A superheated liquid can reach temperatures higher than its boiling point without actually boiling – similar to how liquids can be supercooled below their freezing point without solidifying. The photo sequence above shows how explosive the boiling of a superheated water droplet submersed in sunflower oil can be. Image (a) in the lower left shows the superheated droplet resting on the bottom of its container. Then droplet vaporizes explosively in (b), expanding dramatically. The bubble overexpands and and begins to oscillate around its equilibrium radius. This triggers a Rayleigh-Taylor instability in the bubble’s interface, creating the large lobes in © and enlarged in the upper image. Finally, the bubble fragments in (d). See the original paper for more on superheated droplet boiling. (Image credit: M. A. J. van Limbeek et al.; via @AIP_Publishing)
Tag: fluid dynamics
Fluids Round-up – 20 October 2013
Some very cool fluids applications in this week’s fluids round-up. On to the links!
- Like many colleges, MIT has campus myths about those unbelievably windy spots. But, unlike many others, they have a CFD analysis deconstructing the myths.
- Even rocks can behave like fluids sometimes. Check out this article from @s_i_r_h_c on fluid instabilities left behind in rocks.
- Reader Julian de Charentenay demonstrates some DIY aerodynamic analysis on Pixar’s Lightning McQueen. One of the neat features here is using photos of an object to construct a 3D model, a technique I used in my own research at one point.
- Physics.org explains why teapots drip.
- Phys.org reviews a paper suggesting that fluid dynamics influenced the evolution of lung structure.
- io9 discusses new research on how the brain gets rid of waste products, which includes experiments with flow visualization in mice brains.
- Finally, our lead image shows the airship USS Los Angeles moored to the USS Patoka and comes from The Atlantic’s In Focus series on airships past and present.
ETA: I somehow forgot to include the first of the upcoming APS presentations to get wide media recognition: Law of Urination, which has shown up all over the place.
(Photo credit: San Diego Air and Space Museum Archive/In Focus)
The Cheerios Effect and Tiny Swimmers
Anyone who has eaten a bowl of Cheerios is familiar with the way solid objects floating on a liquid surface will congregate. This is a form of capillary force driven by the wetting of the particles, surface tension, and buoyancy. Using ferromagnetic particles and a vertical magnetic field, one can balance capillary action and lock the particles into a fixed configuration relative to one another. By adding a second, oscillating magnetic field, it’s possible to make the beads dance and swim together. Like all of this week’s videos, this video is an entry in the 2013 Gallery of Fluid Motion. (Video credit: M. Hubert et al.)
Overflowing Foam
Hitting a glass bottle full of a non-carbonated drink can shatter the bottle due to cavitation, but doing the same with a carbonated beverage can make the bottle overflow with foam. The video above breaks down the physics of this bar prank. It all begins with nucleation and the tiny bubbles of carbon dioxide that form in the liquid. Striking the top of the bottle generates a compression wave that travels through the liquid, shrinking bubbles as it passes. When it hits the bottom of the bottle, it gets reflected as an expansion wave that expands the bubbles. This reflection happens several times between the free surface of the liquid and the bottom of the bottle. The rapid collapse-and-expansion of the bubbles makes them implode into a cloud of tinier bubbles that expands until the local supply of carbon dioxide is used up. At this point, the buoyancy of the bubbles carries them upward in plumes, creating more bubbles with the dissolved carbon dioxide nearby. And, all of a sudden, you’ve got foam everywhere. Like all of this week’s videos, this video is an entry in the 2013 Gallery of Fluid Motion. (Video credit: J. Rodriguez-Rodriguez et al.)
Self-Propelled Droplets
Leidenfrost drops hover and move above hot surfaces on a thin layer of their own vapor. Over a flat surface, this vapor flows radially out from under the droplet, but creating rachets in the surface forces the vapor to flow in a single direction. The vapor then acts like exhaust, generating propulsion in the droplet and making it roll. How quickly the drop moves depends both on the droplet’s size and the rachets’ aspect ratio. For a given length, deeper rachets propel a drop faster than their shallower counterparts. The droplet’s size also affects the thrust with different scalings depending on the drop’s initial size. Like all of this week’s videos, this video is an entry in the 2013 Gallery of Fluid Motion. (Video credit: A. G. Marin et al.)
