Every child learns about the water cycle in school, but an academic description of the process often lacks nature’s grandeur. In “Monsoon II” photographer Mike Olbinski captures the majesty of cloud formation and rainfall in a way that rekindles awe for the scale of the process. It begins with bright clouds popping up, the result of warm moist air rising from the ground and cooling at altitude. As more water vapor evaporates, rises, and condenses, water droplets collide in these clouds, coalescing and growing until they grow too large and heavy to stay aloft. These are the droplets that fall in sheets of rain, blurring the air beneath them. There’s an incredible beauty to watching rain fall from a distance; it looks calm and localized in a way that’s utterly at odds with the experience from inside the storm. (Video credit: M. Olbinski; submitted by jshoer)
Search results for: “droplet”
Collecting Water in the Desert
Desert-dwelling plants like cactuses have to be efficient collectors of water. Many types of cactus are particularly good at gathering water from fog that condenses on their spines. Droplets that form near a spine’s tip move slowly but inexorably toward the base of the spine so that the cactus can absorb them. The secret to this clever transport lies in the microstructure of the spine’s surface. The
Gymnocalycium baldianum cactus, for example, has splayed scales along its spines. Capillary interactions with the scales result in differences in curvature on either side of the droplet. Curved fluid surfaces generate what’s known as Laplace pressure, with a tighter radius of curvature causing a larger Laplace pressure. Because the curvature of the droplet varies from the base side to the tip side of the spine, the difference in Laplace pressures across the droplet creates a force that drives the droplet toward the spine’s base. (Image credit: C. Liu et al., source)
Oil Film on Water
This award-winning short film features a thin layer of volatile oil on water. The oil evaporates quickest from shallow pools only microns deep, which appear bluish in the video. Surface instabilities along the edge of the pool create flow that draws oil in, generating the iridescent droplets seen floating among the evaporation pools. The droplets combine and coalesce as they come in contact with one another. Since droplets have a larger volume per surface area than the shallow pools, they evaporate more slowly. The behaviors observed here are important to applications like oil and fuel spills, which can persist longer if the floating fluid layer tends to form droplets. (Video credit: J. Hart; via txchnologist)
From Dripping to Beading
When water drips, it quickly breaks up into a string of smaller droplets due to a surface-tension-driven instability called the Plateau-Rayleigh instability. But adding just a tiny bit of polymer to the fluid changes the behavior entirely. Instead of breaking into droplets, a narrow filament dotted with tiny satellite droplets forms between the larger drops. This is known as the beads-on-a-string instability. The viscoelasticity the polymers add is one key to seeing this behavior. Polymers consist of large molecule chains that, when stretched, act a little like rubber bands–they pull back against the stretch, providing an elastic effect. Without this elasticity, the tiny filament connecting the drops would break up immediately. (Image credit: M. Berman, source; research credit: P. Bhat et al.)
Healing Soap Films
As fragile as a soap bubble seems, these films have remarkable powers of self-healing. The animation above shows a falling water droplet passing through a soap film without bursting it. An important factor here is that the water droplet is wet–passing a dry object through a soap film is a quick way to burst it, as those who have played with bubbles know. The droplet’s inertia deforms the soap film, creating a cavity. If the drop’s momentum were smaller, the film could actually bounce the droplet back like a trampoline, but here the droplet wins out. The film breaks enough to let the drop through, but its cavity quickly pinches off and the film heals thanks to the stabilizing effect of its soapy surfactants. (Image credit: H. Kim, source)
Miniature Bursting Bubbles
Fizzy drinks like soda or champagne contain dissolved carbon dioxide which forms bubbles when the pressure inside its container is released. The tiny bubbles rise to the surface where the liquid film covering them can rupture, creating a small cavity at the surface. The cavity collapses in a matter of milliseconds (bottom animation). Above the surface, the cavity reverses its curvature to create a liquid jet (top animation) which can expel multiple tiny droplets. These droplets can tickle a drinker who hovers too close, but they also carry and distribute the aroma molecules that are part of the experience of a drink like champagne. (Image credit: E. Ghabache et al., source)
(Today’s topic brought to you by my impending nuptials to my favorite physicist/spacecraft engineer.)
