Starfish larvae, like other microorganisms, use tiny hair-like cilia to move the fluid around them. By beating these cilia in opposite directions on different parts of their bodies, the larvae create vortices, as seen in the flow visualization above. The starfish larvae don’t use these vortices for swimming – to swim, you’d want to push all the fluid in the same direction. Instead the vortices help the larvae feed. The more vortices they create, the more it stirs the fluid around them and draws in algae from far away. The larvae actually switch gears regularly, using few vortices when they want to swim and more when they want to eat. Check out the full video below to see the full explanation and more beautiful footage. (Image/video credit: W. Gilpin et al.)
Tag: science
Saturnian Clouds
It may look like an oil slick, but the photo above actually shows the clouds of Saturn. The false-color composite image reveals the gas giant in infrared, at wavelengths longer than those visible to the human eye. NASA uses this infrared photography to identify different chemical compositions in Saturn’s atmosphere based on how they reflect sunlight. You can see an example of how they construct these images here. This detail shot appears to show cloud bands of different compositions mixing. You can see hints of shear instabilities forming along the edges where the light and dark bands meet. (Image credit: NASA; via Gizmodo)
Coarsening in a Soap Film
Flow in a soap film is driven by gravity’s efforts to thin the film and surface tension’s attempts to stabilize variations in thickness. Because evaporation guarantees that the soap film will eventually dry out, gravity typically wins the battle and causes a soap film to rupture. This video takes a close look at what happens in the film just before it ruptures. Black dots form in the thinnest region of the flow. These areas are not holes, but they appear black because they are thinner than any wavelength of visible light. Before rupture, the black dots begin coalescing with one another, first due to diffusion and later more rapidly due to convection in the soap film. Ultimately, the black dots are the harbingers of doom for the fragile bubble. (Video credit: L. Shen et al.)
Oil in Alcohol
A drop of oil impacts and falls through a pool of isopropyl alcohol. Momentum, viscosity, and diffusion combine to deform the drop into a shape that is initially like an upside-down wine glass (top image). Because the oil is both denser than the alcohol and soluble in it, the drop sinks and dissolves as it falls. The drop expands rapidly outward, thinning and formed a concave shape around its denser, sinking core (bottom image). Ultimately, the droplet will deform and fragment as it dissolves into the alcohol. (Image credit: R. La Foy et al.)
APS DFD 2016
It’s the time of year again for the American Physical Society Division of Fluid Dynamics meeting! Tomorrow I’ll fly to Portland, OR for three days of non-stop fluid dynamics. This year I’ll be giving two talks:
– Sunday, November 20th, 3:23pm, Room B117: F*** Yeah Fluid Dynamics: Inisde the science communication process
– Monday, November 21st, 6:01pm, Room E147-148: “In a sea of sticky molasses”: The physics of the Boston Molasses Flood
The latter talk is part of an ongoing project exploring the fluid dynamics of the Boston Molasses Flood of 1919. Since you’ll be hearing more about the project in the coming weeks and months, I’m sharing a sneak peek video I originally made for my Patreon patrons. If you’re interested in following the project’s progress, you may want to become an FYFD patron – otherwise, rest assured that you will see the final results eventually 🙂
I hope to see some of you in Portland, but if you can’t make it, I encourage you to follow the meeting on social media with #APSDFD!
(Video credit: N. Sharp/FYFD)
The Blue Whirl
We wrote earlier this year about the discovery of a new type of fire whirl – the blue whirl – but now the authors have published video of the blue whirl in action! The blue whirl was discovered while investigating the use of fire whirls to more efficiently burn off oil spilled atop water. A tightly spinning yellow fire whirl produces less soot than a non-vortex burn; the blue whirl is even more efficient, producing little to no soot at all. Much remains to be learned about this new type of fire vortex, but in the meantime, enjoy some high-speed video of the blue whirl, particularly from 1:50 onward. (Video credit: M. Gollner et al.)
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)
Surfing on Vapor
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)
Bouncing Droplets
Droplets bouncing on a pool form a beautiful and fascinating system, as recently featured by Physics Girl, Veritasium, and Smarter Every Day. The Lutetium Project – a consortium of French physics, graphic design, and music students – have their own take on the subject with beautiful short videos constructed from experimental research footage. With simple text explanations and lovely original music, they combine science, art, and outreach brilliantly. Also check out their quantum walker video and be sure to subscribe to their channel (in English or French) for more! (Video credit: The Lutetium Project; submitted by @g_durey)
Colors in Macro
Milk, acrylic paints, soap, and oil – all relatively common fluids, but together they form beautiful mixtures worth leaning in to enjoy. Variations in surface tension between the liquids cause much of the motion we see. Soap, in particular, has a low surface tension, which causes nearby colors to get pulled away by areas with higher surface tension, behavior also known as the Marangoni effect. Adding oil creates some immiscibility and lets you appreciate both the coalescence and fragmentation of the fluids. And finally, there’s one of my favorite sequences, where bubbles start popping in slow motion. As the bubble film ruptures, fluid pulls away, breaking into ligaments and then a spray of droplets as the bubble disintegrates. (Video credit: Macro Room; via Gizmodo)
