Fluid dynamics is largely about figuring out the relationship between forces. For a soap bubble sitting still, that’s primarily the effect of gravity, which makes the fluid in the soap film drain downward, and surface tension, which tries to maintain a spherical shape for the bubble.
Once you start spinning the bubble, though, there are new forces that come into play. One is the centrifugal force caused by the rotation, and another is the drag force between the rotating soap bubble and the air inside and outside of it. The addition of these forces drastically changes the bubble’s shape. It becomes wobbly and flattens out. Watch the contact line where the bubble meets the surface and you’ll also see it creeping outward toward the edge of the platform. (Image credit: C. Kalelkar and S. Paul, source)
In the ocean, breaking waves trap air into bubbles that then cluster into foam, but conventional simulations don’t capture this foaminess. For bubbles to cluster into foam, there has to be a force preventing — or at least delaying — their coalescence. Typically, this is caused by impurities in the water that help lower the surface tension and thereby lengthen the bubbles’ lifespans. When these features get added to simulation models, bubbles begin to cluster and breaking waves become foamy. (Image and video credit: P. Karnakov et al.)
High-speed photography gives us an alternate glimpse of reality. Here it provides an all-new perspective on making espresso. Surface tension plays a starring role, first in pulling together the film that forms over the exit, then in creating the drips and drops that follow. The break-up of espresso into individual droplets is an example of the Plateau-Rayleigh instability, where surface tension drives any wobble in the falling jet to pinch off. For more slow-motion espresso, you can also check out this behind-the-scenes video. (Video and image credit: J. Hoffmann; submitted by Jerrod H.)
10 years. 2,590 posts. 21 original videos. 378,000+ followers. Countless hours spent blogging and more than 1,000 journal articles read. When I started FYFD ten years ago as a PhD student, I never imagined the impact the blog would have on my life, my career, or my field. It’s been a wild ride, and I’d like to take a moment today to thank each and every one of you for contributing to this journey, whether it’s by supporting on Patreon, liking a post, sharing content, submitting ideas, leaving a comment, sending an email, or saying hi at an event. FYFD would have petered out long ago if not for your support!
Ten years seems like a good time for a little retrospective, so I went back through the archive in search of the most popular post (based on Tumblr’s notes) from each of those ten years. Here’s what I found:
If you’d rather enjoy something random rather than something “popular”, you can always use the shortcut https://fyfluiddynamics.com/random to explore posts in the archive.
And in case you’re more interested in watching videos, here are the top FYFD videos (by YouTube views):
(Wow, my editing and production skills have evolved since some of those earlier vids!)
So what are your favorite FYFD memories and posts? Let me know in the comments! (Image and video credits: N. Sharp)
I’m not a coffee person, but Thomas Blanchard’s “Oooh !! My Delicious Coffee” manages to capture my favorite part of the beverage – watching cream and coffee mix. From feathery flows driven by surface tension to droplets floating like miniature cappuccinos, the short film features many of the fantastical landscapes we find when staring into a coffee cup. But don’t get too eager to drink it; Blanchard used a combination of coffee, oil, and paint to achieve those effects! (Image and video credit: T. Blanchard)
Vibrate a pool of water, and you’ll get Faraday waves, ripple-like excitations that form their own distinctive pattern compared to the driving vibration. But you don’t have to vibrate a pure liquid to see Faraday waves. A recent study observed them in vibrated earthworms!
Odd as this may sound, the results make sense. When anesthetized (as they were in the experiments), earthworms are essentially a liquid wrapped in an elastic membrane, which is not so different from a droplet held together by surface tension.
But why vibrate earthworms in the first place? It turns out earthworms are a good model organism for studies of vertebrate neural systems, so observing how vibrations propagate through them can provide insight into how our own nervous systems transmit information. (Image, research, and submission credit: I. Maksymov and A. Pototsky)
Making electronics water-resistant can be a challenge, but as this Slow Mo Guys video demonstrates, engineers have some clever ways to deal with unwanted liquids. The Apple Watch, for example, uses its speakers to eject water that gets into the watch during immersion. As seen above, the vibration of the speakers ejects most of the water as tiny droplets. Occasionally, surface tension makes this tough and drops instead coalesce on the watch’s surface. To counter this tendency, the speakers sometimes pause, allowing water to collect before they begin vibrating again. (Video and image credit: The Slow Mo Guys)
While we’re most used to seeing levitating Leidenfrost droplets on a solid surface, such drops can also form above a liquid bath. In fact, the smoothness of the bath’s surface, combined with mechanisms discussed in a new study, means that drops will levitate at a cooler temperature over a liquid than they will over a solid surface.
Researchers found that a donut-shaped vortex forms in the bath beneath a levitating droplet, but the direction of the vortex’s circulation is not always the same. For some liquids, the flow moves radially outward from beneath the drop. In this case, researchers found that the dominant force was shear stress caused by the vapor escaping from under the droplet.
With other droplet liquids, the flow direction instead moved inward, forming a sinking plume beneath the center of the drop. In this situation, researchers found that evaporative cooling dominated. As the liquid beneath the droplet cooled, it became denser and sank. At the same time, the lower temperature changed the bath’s local surface tension, creating the inward surface flow through the Marangoni effect. (Image credit: F. Cavagnon; research credit: B. Sobac et al.)
Rising bubbles and droplets are common in many chemical and industrial applications. But just a tiny concentration of contaminants on their surface can completely alter their behavior, disrupting coalescence and slowing down chemical reactions.
Historically, it’s been hard to measure the level of contamination in these some drops and bubbles, but a new study outlines a way to measure these small concentrations by perturbing the drops and watching how they deform. By analyzing how the drop shimmies and shakes, they’re able to measure its surface tension and, ultimately, the concentration of contaminants. (Image credit: S. Sørensen; research credit: B. Lalanne et al.; via APS Physics)
Placing a mixture of alcohol and water atop a pool of oil creates a stunning effect that pulls droplets apart. The action is driven by the Marangoni effect, where variations in surface tension (caused in this case by the relative evaporation rates of alcohol and water) create flow. David Naylor captures some great stills of the flow, including the only example of a double burst I’ve seen so far. For more on the science behind the effect, check out this previous post or the original research paper. (Image credit: D. Naylor; see also this previous post)
