The splash of a drop may be commonplace, but it is still a mesmerizing and fertile phenomenon. When it comes to splashing, scientists are still learning how to predict the outcome. Here a drop of silicon oil impacts a film of silicon oil with an even higher viscosity. The momentum of that impact creates a crater and a splash curtain that rises and expands from the initial point of impact. Because the film viscosity is higher than the drop’s, the evolution of the corona slows down. Eventually, surface tension and gravity start pulling the splash curtain back down as the crater collapses. Meanwhile at the top of the splash, capillary forces pull fluid into the rim, which becomes unstable and grows cusps that eventually eject a cloud of smaller droplets. (Image and research credit: H. Kittel et al., source)
Tag: surface tension
Spinning Droplet Galaxies
Water flung from a spinning tennis ball takes on a shape reminiscent of a spiral galaxy. As it detaches, water leaves the surface with both the tangential velocity of the spinning ball and a radial velocity due to the centrifugal force flinging it. The continued spin of the ball makes the thin ligaments of water still attached to it spiral and stretch. Eventually, surface tension can no longer hold the water together against the centrifugal forces, and the ligaments split into a spray of droplets. (Image credit: W. Derryberry and K. Tierney)
Bubble Art
Everyone loves soap bubbles, and bubble artist Melody Yang reveals how to make some pretty awesome ones in this video for Wired. The surface tension of bubbles makes them naturally seek a shape that minimizes their surface area relative to the volume they contain. For a single bubble, that’s a sphere. But once you start joining multiple bubbles, as Yang demonstrates, that minimal surface area can change, even to something unexpected like a cube.
Bubbles also have an impressive ability to self-heal. As long as whatever passes through them is wet – whether it’s a hand, a straw, or even a ball bearing – the soap film will probably heal itself rather than break. This is a key feature for many of Yang’s tricks, including the impressive planetary bubble. (Video credit: Wired; image credits: Wired/Colossal; via Colossal)
“Liquid Calligraphy”
In “Liquid Calligraphy,” artist Rus Khasanov’s letters dissolve once he draws them. At first, the white ink spreads in narrow fingers, probably driven by a combination of surface tension gradients, capillary action, and simple diffusion. But then, in flashes, the letters morph faster and flow outward. My best guess is that each jump is a spray from a bottle full of a low surface tension liquid like alcohol. The spray triggers faster outflows than before, like those seen when a strong difference in surface tension activates the Marangoni effect. It’s a beautiful and different artistic take on these important fluid forces. Check out more of his videos here or enjoy high-resolution stills and wallpapers in this style from his Behance page. (Image and video credit: R. Khasanov; submitted by TBBQoC)
Rim Break-Up
Splashing drops often expand into a liquid sheet and spray droplets from an unstable rim. Although this behavior is key to many natural and industrial processes, including disease transmission and printing, the physics of the rim formation and breakup has been difficult to unravel. But a new paper offers some exciting insight into this unsteady process.
The researchers found that if they carefully tracked the instantaneous, local acceleration and thickness of the rim, it always maintained a perfect balance between acceleration-induced forces and surface tension. That means that even though different points on the rim appear very different, there’s a universality to how they behave. They found that this rule held over a remarkably large range of situations, including across fluids of different viscosities and splashes on various surfaces. (Image and research credit: Y. Wang et al.; via MIT News; submitted by Kam-Yung Soh)
Fractal Fingers
Dyed isopropyl alcohol atop a thin layer of acrylic medium spreads in a fractal fingering pattern. Although the shapes are reminiscent of the viscous fingers seen in in the Saffman-Taylor instability, these patterns are most likely a result of surface tension. The lower surface tension of the alcohol causes Marangoni forces to pull it outward. The branching shapes indicate an instability, likely driven by surface tension, but the details of the mechanism behind it are unclear. (Image credits: J. Nahabetian)
The Disintegrating Splash
A drop of blue-dyed glycerine impacts a thin film of isopropanol, creating a spectacular splash and breakup. The drop’s impact flings a layer of the isopropanol into the air, where air currents make the thin sheet buckle inward and break into a spray of droplets. Meanwhile, the liquid from the drop forms a thick, blue crown that rises and expands outward. When tiny droplets of the isopropanol hit the splash crown, their lower surface tension causes the blue glycerine to pull away, due to the Marangoni effect. This opens up holes in the crown, which grow quickly, until the entire sheet breaks apart. (Image and research credit: A. Aljedaani et al., source)
Happy 2000 Posts!