Boiling Water in Oil
Most people know that throwing water into hot oil is a bad idea. But, as dramatic as the results can be, the boiling of a water droplet submerged in oil is remarkably beautiful, as seen in the animations above. The initial water droplet expands as it shifts from liquid to vapor (top). At a critical volume, the expansion occurs explosively (middle), causing the bubble to overexpand relative to the pressure of the surrounding fluid. The higher pressure of the oil around it collapses the drop, which then re-expands, creating the cycle we see in the final two animations. This oscillation triggers a Rayleigh-Taylor type instability along the bubble’s interface, causing the surface corrugations observed. The vapor bubble will continue to rise through the oil, eventually breaking the surface and scattering hot oil droplets. (Image credits: R. Zenit, source)
Carbonation in Space
Astronauts don’t typically drink soda or other carbonated beverages while in space. The reason is probably apparent if you watch this new video of an effervescent tablet in water on the space station (or, you could watch the older classic one from Don Pettit). Unlike on Earth, where the carbon dioxide bubbles are buoyant and rise to the surface, the bubbles in a fluid in microgravity are randomly distributed. Those few bubbles that happen to be located along the edge of the water sphere will sometimes burst, creating the halo of tiny droplets you see in the video. In the case of sodas, though, the bubbles’ behavior creates a foamy mess, and, after ingestion, the bubbles are stuck travelling through the astronaut’s digestive system instead of getting burped out. Sounds rather unpleasant to me. (Video credit: NASA; submitted by entropy-perturbation and buckitdrop)
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LAST CALL: Help us do some science! I’ve teamed up with researcher Paige Brown Jarreau to create a survey of FYFD readers. By participating, you’ll be helping me improve FYFD and contributing to novel academic research on the readers of science blogs. It should only take 10-15 minutes to complete. You can find the survey here.
Soap Bubble Coalescence
Droplets falling onto a bath of the same liquid will sometimes coalesce via a series of increasingly smaller droplets in a process known as the coalescence cascade. Soap bubbles, it turns out, can exhibit a similar partial coalescence. When a bubble nears a soap film and the air between them drains away, coalesce can begin. If the the soap film beneath the bubble ruptures, some air from the inside of the bubble can escape. Part of the bubble coalesces with the soap film and a smaller daughter bubble is left behind. The researchers observed this process happen up to three times before the bubble coalesced completely. Alternatively, if the soap film did not rupture, the air inside the bubble had no escape, and the bubble would coalesce into a hemispherical lens atop the soap film. (Video credit: G. Pucci et al.; via KeSimpulan)
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Don’t forget about our FYFD survey! I’ve teamed up with researcher Paige Brown Jarreau to create a survey of FYFD readers. By participating, you’ll be helping me improve FYFD and contributing to novel academic research on the readers of science blogs. It should only take 10-15 minutes to complete. You can find the survey here. Please take a few minutes to participate and share!
Happy 5th, FYFD!
FYFD is 5 years old! Hard to believe it’s been five whole years. Thank you to everyone who has helped along the way, especially those of you who produce, submit, and share such beautiful fluid dynamics.
Thanks also to everyone who is participating in our reader survey. We’re getting a lot of great feedback. If you haven’t taken it yet, there’s still time!
And, finally, in honor of five years of FYFD, I present you with the five most popular FYFD posts of all time:
1. Swimming through surface tension – Originally posted 7 Feb 2013
2. Bioluminescence as a defense mechanism – Originally posted 4 Sep 2014
3. Liquid mushroom – Originally posted 19 Feb 2013
4. Dancing droplets – Originally posted 30 Mar 2015
5. Stepping on lava – Originally posted 19 Dec 2014