Search results for: “droplet”

Streaming Fire
I’m just going to start this one with a blanket statement: DO NOT TRY THIS. Instead, enjoy the fact that the Internet enables us to enjoy the sight of burning gasoline in slow mo without any danger to ourselves.
In this video, Gav and Dan capture a burning bucket of gasoline as it’s thrown against glass. One thing this stunt really highlights is that it’s not the liquid gasoline that burns, it’s the vapor. However, since gasoline is volatile – in other words, it evaporates easily – the fire is quick to spread, especially as the toss atomizes droplets near the edge of the fluid. That’s why you see distinct streaks near the edge of the spreading flame and a non-burning liquid in the center. (Image and video credit: The Slow Mo Guys)
November 6, 2019
Inside Tears of Wine
Pour wine or liquor into a glass, give it swirl, and you can watch as droplets form and dance on the walls. This well-known phenomena, often called “tears” or “legs” in wine, results from an interplay of surface tension and evaporation. Despite its common occurrence, researchers are still discovering interesting subtleties in the physics, as seen in new research on the subject.
Dianna walks you through the phenomenon step-by-step in this video. The key piece of physics is the Marangoni effect, the tendency of regions with high surface tension to pull flow from areas with lower surface tension. In the wine glass, evaporation creates this surface tension gradient by removing alcohol more quickly from the meniscus than the bulk. That sets up the gradient that lets the wine climb the glass. By preventing or delaying that evaporation, we can see other neat effects, too, like shock fronts that travel through the film. (Video credit: Physics Girl; research credit: Y. Dukler et al.)
November 4, 2019
The Disappearing Cotton Candy
Moisture is cotton candy’s natural enemy. The spun sugar dissolves incredibly quickly under the influence of even a couple drops of water. Why that’s so is clearer when looking at a single fiber. Inside the droplet there’s a gradient in the sugar concentration. The more sugary water sinks, and the sugar fiber dissolves more quickly in the upper part of the droplet, where the less sugary water can more easily take up new sugar.
Once the fiber breaks, capillary forces draw the droplet upward, giving it a fresh section of fiber to dissolve. In a web of fibers, this process can pull droplets apart and together as they quickly eat through the spun sugar. (Image and video credit: S. Dorbolo et al.; submitted by Alexis D.)

October 21, 2019
Sliding Down a Pitcher Plant
Carnivorous pitcher plants supplement their nutrient-poor environments by capturing and consuming insects. The viscoelastic fluid inside them helps trap prey, but fluid dynamics plays a role elsewhere on the plant as well. The inner and outer surfaces of the pitcher are covered in macroscopic and microscopic grooves, seen above, oriented toward the interior of the plant.
Researchers found that these grooves trap droplets on the slippery plant through capillary action. Once adhered, the droplet cannot easily move across the grooves, but it can slip along them, carrying the droplet and any insect stuck to it, into the plant. By replicating pitcher-plant-inspired grooves on manmade surfaces, researchers found they were able to better control droplet motion on slippery, lubricant-infused surfaces than in previous work. (Image and research credit: F. Box et al.; via Royal Society; submitted by Kam-Yung Soh)
October 15, 2019
“Emergence”
Artist Susi Sie explores fluidic worlds through her macro lens. In “Emergence,” her focus is on ferrofluids immersed in other liquids. Beginning with tiny droplets traversing the thin fluid channels of a foam, she allows the unique qualities of ferrofluids to slowly take center stage. Dark blobs grow into curvy labyrinths as a magnetic fields come into play. Until ultimately the magnetic nature of the fluid becomes undeniable as scattered droplets elongate into miniature compass needles and swing around to follow the field lines. (Video and image credit: S. Sie)
September 19, 2019
“Unity”
Rus Khasanov’s latest short film, “Unity,” is all about coming together with droplets coalescing, globules bursting, and colors mixing. Take a glittery, paint-filled break and enjoy some macro-filmed fluid dynamics in action. (Video and image credit: R. Khasanov)
September 3, 2019
Breaking Up
The dripping of a faucet and the break-up of a jet into droplets is universal. That means that the forces – the inertia of the fluid, the capillary forces governed by surface tension, and the viscous dissipation – balance in such a way that the initial conditions of the jet – its size, speed, etc. – don’t matter to the process of break-up.
We’d expect that the inverse situation – the breakup of a gas into bubbles in a liquid – would be similarly universal, but it’s not. When unconfined bubbles pinch off, the way they do so is heavily influenced by initial conditions. But that changes, according to a new study, if you confine the gas to a liquid-filled tube before pinch-off. Confinement forces a different balance between viscous and capillary effects, one which effectively erases the initial conditions of the flow and restores universality to the pinch-off process. (Image and research credit: A. Pahlavan et al.; via phys.org)
August 23, 2019
Splashes on Hairy Surfaces
The question of whether a droplet will splash is a complicated one, even for smooth surfaces, but researchers are also interested in what happens to hairy surfaces when droplets strike. By varying the droplet viscosity and speed, along with the spacing of the hairs, researchers sketched out the variety of impacts one can get.
What happens during impact depends largely on how the kinetic energy of the droplet compares to the dissipation caused by interaction with the hairs. When the two balance, the droplet gets captured, like in the upper right image. If the hairy dissipation wins, you get a drop that stays mostly on the surface of the hairs. And if the kinetic energy outweighs the dissipation, you end up with a star-shaped splash that spreads between the hairs. (Image and research credit: A. Nasto et al.)
August 20, 2019
How Ant Stingers Work
Anyone who’s felt the sting of a fire ant knows it only takes an instant for this species to deliver a painful blow. Scientists are uncovering why that is using some of the first-ever high-speed footage of ant stingers in action. Stingers are actually made up of multiple separate pieces, including a central stylet and a pair of lancets that move up and down along the stylet. This lancet motion pulls the stinger deeper and helps form and deliver droplets of venom. The back-and-forth motion helps ants release up to 13 venom droplets per second, a level of speed that’s key for some of its high-speed, small-scale battles. (Image and video credit: Ant Lab; research credit: A. Smith)
August 14, 2019
Collecting Dew
In areas of the world where fresh water is scarce, one potential source is dew collection. Scientists have been working in recent years on making overnight dew collection more efficient. The challenge is that drops won’t begin to slide down an inclined surface until they are large enough for gravity to overcome the surface tension forces that pin the drop. Most efforts have focused on reducing the critical size where drops begin to slide through surface treatments and chemical coatings.
A recent study, however, uses a different tactic. Instead of aiming to reduce the critical drop size, these researchers built a grooved surface designed to encourage drops to grow faster. By helping the droplets coalesce quickly, their surface (right side) is able to start shedding droplets much faster than a smooth surface (left side). Under test conditions, the grooved surface was shedding droplets after only 30 minutes, whereas the smooth surface shed its first drops after 2 hours. (Image and research credit: P. Bintein et al.; see also APS Physics)
August 12, 2019

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