The brilliant colors of a soap film are directly related to the film’s thickness. Black regions, like the one in the upper right of this image, are the thinnest regions and may be less than 100 nanometers thick. (That’s smaller than the shortest wavelength of visible light!) The colors of the peacock-feather-like blooms along the bottom of the image demonstrate significant variations in film thickness. This is caused by uneven concentrations of surfactants in the film. The variations in concentration causes differences in local surface tension, which in turn moves fluid around within the film. This is known as a Marangoni effect. (Image credit: S. Berg and S. Troian)
Category: Phenomena
Jovian Belts and Zones
Jupiter’s colorful cloud bands alternate between dark belts and light zones. The bands mark convection cells in Jupiter’s atmosphere, and, like on Earth, powerful jet streams form due to this atmospheric heating and the planet’s rotation. The jet winds can even move in opposite directions, creating strong shear forces between neighboring cloud bands. The shear helps drive Kelvin-Helmholtz instabilities in the clouds, resulting in the regularly spaced waves and vortices seen along the edges of some bands. (Image credit: NASA/ESA; via APOD)
Cream in Coffee
Pouring cream in coffee produces some of the most mesmerizing displays of fluid dynamics. The density difference between the two fluids sets up Rayleigh-Taylor instabilities that mushroom out and help create the turbulence that eventually mixes the drink. You can learn more about Rayleigh-Taylor instabilities in this FYFD video, and, if you need more awesome caffeine-filled examples of fluids, check out the coffee dynamics blog. (Video credit: S. Geraldine and L. Kang)
Waves Over the Rockies
These spectacular wave-like clouds are the result of the Kelvin-Helmholtz instability. When two layers of air move past one another at different velocities, an unstable shear layer forms at their interface. Disturbances in this shear layer grow exponentially, creating these short-lived overturning waves that quickly turn turbulent. The strong resemblance of these clouds to breaking ocean waves is no coincidence–the Kelvin-Helmholtz instability occurring between the wind and water is what generates many ocean waves. Kelvin-Helmholtz patterns are also common on other planets, like Jupiter, Saturn, and Mars. (Image credit: Breckenridge Resort; submitted by jshoer)
Re-Entry
Atmospheric re-entry subjects vehicles to extreme conditions. At high Mach numbers, the leading shock wave compresses the air so strongly that it reaches temperatures hotter than the surface of the sun. At these temperatures, oxygen and nitrogen molecules in the air dissociate, bathing a vehicle in a plasma of ionized gas molecules. Often these atoms chemically react with the surface materials of a vehicle causing ablation that removes mass from the vehicle while helping protect the vehicle substructure from re-entry heating. Tests in specialized ground facilities like arc-jet plasma tunnels are necessary to develop thermal protection systems capable of shielding a vehicle during hypersonic flight. (Image credit: D. Ponseggi/NASA)
Glow-Stick Ferrofluids
Ferrofluids create all kinds of fascinating shapes when exposed to magnetic fields. In this video, Dianna from Physics Girl shows off what happens when you combine a ferrofluid with glowsticks and explains how ferrofluids get some of their unique properties. Ferrofluids consist of tiny nanoparticles of magnetic material that are surrounded by surfactants and suspended in a carrier fluid. This creates a fluid whose shape depends on gravity, surface tension, and the local magnetic field. By manipulating the relative strength of these forces, you can create everything from spikes to maze-like patterns to whatever this is. (Video credit and submission: Physics Girl)
Mammatus Clouds
Mammatus clouds, the bubble-shaped protrusions sometimes seen underneath cumulus clouds, are a rare and dramatic type of cloud. The mammatus is typically short-lived, with lobes lasting only 10 minutes or so. Their rarity and short appearances are among the reasons why this cloud type has been little studied. As a result, there are many theories as to how the clouds form their distinctive, bulbous lobes, but, to my knowledge, there is no single widely accepted explanation. Mammatus often appear before or after severe thunderstorms and are associated with strong turbulence, so this may play a factor in their formation. (Photo credit: C. Lindsey; via APOD)
How Plants Move
Though most plants don’t move at speeds that we humans notice, many plants are remarkably active, as seen in the timelapse animations above. Much of this motion is driven by water flow inside the plant. The two plants above are phototropic–they move in response to light. The motion is actuated via a specialized motor cell called the pulvinus, which is located at the base of the leaf where it meets the stem. Unlike animal cells, plant cells have stiff outer walls that allow them to maintain an internal pressure–or turgor pressure–that differs from the outside environment. In fact, it’s not unusual for a plant’s cell to hold a pressure equivalent to 5 atmospheres! The plant manipulates this turgor pressure by controlling the transport of ions across cell membranes. Pump more ions into a cell, and osmosis will cause water to flow into the area of high solute (ion) concentration. This causes the cell to swell and raises the turgor pressure, resulting in the plant’s leaf moving. (Image credit: L. Miller and A. Hoover, source; additional research credit: J. Dumais and Y. Forterre)
Early Rocket Launch
Pre-dawn launches provide some of the most dramatic rocket footage. This video is from an October 2nd Atlas V launch, and the really fun stuff starts at about 0:34. As the rocket climbs to higher altitudes, the atmospheric pressure around it decreases. As a result of this low pressure, the rocket’s exhaust gases balloon outward in a giant plume many times larger than the rocket. This happens in every launch, but it’s visible here because the rocket is at such a high altitude that its exhaust is being lit by sunlight while the observers on the ground are still in the dark. The ice crystals in the exhaust–much of the rocket’s exhaust is water vapor–reflect sunlight down to the earth. Around 0:47, a cascade of shock waves ripples through the plume just before the first-stage’s main engine cuts off. Once the engine stops firing, there’s no more exhaust and the plume ends. (Video credit: Tampa Bay Fox 13 News; submitted by Kyle C)
Shock Diamonds
Rocket engine exhaust often contains a distinctive pattern known as shock diamonds or Mach diamonds. These are a series of shock waves and expansion fans that increase and decrease, respectively, the supersonic exhaust gases’ pressure until it equalizes with atmospheric pressure. The bright glowing spots visible to the naked eye are caused by excess fuel in the exhaust igniting. As awesome as shock diamonds look, they’re actually an indication of inefficiencies in the rocket: first, because the exhaust is over- or underexpanded, and second, because combustion inside the engine is incomplete. Both factors reduce a rocket engine’s efficiency (and both are, to some extent, inescapable). (Photo credit: XCOR)
