Sometimes the similarity between fluid flow and granular flows is quite striking. This video shows a stream of sand falling down a tube and impacting a rod. (Note: the view is rotated 90 degrees counter-clockwise, so down points to the right.) As the sand strikes the rod, it’s deflected into a conical sheet, very much like a water bell. There are even ripple-like instabilities that form in the granular sheet, though they move differently than in a liquid due to the sand’s lack of surface tension. (Video credit: S. Nagel et al.)
Search results for: “surface tension”
Inside a Splash
When a droplet strikes a pool, a thin, fast-moving sheet of liquid expands outward from the region of contact. These ejecta sheets come in many forms depending on surface tension, viscosity, air pressure, and droplet momentum. When the ejecta sheet curls downward to touch the pool, it can spray microdroplets outward or trap a layer of air underneath the droplet. For more, see this video by S. Nagel et al., and the papers Thoroddsen (2002) and Thoroddsen et al. (2008). (Photo credits: S. Thoroddsen et al.; GIF from this video by S. Thoroddsen et al.)
Bursting a Bubble
Though seemingly instantaneous to the naked eye, the bursting of a soap bubble is fascinating when slowed down. Here it is at about 2200 frames per second. Initially, the bubble is approximately spherical – its shape determined by a balance between surface tension, gravity, and pressure. The prick of a pinpoint disrupts the balance, and surface tension pulls the thin film away from the defect. The liquid sheet of the bubble retracts swiftly into a filament of fluid and a cloud of tiny droplets. (Video credit: soapbubble.dk)
Coalescence
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)
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)
Sochi 2014: Downhill Skiing
Like the athletes who compete on ice, skiers rely on a film of liquid beneath their skis to provide the low friction necessary to glide. The moisture results from the friction of the ski’s base and edges cutting into the snow, and, depending on the conditions of the snow, different surface treatments are recommended for the skis to help control and direct this lubricating film. Similarly, skiers uses various waxes on their skis to lower surface tension and provide additional lubrication. Fluid dynamics can also play a role in tactics for various ski-based events. In endurance events like cross-country skiing, drafting behind other skiers can help an athlete avoid drag and save energy. When drafting, cross-country skiers have lower heart rates. Drag and aerodynamics can also play a significant roles in alpine skiing, especially in speed events like the downhill or super G. In these events solo skiers reach speeds of 125 kph, where drag is a major factor in slowing their descent. Between turns smart skiers will tuck, decreasing their frontal area and reducing drag’s effects. Athletes use wind tunnel testing to dial in their tuck position for maximum effect, and, like speedskaters, skiers may also wear special aerodynamic suits. (Photo credits: F. Cofferini/AFP/Getty Images, C. Onerati; h/t to @YvesDubief)
Inksplosion
Chemical Bouillon are a trio of artists who use the chemistry of surface reactions to create abstract videos full of exploding and imploding droplets and colors. As chemicals react, local concentrations at the interface vary, which changes the local surface tension. These gradients drive flow from areas of low surface tension to those of higher surface tension. This is called the Marangoni effect – the same behavior that drives tears in a glass of wine. Chemical Bouillon have a whole YouTube channel dedicated to these kinds of videos, with everything from inks to ferrofluids. Be sure to take a look at some of their other videos and, if you like them, subscribe. (Video credit: Chemical Bouillon)
“Porgrave”
Artist Sandro Bocci uses macro imagery of fluids in his new piece “Porgrave” to create scenes reminiscent of celestial landscapes and the first moments of life. Surface tension, the Marangoni effect, and diffusion create pulsating motion in some frames whereas immiscible liquids form untouchable islands in others. “Porgrave” reminds me of work by Pery Burge and Julia Cuddy as well as sequences from films like 2001 and The Fountain, both of which created some of their effects with macro photography of fluids. Still images from “Porgrave” are available on Bocci’s site. (Video credit and submission: S. Bocci)
ETA: This article originally misprinted the artist’s name as “Sandro Bocchi” and has been updated with the correct spelling.
Hydrophobia
On a recent trip to G.E., the Slow Mo Guys used their high-speed camera to capture some great footage of dyed water on a superhydrophobic surface. Upon impact, the water streams spread outward, flat except for a crownlike rim around the edges. Then, because air trapped between the liquid and the superhydrophobic solid prevents the liquid from wetting the surface, surface tension pulls the water back together. If this were a droplet rather than a stream, it would rebound off the surface at this point. Instead, the jet breaks up into droplets that scatter and skitter across the surface. There’s footage of smaller droplets bouncing and rebounding, too. Superhydrophobic surfaces aren’t the only way to generate this behavior, though; the same rebounding is found for very hot substrates due to the Leidenfrost effect and very cold substrates due to sublimation. As a bonus, the video includes ferrofluids at high-speed, too. (Video credit: The Slow Mo Guys/G.E.)
Impacts on Sand
Granular materials like sand are sometimes very fluid-like in their behaviors. The high-speed video above shows a ball bearing being dropped into packed sand. Many features of the splash are fluid-like; the initial impact creates a spreading crownlike splash, followed by a strong upward jet that eventually collapses back into the medium. At the same time, many of the impact characteristics are decidedly non-fluidic. Sand has no surface tension, so both the crown and the jet readily break up into small particles. The granular jet is very narrow and energetic, reaching heights greater than the impacter’s drop height. Interestingly, the column begins collapsing on its lower end before the jet even reaches its highest peak. This may be due to the lower energy of the sand particles that were ejected later in the crater formation process. (Video credit: J. Verschuur, B. van Capelleveen, R. Lammerink and T. Nguyen)