Tag: surface tension

Gathering Droplets
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.)
March 4, 2021
Why Food Sticks to Nonstick Pans
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)
March 1, 2021
Bubble Array
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)
January 22, 2021
Stabilizing Foams
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)
January 20, 2021
The Best of FYFD 2020
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)
January 1, 2021
Wrinkles on Bubble Collapse
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.)
December 15, 2020
Spinning Bubbles
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)
November 9, 2020
Freezing Splats
When a drop hits a surface colder than its freezing point, there’s a competition between retraction and solidification that determines the final shape of the splat. For many materials, like wax or soldering metals, the contact angle between their liquid and solid phase is zero, so there’s no major shape change once solidification begins. But water — as is so often the case — is an exception.
Water and ice have a non-zero contact angle, which means that retraction can continue even after the drop begins freezing. As a result, the final shape of the splat varies depending on how cold the surface is. For a surface only a little colder than the freezing point, the final splat forms a spherical cap (Image 1). But once the surface is colder, freezing happens before the water can fully retract and the final splat forms a ring (Image 2). (Image and research credit: V. Thiévenaz et al.)
October 19, 2020
Simulating Better Breaking Waves
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.)
October 5, 2020
Slow Mo Espresso
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.)
August 21, 2020