Shaping and Levitating Droplets
Opposing ultrasonic speakers can be used to trap and levitate droplets against gravity using acoustic pressure. Changes to field strength can do things like bring separate objects together or flatten droplets. The squished shape of the droplet is the result of a balance between acoustic pressure trying to flatten the drop and surface tension, which tries to pull the drop into a sphere. If the acoustic field strength changes with a frequency that is a harmonic of the drop’s resonant frequency, the drop will oscillate in a star-like shape dependent on the harmonic. The video above demonstrates this for many harmonic frequencies. It also shows how alterations to the drop’s surface tension (by adding water at 2:19) can trigger the instability. Finally, if the field strength is increased even further, the drop’s behavior becomes chaotic as the acoustic pressure overwhelms surface tension’s ability to hold the drop together. Like all of this week’s videos, this video is a submission to the 2103 Gallery of Fluid Motion. (Video credit: W. Ran and S. Fredericks)
Fluid Juggling
It’s that time of the year – the 2013 APS Division of Fluid Dynamics meeting is not far off, and entries to this year’s Gallery of Fluid Motion are starting to appear. This week we’ll be taking a look at some of the early video submissions, beginning with one that you can recreate at home. This video demonstrates a neat interaction between a slightly-inclined liquid jet and a lightweight ball. The jet can stably support–or, as the authors suggest, juggle–the ball under many circumstances, as seen in the video. Initially, the jet impacts near the bottom of the ball and then spreads into a thin film over the surface. This decrease in thickness between the jet and the film is accompanied by an increase in speed due to conservation of mass. That velocity increase in the film corresponds to a pressure decrease because of Bernoulli’s principle. This means that there is a region of higher pressure where the jet impacts the ball and lower pressure where the film flows around the ball. Just as with airflow over an airfoil, this generates a lift force that holds the ball aloft. (Video credit: E. Soto and R. Zenit)
Fluids Round-up – 13 October 2013
There were so many good fluids links this week that I decided for an off-week fluids round-up. Here we go!
- Jefferson Lab has a cool demo on how to make a cloud chamber using dry ice, isopropyl alcohol, and a radioactive source. There is all kinds of fun physics to explain in this one!
- io9 has a great article and videos on the efficiency of jellyfish propulsion (spoiler alert: there are vortex rings).
- Half-blimp, half-jet transport option could change shipping landscape. In a similar vein, Jalopnik takes a look back at the golden age of the dirigible.
- For the armchair daredevils, check out this 360-degree view of BASE jumping with a wingsuit off a Swiss mountain. (via Janeen M)
- Also from io9, an article on my favorite fluids demo: reversible laminar flow. If you’d like to try this at home, here’s a DIY version.
- National Geographic talks about the differences between hurricanes, cyclones, and typhoons.
- Finally, our lead video comes from #5facts and Sesame Street’s Grover who bring us 5 DIY science experiments, 4 of which are fluids demos. Sit back and enjoy!
(Video credit: #5facts/Sesame Street)
Lakes Upon Glaciers
Supraglacial lakes–ephemeral bodies of water that form atop glaciers–can form and empty in a matter of hours. The lakes typically empty either by overflowing their banks or by discharging through a moulin, a well-like crevasse in the ice. When this happens, the water from the lake drains into the bed beneath the glacier, acting like a lubricant between the ice and the land and thus accelerating the glacier’s movement. The team in the video studied the draining of two different lakes, one which voided within 2 hours and the other slower one which drained over 45 hours. The faster of the two accelerated the glacier’s movement to a maximum of 1600 meters/year, far higher than its baseline velocity of 90-100 meters/year. For more see Laboratory Equipment and this post on ice flow. (Video credit: City College of New York)
Schlieren in Flight
Schlieren photography is a common method of visualizing shock waves in wind tunnel experiments, but it’s much harder to pull off for aircraft in the sky. This video from NASA shows off some stunning work out of NASA Dryden capturing schlieren video of shock waves from a F-15B aircraft at Mach 1.38. You’ll notice that shock waves extend off the nose, wings, tail, and other parts of the airplane and extend well beyond the camera’s field of view. It’s these shock waves hitting the ground level that causes distinctive sonic booms. These tests are part of NASA’s on-going research into minimizing the effects of sonic boom so that civilian supersonic flight over land is feasible in the future. When the U.S. government shutdown ends, you’ll be able to learn more about this work at NASA Dryden’s GASPS page. (Video credit: NASA Dryden)
