The coalescence of two liquid droplets takes less than the blink of an eye, but it is the result of an intricate interplay between surface tension, viscosity, and inertia. The high-speed video above was filmed at 16000 frames per second, yet the initial coalescence of the silicone oil drops is still nearly instantaneous. At the very instant the drops meet, an infinitesimally small neck is formed between the droplets. Mathematically speaking, the pressure and curvature of the droplets diverge as a result of this tiny contact area. This is an example of a singularity. Surface tension rapidly expands the neck, sending capillary waves rippling along the drops as they become one. (Video credit: S. Nagel et al.; research credit: J. Paulsen)
Videos
Convection Cells
Human eyesight is not always the best for observing how nature behaves around us. Fortunately, we’ve developed cameras and sensors that allow us to effectively see in wavelengths beyond those of visible light. What’s shown here is a frying pan with a thin layer of cooking oil. To the human eye, this would be nothing special, but in the infrared, we can see Rayeigh-Benard convection cells as they form. This instability is a function of the temperature gradient across the oil layer, gravity, and surface tension. As the oil near the bottom of the pan heats up, its density decreases and buoyancy causes it to rise to the surface while cooler oil sinks to replace it. Here the center of the cells is the hot rising oil and the edges are the cooler sinking fluid. The convection cells are reasonably stable when the pan is moved, but, even if they are obscured, they will reform very quickly. (Video credit: C. Xie)
Water and Aerogel
Aerogel is an extremely light porous material formed when the liquid inside a gel is replaced with gas. When combined with water, aerogel powders can have some wild superhydrophobic effects. Here water condensed on a liquid nitrogen cooler has dripped onto a floor scattered with aerogel powder from the nitrogen’s shipping container. The result is that the water gets partially coated in aerogel powder and takes on some neat properties. Its contact angle with the surface increases – in other words, it beads up – which is typical of superhydrophobicity. When disturbed, the water breaks easily into droplets which do not immediately recombine upon contact. With sufficient distortion, they can rejoin. You can see some other neat examples of aerogel-coated water behaviors in this second video as well. (Video credit: ophilcial; submitted by Jason I.)
The Kaye Effect
The Kaye effect is particular to shear-thinning non-Newtonian fluids – that is, fluids with a viscosity that decreases under deformation. The video above includes high-speed footage of the phenomenon using shampoo. When drizzled, the viscous liquid forms a heap. The incoming jet causes a dimple in the heap, and the local viscosity in this dimple drops due to the shear caused by the incoming jet. Instead of merging with the heap, the jet slips off, creating a streamer that redirects the fluid. This streamer can rise as the dimple deepens, but, in this configuration, it is unstable. Eventually, it will strike the incoming jet and collapse. It’s possible to create a stable version of the Kaye effect by directing the streamer down an incline. (Video credit: S. Lee)
Harnessing Ocean Waves
Ocean waves contain substantial amounts of energy, and many projects are underway to harness them as renewable energy sources. Most of these projects use the motion caused by waves to generate electrical energy. In this example, a flexible carpet is attached to hydraulic pumps. As the waves move over the carpet, it oscillates, raising and lowering the piston of the pumps. This adds hydraulic pressure to the discharge lines that run from the wave carpet to the shore. Once on dry land, that hydraulic pressure can be converted to electrical energy. This design addresses one of the major challenges in ocean-wave-energy technologies–namely how to safely transmit power from the wave farm to the shore. (Video credit: University of California Television)
How Dogs Drink
This high-speed footage shows how a dog drinks. The dog’s tongue curls backwards, creating a large area of surface contact with the water. When the dog pulls its tongue back up, water adheres to it and is drawn upward in a column. The dog then closes its mouth around the water before it falls. Fundamentally, this is the same mechanism as the one cats use. Part of the reason that dogs are messier drinkers, though, is that the backwards curl of their tongue picks up extra water. Because the dog has no cheeks, there’s no way to move this water from the underside to the top of the tongue and so the water just falls back out. (Video credit: Oxford Scientific Films; submitted by Carolyn W.)
Ice in Engines
Ice build-up is a major hazard on airplane wings and control surfaces, but ice can accrete on internal engine components, too. When this happens, the turbofan jet engine can lose power. Such incidents have been observed in high-altitude flight even when pilots observed little to no inclement weather. Researchers think this ice accretion may occur when the plane flies through a cloud of tiny ice crystals. These ice crystals get ingested into the engine, where they hit the warmer internal surfaces and melt. Over the course of the flight, the engine components cool off due to this influx of ice and water. Eventually, ice begins to form and grow inside the engine, ultimately resulting in power loss. Researchers have recreated such ice cloud conditions in a facility at NASA Glenn Research Center and tested a full-scale jet engine for ice accretion. They aim to gather the data necessary to improve commercial engine capabilities under ice ingestion. (Video credit: NASA Glenn Research Center)
Flowing Uphill
Science Friday takes an inside look at self-propelled Leidenfrost droplets like those we’ve featured previously. The Leidenfrost effect takes place when a liquid comes in contact with a surface much, much hotter than its boiling point. Part of the liquid is vaporized, creating a thin gas layer that both insulates the remaining liquid and causes it to move with very little friction. Over a flat surface, this underlying vapor will spread in any direction. But by covering the surface with ratchets, it’s possible to direct the vapor in a particular direction, which propels the droplet in the opposite direction. Check out the video and our previous posts for more! (Video credit: Science Friday; via io9 and submitted by Urs)
Greening the River
Every year Chicago dyes its river green in honor of St. Patrick’s Day. This timelapse video shows this year’s dyeing, including several passes from a boat distributing the green dye. The color is remarkably slow to diffuse. The boat’s passage does little to affect the motion of the dye already in the river. This is because the boat mainly disturbs the surface and most of the color comes from dye spread throughout the water. It’s like if you tried to stir milk into your coffee just by tapping the surface with your spoon. Instead, the slower, large-scale turbulent motion of the river distributes the dye. For more St. Patrick’s Day physics, be sure to check out Guinness physics and why tapping a beer makes it foam. (Video credit: P. Tsai; submitted by Bobby E.)
Vortex Ring Tricks
Vortex rings are wonderful at maintaining coherent vorticity while moving over significant distances. If you stand several meters from a foam cup and try blowing to knock it over, it’s not likely to budge. But move the air impulsively with a vortex cannon, and you can knock it over from the opposite side of the room. The same principle works underwater with added visual effect. Here an impulsive burst of air exhaled by the diver forms a bubble ring with vorticity strong enough to knock over a stack of rocks. It may look like a superpower, but this is science! Dolphins and whales are also known to play with this trick. For the non-scuba-divers among you, it’s also possible to learn to do it in a swimming pool. (Video credit: DjDeutchTv; h/t to coolsciencegifs)