“Liquid Skies” by Roman De Giuli is full of colorful but nebulous fluid imagery. The visuals consist of liquids like paint, ink, and alcohol filmed in macro atop paper. You can catch a behind-the-scenes glimpse of De Giuli at work here. (Image and video credit: R. De Giuli)
The Brazilian pygmy gecko is a tiny inhabitant of the Amazon rainforest, growing to no longer than 24 mm. But these tiny lizards have some incredible superpowers when it comes to surviving the rainforest’s deluges. The gecko’s surface is superhydrophobic — water repellent — thanks to millions of tiny hairs that create air pockets between water and the gecko’s skin. This superhydrophobic surface, combined with the gecko’s tiny stature, allow it to sit atop water, supported entirely by surface tension. (Image and video credit: BBC Earth)
Sandro Bocci’s short film “Flux Capacitor” explores the geometry and dynamics of soap films. When you dip wire models into soapy solution, the films that cling to the model can form complicated shapes as surface tension works to minimize the overall surface area. Bocci’s macro photography highlights the intense flows going on in the narrow regions where films meet. It’s a different take on soap films and neat to see! (Image, video, and submission credit: S. Bocci et al.)
In deserts around the world, plants have adapted to collect as much moisture as they can. Geometry aids them in this endeavor because droplets on the tip of a cone will move toward its thicker base. The motion takes place due to a imbalance in surface tension forces on either end of the droplet.
As the droplet moves up a cone, it changes shape from a barrel-like drop that fully covers the conical surface to a clamshell-shaped droplet that hangs only from the bottom of the cone. (Image and research credit: J. Van Hulle et al.)
Whether you’re cooking with ceramic, Teflon, or a well-seasoned cast iron pan, it seems like food always wants to stick. It’s not your imagination: it’s fluid dynamics.
As the thin layer of oil in your pan heats up, it doesn’t heat evenly. The oil will be hotter near the center of the burner, which lowers the surface tension of the oil there. The relatively higher surface tension toward the outside of the pan then pulls the oil away from the hotter center, creating a hot dry spot where food can stick.
To avoid this fate, the authors recommend a thicker layer of oil, keeping the burner heat moderate, using a thicker bottomed pan (to better distribute heat), and stirring regularly. (Image and research credit: A. Fedorchenko and J. Hruby)
Surface tension tries to minimize a bubble‘s surface area, which is why bubbles assume a spherical shape. But when many bubbles clump together, a curved interface is not always the most energy efficient one. In this case, bubbles can take on many shapes and sizes while still minimizing the overall surface energy. Take a close look at this image and see what shapes you discover! (Image credit: M. Adil)
Bubbles in a pure liquid don’t last long, but with added surfactants or multiple miscible liquids, bubbles can form long-lasting foams. In soapy foams, surfactants provide the surface tension gradients necessary to keep the thin liquid layers between bubbles from popping. But what stabilizes a surfactant-free foam?
New work finds that foams in mixtures of two miscible fluids only form when the surface tension depends nonlinearly on the concentration of the component liquids. When this is true, thinning the wall between bubbles creates changes in surface tension that stabilize the barrier and keep it from popping.
In mixtures without this nonlinearity, foams just won’t form. The new results are valuable for manufacturing, where companies can avoid unintentional foams simply by careful selection of their fluids. (Image credit: G. Trovato; research credit: H. Tran et al.; via APS Physics; see also Ars Technica, submitted by Kam-Yung Soh)
2020 was certainly a strange year, and I confess that I mostly want to congratulate all of us for making it through and then look forward to a better, happier, healthier 2021. But for tradition and posterity’s sake, here were your top FYFD posts of 2020:
There’s a good mix of topics here! A little bit of biophysics, some research, some phenomena, and some good, old-fashioned fluid dynamics.
If you enjoy FYFD, please remember that it’s primarily reader-supported. You can help support the site by becoming a patron, making a one-time donation, buying some merch, or simply by sharing on social media. Happy New Year!
(Image credits: catfish – Abyss Dive Center, owl – J. Usherwood et al., masks – It’s Okay to Be Smart, droplet – C. Kalelkar and H. Sai, boundary layer – J. Lienhard, bubble – A. Patsyk et al., boiling – S. Mould, ice – D. Spitzer, defects – The Lutetium Project, tadpoles – K. Schwenk and J. Phillips)
A viscous bubble wrinkles when it collapses, and scientists long assumed this behavior was caused by gravity. But a new experiment shows that the buckling is, instead, driven by surface tension.
To test gravity’s influence on bubble collapse, the researchers popped bubbles in three orientations: the (normal) upright orientation (Images 1 and 2), upside-down (Image 3), and sideways (Image 4). In all cases, the bubble’s thin film wrinkled as it collapsed, indicating that gravity had little influence on the process. Instead the authors concluded that surface-tension-driven collapse causes the dynamic buckling of the film. (Image and research credit: A. Oratis et al.; submitted by Zander B.)
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)