For a little Friday fun, enjoy this timelapse of magnetic putty consuming magnets. Really this is a bit of slow-motion magnetohydrodynamics. The magnet’s field exerts a force on the iron-containing putty, which, because it is a fluid, cannot resist deformation under a force. As a result, the putty will flow around the magnet, eventually coming to a stop once it reaches equilibrium, with its iron equally distributed around the magnet. Assuming the putty is homogeneously ferrous (i.e. the iron is mixed equally in the putty), that means the putty will stop moving when the magnet is at its center of mass. (Video credit: J. Shanks; submitted by Neil K.)
Videos
Watching the Boundary Layer Go By
In experiments, it can be difficult to track individual fluid structures as they flow downstream. Here researchers capture this spatial development by towing a 5-meter flat plate past a stationary camera while visualizing the boundary layer – the area close to the plate. The result is that we see turbulent eddies evolving as they advect downstream. Despite the complicated and seemingly chaotic flow field, the eye is able to pick out patterns and structure, like the merging of vortices that lifts eddies up into turbulent bulges and the entrainment of freestream fluid into the boundary layer as the eddies turn over or collapse. It is also a great demonstration of how the Reynolds number relates to the separation of scales in a turbulent flow. Notice how much richer the variety of length-scale is for the higher Reynolds number case and how thoroughly this mixes the boundary layer. (Video credit: J. H. Lee et al.)
Lifting Liquids
At very small scales, the interaction of solids and liquids is governed by molecular forces. Here researchers demonstrate how carbon nanowires of only a few nanometers in diameter draw liquid up in a film or bead when inserted in a pool. Capillary action is the name we give this gravity-defying force generated between the liquid and solid molecules. Although this behavior was predicted theoretically, it had not been previously observed at this scale due to the need for electron microscopy. Such microscopes require a vacuum, which boils off almost any liquid instantaneously. Researchers used a special fluid that remained in a liquid state even under near-vacuum pressures in order to make these observations. (Video credit: J. Li et al/MIT News; submitted by 20percentvitaminc)
Wake Vortices at Night
The ends of an airplane’s wings generate vortices that stretch back in the wake of the plane. Most of the time these vortices are invisible, even if their effects on lift are distinctive. Here an A-340 coming in for a foggy landing demonstrates the size and strength of these vortices. Notice how the fog gets swept up and away by the vortices. Pilots will sometimes use this effect to their advantage in clearing a runway of fog by making repeated low-passes to clear the fog before landing. (Video credit: A. Ruesch; submitted by Jens F.)
Tears of Wine
Wine drinkers may be familiar with the “tears of wine” often seen on the wall’s of a glass. The effect is a combination of evaporation and surface tension. As the low-surface-tension alcohol evaporates from the wine film left by swirling the glass, the higher local surface tension draws wine up the walls of the glass. Eventually enough wine gathers that droplets form and slide back down. This timelapse video shows how the beads form and move, almost dancing around the glass. The video’s author, Dan Quinn, has a second video with an awesome visual explanation of the behavior that’s well worth watching, too! (Video credit and submission: D. Quinn)
Entering a Viscous Liquid
When a solid object impacts on a liquid a cavity typically forms, entraining air into the pool. But this behavior varies widely according to the surface of the solid as well as the fluid’s properties. This video shows a sphere impacting a highly viscous liquid. The sphere stops shortly after impact while the cavity continues expanding in its wake. With a fluid like water, a long and thin cavity will typically pinch off before the object is decelerated, causing bubbles to form. No such behavior here. Instead the wide cavity pinches off at the surface of the motionless sphere and begins its rebound upwards. It even appears to pull the sphere partially back towards the surface! (Video credit: A. Le Goff et al.)
Internal Wave Demo
This video has a fun and simple demonstration of the importance of fluid density in buoyancy and stratification. Fresh water (red) and salt water (blue) are released together into a small tank. Being lighter and less dense, the red water settles on top of the blue water, though some internal waves muddy their interface. After the water settles, a gate is placed between them once more and one side is thoroughly mixed to create a third fluid density (purple), which, when released, settles between the red and blue layers. In addition to displaying buoyancy, this demo does a great job ofaa showing the internal waves that can occur within a fluid, especially one of varying density like the ocean. (Video credit: UVic Climate Modeling Group)
Washing Your Face in Space
What happens to a wet washcloth when wrung out in space? Astronaut Chris Hadfield answers this question from students with a demonstration. Without gravity to pull the water downward, surface tension effects dominate and the wrung cloth forms a tube of water around it. Surface tension and capillary action draw the fluid up and onto Hadfield’s hands as long as he holds the cloth. After he lets go, we see that the water remaining around the cloth soaks back in (again due to capillary action) and the wet, twisted washcloth simply floats without releasing water or relaxing its shape. While pretty much what I would have expected, this was a very cool result to see! (Video credit: C. Hadfield/CSA; submitted by Bobby E)
Explosions Underwater
Underwater explosions are, in general, much more dangerous than those in air. This video shows an underwater blast at 30,000 fps. During the initial blast, a hot sphere of gas expands outward in a shock wave. In air, some of the energy of this pressure wave would be dissipated by compressing the air. Since water is incompressible, however, the blast instead moves water aside as the bubble expands. Eventually, the bubble expands to the point where its pressure is less than that of the water around it, which causes the bubble to collapse. But the collapse increases the gas pressure once more, kicking off a series of expansions and collapses. Each bubble contains less energy than the previous, thanks to the loss of pushing the water aside. (Video credit: K. Kitagawa)
Shocking Droplets
Typical liquid drops will break apart into long, stretched ligaments and a spray of tiny droplets when deformed. But with just a small addition of polymers, these same liquids become viscoelastic and capable of some pretty incredible behaviors. This video shows a viscoelastic drop being struck by a shock wave that passes from right to left. The droplet is smashed and deformed, then stretches into jellyfish-like sheet of liquid. But incredibly, the elastic forces in the droplet are enough to hold it together. Researchers are interested in understanding these behaviors for many applications, including preventing accidental explosions caused by explosive fuels atomizing in air. (Video credit: T. Theofanous et al.)