plume is quickly disturbed, rolling up into vortices shaped like the scroll on the end of a violin. The vortices are a precursor to the turbulence that follows, mixing the smoke and ambient air so effectively that the smoke diffuses into invisibility. To see the full series, see Herbich’s website.  (Image credits: T. Herbich; via Colossal; submitted by @jchawner@__pj, and Larry B)

P.S. - FYFD now has a page listing all entries by topic, which should make it easier for everyone to find specific topics of interest. Check it out!


Just about everyone wishes for a White Christmas, but even when that happens, it’s rare to get a good look at the beauty of individual snowflakes. Alexey Kljatov’s macro photography of snowflakes is simply stunning and highlights the incredible variety of forms snowflakes take. A snowflake forms when a water droplet freezes onto dust or other particles and grows as more water vapor freezes onto the initial crystal. The symmetry of the snowflakes, as with any crystal, comes from the internal order of its water molecules. The shape and features that form vary due to the local temperature and humidity level while vapor is freezing onto the crystal. Check out this handy graph showing which shapes form for various situations. Since snowflakes can encounter wildly different conditions on their path to the ground, it’s rare or next-to-impossible to find any two alike. Join us all this week at FYFD as we look at holiday-themed fluid dynamics. (Photo credit: A. Kljatov)