How liquid droplets spread on solid surfaces is pretty well understood, but researchers have looked less at the related problem of how a gas spreads. In a recent paper, scientists have examined the spreading dynamics of bubbles impacting an immersed solid. As the bubble contacts the surface, it quickly squeezes out water trapped between the bubble and the gas layer trapped at the solid surface. The bubble squishes as surface tension tries to flatten the liquid-gas interface. Buoyancy also helps flatten the bubble. The spreading is remarkably fast, taking only about 10 milliseconds. That’s good news for the many insects who use trapped air bubbles like these to breathe underwater. Check out the video below to learn about some of these natural scuba divers. (Image credit: H. de Maleprade et al., source; video credit: Deep Look)
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Icy Spikes
Water is one of those strange materials that expands when it freezes, which raises an interesting question: what happens to a water drop that freezes from the outside in? A freezing water droplet quickly forms an ice shell (top image) that expands inward, squeezing the water inside. As the pressure rises, the droplet develops a spicule – a lance-like projection that helps relieve some of the pressure.
Eventually the spicule stops growing and pressure rises inside the freezing drop. Cracks split the shell, and, as they pull open, the cracks cause a sudden drop in pressure for the water inside (middle image). If the droplet is large enough, the pressure drop is enough for cavitation bubbles to form. You can see them in the middle image just as the cracks appear.
After an extended cycle of cracking and healing, the elastic energy released from a crack can finally overcome surface energy’s ability to hold the drop together and it will explode spectacularly (bottom image). This only happens for drops larger than a millimeter, though. Smaller drops – like those found in clouds – won’t explode thanks to the added effects of surface tension. (Image credit: S. Wildeman et al., source)
ETA: A previous version of this post erroneously said this was freezing from the “inside out” instead of “outside in”.
Self-Propelling Drops
Droplets of acetone deposited on a bath of warm water can float along on a Leidenfrost-like vapor layer. The droplets are self-propelling, too, thanks to interactions between the acetone and water. Acetone can dissolve in water, and when acetone vapor beneath the drop gets absorbed into the water bath, it lowers the local surface tension. That drop in surface tension creates a pull in the direction of a higher surface tension; this is what is known as the Marangoni effect. Because of that flow in the direction of higher surface tension, the acetone drop accelerates away. (Image credit: S. Janssens et al., source)
Hot Versus Cold
Did you know that you can hear the difference between hot and cold water when they’re poured? Go ahead and give the video above a listen to try it out. I’ll wait.
As explained in the video, the viscosity of water changes with temperature – the higher the temperature, the lower the viscosity. In fact, the viscosity of water at 10 degrees Celsius is more than 4 times higher than the viscosity at 100 degrees Celsius! That’s pretty significant, and it’s a big enough difference that we can hear it in the splash, even if we don’t see the difference when pouring.
Surface tension also decreases with temperature but not nearly as strongly. That 100 degrees Celsius water has 25% less surface tension than the 10 degrees Celsius water. But the combination of this change in viscosity and change in surface tension is why your cold water is more likely to dribble down the spout of your coffee pot when you’re filling the coffee machine than when you’re pouring coffee from the same pot. (Video credit: Steve Mould and Tom Scott; submitted by entropy-perturbation)
Sloshing in Space
Last month, French astronaut Thomas Pesquet posted a video of some experiments he did on the International Space Station exploring the movement of fluids in microgravity. He filmed the experiments as part of the SPHERES Slosh project. Sloshing is the technical term for how liquids respond to the motion of their container, and it’s a tough problem whether you’re carrying a full coffee mug on Earth or dealing with a partially-emptied fuel canister in orbit.