Happy Friday and happy 2000th FYFD post! To celebrate, I played with surface tension and the Marangoni effect to make some art. For a run-down on the physics, check out this previous post on water calligraphy. Two thousand posts feels like a major milestone. Not everyone realizes this, but FYFD is a one-woman operation, so 2000 posts is a whole lot of research, image editing, and writing. For fun, I’m including here eight completely random FYFD entries, representing less than one-half of one percent of my total archives:
1. Why did Chinese junks put holes in their rudders?
2. Making droplets in an ultrasonic humidifier
3. Floating on a granular raft
4. Air-trapping fur keeps otters warm
5. The physics of the knuckleball
6. What makes badminton so fast?
7. Playing with fluorescein
8. How frost formsWant to keep up the random walk? Use https://fyfluiddynamics.com/random to find random entries, or if you prefer your browsing to be more directed, check out the visual archive or the themed series page.
As always, a special thanks to those who help support FYFD through Patreon subscriptions – I couldn’t keep writing and making videos without your help! And thank you to all of you who read and share FYFD. Whether you’ve been following along for a week or for the last eight years, your enthusiasm keeps me motivated! Thank you!
(Image credits: 2k animation – N. Sharp; Chinese junk ship – Premier Ship Models; ultrasonic humidifier – S. J. Kim et al.; granular raft – E. Jambon-Puillet and S. Protiere; 3D-printed “fur” – F. Frankel; knuckleball – L. Kang; shuttlecock – Science Friday; fluorescein – Shanks FX; freezing droplets – J. Boreyko et al.)
Spinning Paint
Several years ago Fabian Oefner started spinning paint, and it’s been a perennial favorite online ever since. Here the Slow Mo Guys revisit their own paint-spinning antics by super-sizing their set-up. In some respects, it’s a little dissatisfying; as with their first time around, they don’t moderate the drill speed at all, so after the initial spin-up, the centrifugal acceleration is so strong that it just shreds the paint instead of showing off the interplay between the acceleration and surface tension’s efforts to keep the paint together.
In their largest experiment, though, the Slow Mo Guys get some interesting physics. Here there’s only a single slot for paint to exit, so the set-up doesn’t lose all its paint at once. The centrifugal acceleration flings the paint out in sheets that stretch into ligaments and then tear into droplets as they move further out. But there’s some more complicated phenomena, too. Notice the bubble-like shapes forming in the yellow paint on the lower right. These are known as bags, and they form because of the relative speed of the paint and the air it’s moving through. This is actually the same thing that happens to falling drops of rain! (Video and image credit: The Slow Mo Guys)
Nestling Droplets
Pay attention after a rainfall, and you may notice beads of water gathering in the corners of a spider’s web or along the leaves of a cypress tree (bottom right). Look closely and you’ll notice that the largest droplets don’t form along a straight fiber. Instead they nestle into the corners of a bent fiber (top image). Researchers recently characterized this corner mechanism and found that the angle at which the largest droplets form is about 36 degrees. This angle provides the optimal conditions for capillary action and surface tension to hold large drops in place. At smaller angles, a growing droplet’s weight pulls it down until the thin film holding the droplet near the top ruptures and the droplet falls. At larger angles, a heavy droplet will slowly detach from one side of its fiber and shift toward the other side until its weight is too great for the wetted length of fiber to hold. Then it detaches completely and falls. (Research and image credit: Z. Pan et al.; via T. Truscott)
