Tag: science

Reader Question: Snow from Boiling Water?
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Reader kylewpppd asks:
Have you seen the post of a man in Siberia throwing boiling water off of his balcony? Can you provide a better explanation of what’s going on?
As you can see in the video (and in many similar examples on YouTube), tossing near boiling water into extremely cold air results in an instant snowstorm. Several effects are going on here. The first thing to understand is how heat is transferred between objects or fluids of differing temperatures. The rate at which heat is transferred depends on the temperature difference between the air and the water; the larger that temperature difference is the faster heat is transferred. However, as that temperature difference decreases, so does the rate of heat transfer. So even though hot water will initially lose heat very quickly to its surroundings, water that is initially cold will still reach equilibrium with the cold air faster. Therefore, all things being equal, hot water does not freeze faster than cold water, as one might suspect from the video.
The key to the hot water’s fast-freeze here is not just the large temperature difference, though. It’s the fact that the water is being tossed. When the water leaves the pot, it tends to break up into droplets, which quickly increases the surface area exposed to the cold air, and the rate of heat transfer depends on surface area as well! A smaller droplet will also freeze much more quickly than a larger droplet.
What would happen if room temperature water were used instead of boiling water? In all likelihood, a big cold bunch of water would hit the ground. Why? It turns out that both the viscosity and the surface tension of water decrease with increasing temperature. This means that a pot of hot water will tend to break into smaller droplets when tossed than the cold water would. Smaller droplets means less mass to freeze per droplet and a larger surface area (adding up all the surface area of all the droplets) exposed. Hence, faster freezing!
December 21, 2012
Laminar Fountain
In the midst of holiday travels, take a moment (particularly if you’re flying through Detroit) to enjoy the simple beauty of WET Design’s fountain in the McNamara Terminal. Laminar jets arc through the air almost like perfect crystalline columns of fluid. Watch closely and you’ll see a few wavy variations–like a Plateau-Rayleigh instability creeping in–but there will be no turbulence to distress passengers and passers-by. (Video credit: WET Design)
December 20, 2012
Airborne Aerosols
This numerical simulation from NASA Goddard shows the motion of particulates in Earth’s atmosphere between August 2006 and April 2007. These aerosols come from various sources including smoke, soot, dust, and sea salt. As these fine particles move through atmosphere, they can have significant effects on weather as well as climate. For example, the particles serve as nucleation sites for the condensation and formation of rain drops. (Video credit: NASA Goddard SFC)
December 19, 2012
Ferrofluid Sculptures
Artist Sachiko Kodama is known for her mesmerizing ferrofluid sculptures. Ferrofluids are a colloidal liquid consisting of nanoscale ferromagnetic particles and a carrier fluid such as water or oil. They can react strongly to magnetic fields, forming spikes, brain-like whorls, and even labyrinths. (Photo credits: Sachiko Kodama; via freshphotons)
December 18, 2012
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)
December 17, 2012
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)
December 14, 2012
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)
December 13, 2012
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)
December 12, 2012
Bouncing Jet
For the right flow speeds and incidence angles, a jet of Newtonian fluid can bounce off the surface of a bath of the same fluid. This is shown in the photo above with a laser incorporated in the jet to show its integrity throughout the bounce. The walls of the jet direct the laser much the way an optical fiber does. The jet stays separated from the bath by a thin layer of air, which is constantly replenished by the air being entrained by the flowing jet. The rebound is a result of the surface tension of the bath providing force for the bounce. (Photo credit: T. Lockhart et al.)
December 11, 2012
Stirring Faces
This video features simulation of the laminar flow around a plate plunging sinusoidally in a quiescent flow. As the plate moves up and down, it mixes the fluid around it. This is visualized in several ways, beginning with the vorticity. Clockwise and anti-clockwise vortices are shed by the edges of the plate as it moves. The flow is also visualized using particle trajectories, which are classified by their tendency to accumulate (attract) or lose (repel) particles. These trajectories are particularly intriguing to watch develop as they appear to show ornate faces and designs. (Video credit: S. L. Brunton and C. W. Rowley)
December 10, 2012