Here on Earth, gravitational forces dominate how fluids respond, but in microgravity, surface tension is a more powerful player. Pesquet’s demonstrations help scientists here on Earth better understand and model how liquids respond to movement in space. One major application for this is in spacecraft fuel tanks, which engineers must be able to design so that they empty themselves consistently with or without the added complications of spinning, maneuvering, or impulsive kicks of acceleration. (Video and image credit: ESA; submitted by gdurey)
Molten Copper
In this video, the Slow Mo Guys prove that pouring molten copper in slow motion is every bit as satisfying as one would imagine. Because they pour the metal from fairly high up, they get a nice break-up from a jet into a series of droplets; that’s due to the Plateau-Rayleigh instability, in which surface tension drives the fluid to break up into drops. Upon impact, the copper splashes and splatters very nicely, forming the crown-like splash many are familiar with from famous photos like Doc Edgerton’s milk drop. The key difference between the molten copper and any other liquid’s splash comes from cooling; watch closely and you’ll see some of the copper solidifying along the edges and surface of the fluid as it cools. In this respect, watching the molten copper is more like watching lava flow than seeing water splash. (Video and image credit: The Slow Mo Guys)
Bursting Droplets
Mixing multiple fluids can often lead to surprising and mesmerizing effects, whether it’s droplets that dance or tears along the walls of a wine glass. A recent paper highlights another such mixture-driven instability – the bursting of a water-alcohol droplet deposited on an oil bath. The Lutetium Project tackles the physics behind this colorful burst in the short video above. The behavior is driven by the quick evaporation rate of alcohol in the droplet and the way this changing chemical concentration affects surface tension in the droplet. Alcohol evaporates more quickly from the edges of the drop, creating a region of higher surface tension around the edge. This pulls fluid to the rim of the drop, where it breaks up into droplets that get pulled outward as the inner drop shrinks.
The oil bath plays an important role in the instability, too. Without it, friction between the drop and a wall is too high for the droplet to “burst”. A thick layer of oil acts as a lubricant, allowing the escaping satellite drops to speed away. (Video and image credit: The Lutetium Project; research credit: L. Keiser et al.; submitted by G. Durey)
Breaking the Wave Speed Limit
Whirligig beetles are small surface swimming insects. As they race across the water surface, they create both visible and unnoticeable waves on the water. These waves are the result of both surface tension and gravity. Typically, it’s the wavelength of the gravity waves that limit a swimmer or boat’s speed. When the wavelength of the gravity waves a swimmer creates meets the size of the swimmer, the waves generated ahead of the swimmer start to reinforce the waves forming at the back of the swimmer. This traps the swimmer (or boat) in a trough between its bow and stern waves and limits the max speed of the swimmer since overcoming this critical hull speed requires excessive amounts of power.
The tiny whirligig beetle overcomes this natural speed limit cleverly. It is smaller than the shortest possible gravity wave in water. Thus, it can never be trapped between its bow and stern waves! This allows the tiny swimmer to zip across the water’s surface at speeds above 0.5 m/s. That’s over 30 beetle body lengths per second! (Image credit: H. L. Drake, source; research credit: V. Tucker; submitted by Marc A.)
Supporting Bubbles
Surface tension holds small droplets in a partial sphere known as a spherical cap. But when droplets become larger, they flatten out into puddles due to the influence of gravity. In contrast, soap bubbles remain spherical to much larger sizes. The bubble pictured above, for example, is more than 1 meter in radius and nearly 1 meter in height.
There is a maximum height for a soap bubble, though, and it’s set by the physical chemistry of the surfactants used in the soap. To support itself, the bubble requires a difference in surface tension between the top and bottom of the bubble. A higher surface tension is necessary at the top of the bubble to help prevent fluid from draining away. The difference in surface tension between the top and bottom of the bubble can never be greater than the difference in surface tension between pure water and the soap mixture – thus those values set a maximum height for a bubble. The researchers found their bubbles maxed out at a height of about 2 meters, consistent with their theoretical predictions. (Image credit: C. Cohen et al.; via freshphotons)
Leidenfrost Atop a Fluid
Leidenfrost droplets typically hover on a thin layer of vapor above a surface that is much hotter than the boiling point of the liquid. Such drops move almost frictionlessly across these surfaces and can even propel themselves. The question of how hot is hot enough to produce the Leidenfrost effect is still being debated, but recent research suggests that the answer may depend strongly on surface roughness.
To test the role of surface roughness, one group tested drops of ethanol atop a heated pool of silicone oil, as pictured above. Ethanol’s boiling point is 78 degrees Celsius, and the researchers found they could hold the ethanol drop in a Leidenfrost state by heating the pool to 79 degrees Celsius – only 1 degree above ethanol’s boiling point! Thanks to surface tension, a liquid surface is essentially molecularly smooth. The fact that solid surfaces require much higher temperatures before the Leidenfrost effect is observed indicates that even the slightest roughness can have a large impact on the Leidenfrost temperature. (Image credit: F. Cavagnon; research credit: L. Maquet et al., pdf)
Heads-up for Boston-area folks! I’ll be taking part this Saturday evening in the Improbable Research show at the AAAS conference. The show is free and open to the public but fills up quickly, so be sure to come early for a seat.