Milk, acrylic paints, soap, and oil – all relatively common fluids, but together they form beautiful mixtures worth leaning in to enjoy. Variations in surface tension between the liquids cause much of the motion we see. Soap, in particular, has a low surface tension, which causes nearby colors to get pulled away by areas with higher surface tension, behavior also known as the Marangoni effect. Adding oil creates some immiscibility and lets you appreciate both the coalescence and fragmentation of the fluids. And finally, there’s one of my favorite sequences, where bubbles start popping in slow motion. As the bubble film ruptures, fluid pulls away, breaking into ligaments and then a spray of droplets as the bubble disintegrates. (Video credit: Macro Room; via Gizmodo)
Tag: surface tension
Living Fluid Dynamics
This short film for the 2016 Gallery of Fluid Motion features Montana State University students experiencing fluid dynamics in the classroom and in their daily lives. As in her previous film (which we deconstructed), Shanon Reckinger aims to illustrate some of our everyday interactions with fluids. This time identifying individual phenomena is left as an exercise for the viewer, but there are hints hidden in the classroom scenes. How many can you catch? I’ve labeled some of the ones I noticed in the tags. (Video credit: S. Reckinger et al.)
Dripping, Frozen
The simple drip of a faucet is more complicated when frozen in time. Any elongated strand of water tends to break up into droplets due to surface tension and the Plateau-Rayleigh instability. Whenever the radius of the water column shrinks, surface tension tends to drive water away from the narrow region and toward a wider point. This exaggerates the profile, making narrow regions skinnier and wider regions fatter. Eventually, the neck connecting the droplets becomes so thin that it pinches off completely, leaving a string of falling droplets. (Image credit: N. Sharp)
Droplet Bounce
Water droplets don’t always immediately disappear into a pool they’re dropped onto. If the droplet is small and doesn’t have much momentum, it will join the pool gradually through a process known as the coalescence cascade, seen here in high speed video. The droplet bounces off the surface, then settles. A thin layer of air is caught between it and the pool. Slowly the weight of the drop pushes that air out until there is contact between the drop and pool. Before the drop can merge completely, though, surface tension pinches it off, creating a smaller daughter droplet. Ripples caused by the merger help bounce the little droplet, which repeats the same process until the tiniest droplet merges completely. (Video credit: B. ter Huume)
“Memories of Paintings”
In “Memories of Paintings,” Thomas Blanchard gives us an up-close view of fluids and mixing. It’s a calming and curious video made from combinations of paint, oil, oat milk, and soap. The fluids feather and intertwine, driven by differences in surface tension. Paint gets encapsulated by immiscible oil to create little islands of color that float and dance against the background. It’s a fun journey through effects that we witness daily but rarely take the time to watch. (Video credit: T. Blanchard; via Gizmodo)
Paper Marbling
Fluid dynamics and art have gone hand-in-hand for centuries. In this video, artist Garip Ay demonstrates one of the coolest fluids-based art techniques: paper marbling. In this technique, artists float ink or paints on a liquid surface, manipulate the colors as desired–in this case to recreate Van Gogh’s “Starry Night”–and then float a piece of paper atop the surface to transfer the image. Multiple cultures around the world developed marbling techniques, dating all the way back to the Middle Ages. Ay is an expert in ebru, a Turkish form of the art. For more of Ay’s art, check out his website and YouTube channel. (Video credit: G. Ay; via Gizmodo)
Drawing Up Dew
Desert plants have evolved to efficiently collect and capture whatever water they can. Each leaf of the moss Syntrichia caninervis ends in a hairlike fiber called an awn (seen in white in the top image). Tiny as they are, awns are vital to the moss’s water collection, correlating to more than 20% of their dew collection. Extremely tiny grooves on the surface of the awn provide nucleation sites where dew condensed from fog collects. Once a droplet forms on the awn, it grows larger as more fog condenses (middle image). When the droplet grows large enough, the conical shape of the awn will cause surface tension to draw the droplets along the awn and toward the leaf (bottom image).
(Credits: Syntrichia caninervis moss image – M. Lüth; videos and research – Z. Pan et al., Supplementary Videos 3 and 4; h/t to T. Truscott)
“Bubble Circus”
The “Bubble Circus” is a delightful outreach device equipped for all manner of physics demos, as seen in the video above. Many of its exercises explore surface tension, a force observed at the interface of a fluid. Surface tension is what provides bubbles with their surface-minimizing spherical shape. That same property determines the minimal distance between the four vertices of a pyramid (0:54). Changing the surface tension causes fluid at the interface to move. At 1:16 adding a lower surface tension fluid makes the water and black pepper pull away; the same physics drives the boat away at 2:09. For more on the Bubble Circus, see here. (Video credit: A. Echasseriau et al.; via J. Ouellette)
Pearls of Mezcal
Mezcal is a traditional Mexican liquor distilled from agave. (The more commonly known tequila is actually a special type of mezcal.) As a part of the production process, distillers pour a stream of mezcal into a bowl, creating a flotilla of small bubbles called pearls. Strange as it sounds, these pearls let the distiller judge the alcohol content of the liquor! When the ratio of alcohol and water in the mixture is just right, the bubbles will have a longer lifetime before they coalesce. If there’s too little or too much alcohol, the bubbles won’t last as long. The effect depends on both the viscosity and the surface tension of the liquor, but it’s the odd way that viscosity changes in water/alcohol mixtures that creates this Goldilocks behavior. It’s a fascinating demonstration of how traditional techniques often have true scientific underpinnings. (Video credit: M. Wilhelmus et al.)
Coffee-Making in Space
In this video, Kjell Lindgren demonstrates his technique for making coffee aboard the Space Station. Astronauts usually drink coffee reconstituted from powder, or, on special occasions, enjoy a beverage from their special espresso machine. But Lindgren uses a pour-over method by attaching a pod of coffee grounds to the underside of a Capillary Beverage Experiment cup – a specially-designed 3D printed cup that uses capillary action and surface tension to guide fluids. Then, by forcing hot water from a syringe through the grounds and into the cup, he gets a result that’s not too different from the way many people enjoy their coffee here on Earth. I must say, though, that my favorite part of this video is how he just starts spinning to separate the air and water in the syringe! (Video credit: NASA; via IRPI LLC)
