In this video, a thin film of viscous glycerin sits between two glass plates. As the plates are forced apart, air gets entrained from either side, causing finger-like instabilities to form between the two fluids. This is a result of the Saffman-Taylor mechanism. The final dendritic pattern depends on the fluid viscosities, surface tension, and any non-uniformities in the apparatus. (Video credit and submission by M. Goodman)
Category: Phenomena
Flame Thrower Physics
This high-speed video–which we do not recommend recreating yourself–features burning gasoline flying through the air. In addition to the sheer entertainment value, there are some neat physics. In the first segment, when they kick a tray of gasoline, one can see lovely fiery vortices forming around the backside of the tray as it’s launched. This is the start of the tray’s wake. In the latter half of the video, they launch the flaming gasoline from a bucket. Notice how the flames are in the wake while liquid gasoline streams out ahead without burning. This is because it is primarily gaseous petrol that is flammable. As the liquid fuel breaks up into droplets heated by the burning gasoline vapors nearby, the rest of the fuel changes to a vapor state and catches flame. (Video credit: The Slow Mo Guys; submitted by Will T)
Lenticular Clouds
Lenticular clouds, such as the one shown above, are stationary lens-shaped clouds that form over a mountain or range of mountains. Moist air is deflected up over the mountain, and, if the temperature at higher altitudes is below that of the dew point, the water vapor in the air can condense, forming a cloud that sits over the peak of the mountain. Once the air traverses the mountain and reaches warmer, lower altitudes on the far side, it will often transition back to a gaseous state. Lenticular clouds are sometimes also called UFO clouds, due to their distinctive shape and the way they seem to hover over a peak. (Photo credit: James Woodcock, Billings Gazette via Associated Press)
Perpetual Motion?
In the 17th century, scientist Robert Boyle proposed a perpetual motion machine consisting of a self-filling flask. The concept was that capillary action, which creates the meniscus of liquid seen in containers and is responsible for the flow of water from a tree’s roots upward against gravity, would allow the thin side of the flask to draw fluid up and refill the cup side. In reality, this is not possible because surface tension will hold it in a droplet at the end of the tube rather than letting it fall. In the video above, the hydrostatic equation is used to suggest that the device works with carbonated beverages (it doesn’t; the video’s apparatus has a hidden pump) because the weight of the liquid is much greater than that of the foam. Of course, the hydrostatic equation doesn’t apply to a flowing liquid! The closest one can come to the hypothetical perpetual fluid motion suggested by Boyle is the superfluid fountain, which flows without viscosity and can continue indefinitely so long as the superfluid state is maintained. (Video credit: Visual Education Project; submission by zible)
Merry Christmas
[original media no longer available]
Sit back, relax, and enjoy some science-y goodness with Bill Nye as he explains fluids. Happy holidays, everyone!
Santa and the Egg
[original media no longer available]
If I were Santa–or the egg in this video–I don’t think I’d particularly like getting sucked through a chimney in this fashion. I wonder if Santa re-kindles the fire and tries to increase air pressure in the house relative to the outside in order to get back out the chimney. (Video credit: Hooked on Science)
Pancake Vortex
In large-scale geophysical flows, rotation and density gradients often play major roles in the structures that form. Here the UCLA SPINLab demonstrates how large, essentially flat vortices–pancake vortices–form in rotating, stratified fluids. The stratification, in this case, is due to the density difference between salt water and fresh water; salt water is denser and therefore less buoyant, so it sinks toward the bottom of the tank. Note how the pancake vortex only forms when the fluid is both stratified and rotating. If it lacks one of the two, the structures will be very different. (Video credit: O. Aubert et al./SPINLab UCLA)
Freezing Bubbles
If you find yourself some place really cold this holiday season, may I suggest stepping outside and having some fun freezing soap bubbles? The crystal growth is quite lovely, as seen in this photograph. If you live in warmer climes, fear not, you can always experiment in your freezer. It would be particularly fun, I think, to see how a half-bubble sitting on a cold plate freezes in comparison to a droplet like this one. (Video credit: Mount Washington Observatory)
Tears of Wine
Physicist Richard Feynman once famously ended a lecture by describing how the whole universe can be found in a glass of wine. And there is certainly plenty of fluid dynamics in one. In the photo above, we see in the shadows how a film of wine drips down into the main pool below. This effect is known by many names, including tears of wine and wine legs; it can also be found in other high alcohol content beverages. Several effects are at play. Capillary action, the same effect that allows plants to draw water up from their roots, helps the wine flow up the wall of the glass. At the same time, the alcohol in this wine film evaporates faster than the water, raising the surface tension of the wine film relative to the main pool of wine below. Because of this gradient in surface tension, the wine will tend to flow up the walls of the glass away from the area of lower surface tension. This Marangoni effect also helps draw the wine upward. When the weight of the wine film is too great for capillary action and surface tension to hold it in place, droplets of wine–the legs themselves–flow back downward. (Photo credit: Greg Emel)
Underwater Gunfire
When a projectile is fired from a gun or other firearm, it is propelled by the expansion of high-temperature, high-pressure gases resulting from the combustion of a propellant, like gunpowder, inside the weapon. The explosive expansion of these gases transfers momentum to the bullet; however, the gases will continue to expand outward from the gun even after the bullet is fired. They do so in the form of a supersonic blast wave; it’s this blast wave that’s responsible for the noise of the firearm. Firing a gun underwater is one way to see the blast wave, though it is far from the only way. In fact, a blast wave viewed underwater is not equivalent to one in air. The differences in density and compressibility between the two fluids mean that, while the general form may be similar, the specifics and the results may not be. In general, a blast wave underwater is much more damaging than one in air. (Video credit: destinsw2/Smarter Every Day; requested by nikhilism)
