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)
Tag: surface tension
“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)
Rotating Jet
This photo, one of the winners of the Engineering and Physical Sciences Research Council’s (EPSRC) annual photography contest, shows a rotating viscoelastic jet. Rotating liquid jets are common to many manufacturing processes, and their sometimes-wild appearance comes from a balance of gravitational forces and centrifugal force against surface tension. But because this fluid contains a small amount of polymer additive, surface tension has the additional aid of some elasticity to help hold the jet together and keep the globules and ligaments you see from flying off. As centrifugal forces fling the fluid outward, it stretches the polymer chains within the fluid, and they pull back against that tension like a stretched rubber band. To see some of the other contest winners–including other fluids entries!–check out the Guardian’s run-down. (Image credit and submission: O. Matar et al., ICL press release)
Dancing Droplets
The seemingly-alive dancing droplets are back in a new video from Veritasium. These droplets of food coloring attract, merge, and chase one another due to evaporation and surface tension interactions between their two components: water and
propylene glycol. Because the droplets are constantly evaporating, they are surrounded by a cloud of vapor that helps determine a drop’s surface tension. These localized differences in surface tension are what causes the drops to attract. The chasing is also surface-tension-driven. Like any liquid, the drops will flow from areas of low surface tension to those of higher surface tension due to the Marangoni effect. Thus drops of different concentration appear to chase one another. This is a relatively simple experiment to try yourself at home, and Derek outlines what you need to know for it. (Video credit: Veritasium; research credit: N. Cira et al.; submitted by @g_durey)
Molten Salt in Water
In his latest video, The Backyard Scientist explores what happens when molten salt (sodium chloride) gets poured into water. As you can see, the results are quite dramatic! He demonstrates pretty convincingly that the effect is physical – not chemical. The extreme difference in temperature between the liquid water (< 100 degrees Celsius) and the molten salt (> 800 degrees Celsius) causes the water to instantly vaporize due to the Leidenfrost effect. This vapor layer protects the liquid water from the molten salt – until it doesn’t. When some driving force causes a drop of water to touch the salt without that protective vapor layer, the extreme temperature difference superheats the water, causing it to expand violently, which drives more water into salt and feeds the explosion.
But why don’t the other molten salts he tests explode? Sodium carbonate, the third salt he tests, has a melting point of 851 degrees Celsius, 50 degrees hotter than sodium chloride. Yet for that test, the Leidenfrost effect prevents any contact between the two liquids. The key in this case, I hypothesize, is not simply the temperature difference between the water and salt, but the difference in fluid properties between sodium chloride and sodium carbonate. The breakdown of the vapor layer and subsequent contact between the water and the molten salt depends in part on instabilities in the fluids. A cavity where instabilities can grow more easily is one where the Leidenfrost effect is less likely to protect and separate the two fluids. And, in fact, it turns out that the surface tension of molten sodium chloride is significantly lower than that of molten sodium carbonate! A lower surface tension value means that the molten sodium chloride breaks into droplets more easily and its vapor cavity will respond more strongly to fluid instabilities, making it more likely to come in contact with liquid water and, thus, cause explosions. (Image/video credit: The Backyard Scientist; submitted by Simon H)
