When working at the microscale, engineering structures like those used for drug delivery systems requires ingenuity. Since it isn’t possible to manipulate particles manually, researchers harness physical effects to do the work for them. Here a droplet filled with millions of polystyrene microparticles sits on a hydrophobic surface, which helps keep the drop’s spherical shape. As the drop evaporates, surface tension and internal flow in the drop help the microparticles self-assemble into a microscopic soccer-ball-like shape. (Video credit: A. Marin et al.; submission by A. Marin)
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Magnetic Putty
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.)
Reader Question: Energy from Whirlpools?
shiftymctwizz asks:
So I just read your post about vortices, and now I’m wondering if we could build structures similar to the Corryvreckan and put turbines in them for energy production? Would it be any more efficient than hydroelectric dams? Are you the right person to ask?
I can’t give you numbers off the top of my head, but I suspect that your typical hydroelectric dam will be more reliable if not more efficient. The trouble with things like the Corryvreckan, aside from the randomness of where the vortices pop up, is that they aren’t there every single day the way, say, Niagara Falls is.
That said, there is on-going work to effectively harness ocean waves for power, with ideas like buoy generators or sea snake generators. As with most concepts one of the difficulties in implementation is determining a safe and efficient manner to transmit the electricity generated from these offshore sites (we’re generally talking miles from shore) to where it’s needed. This problem is often similarly faced by solar and wind energy producers. There are already wave farms in place around the world, though, and it’s a promising field of renewable energy. (Photo credit: Wikimedia)
Real-Life Whirlpools
Literature is full of descriptions of monstrous whirlpools like Charybdis, which threatens Homer’s Odysseus. While it’s not unusual to see a small free vortex in bodies of water, most people would chalk boat-swallowing maelstroms up to literary device. But it turns out that, while there may not be permanent Hollywood-style whirlpools, there are several places in the world where the local tides, currents, and topology combine to produce turbulence, dangerously vortical waters, and even standing vortices on a regular basis.
One example is the Corryvreckan, between the islands of Jura and Scarba off Scotland. In this narrow strait, Atlantic currents are funneled down a deep hole and then thrust upward by a pinnacle of rock that rises some 170 m to only 30 m below the surface. The swift waters and unusual topology produce strong turbulence near the surface and whirlpools pop up throughout the strait. Other “permanent” maelstroms, such as those in Norway and Japan, arise from tidal interactions with similar structures rising from the sea floor.
For more, check out this Smithsonian article, Gjevik et al., Moe et al., and the videos linked above! (Photo credits: Manipula, Tokushima Gov’t, Wikimedia, and W. Baxter; requested by @kb8s)
SpaceShipTwo Lights It Up
Monday morning Virgin Galactic and their partners at Scaled Composites reached a new milestone in their commercial sub-orbital spaceflight program, firing SpaceShipTwo’s main engine for the first time and accelerating to supersonic speeds. The upper image shows hints of Mach diamonds, formed by a series of shock waves and expansions, in its exhaust. This is very common for rockets since most have a fixed geometry, and, by extension, a fixed Mach number and exhaust pressure. (Photo credits: Virgin Galactic and Mars Scientific)
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.)
Turning Sound Into Imagery
The acoustic signatures of many animals contain features we humans cannot appreciate, given the limited range of frequencies we can hear. In fluid dynamics and many other fields, scientists and engineers have to find ways to analyze and decompose time-series data–like acoustic pressure signals–into useful quantities. Mark Fischer uses one tool for such analysis, a wavelet transform, to turn the calls of whales, birds, and insects into the colorful snapshots seen here. Wavelet transforms are somewhat similar to Fourier transforms but represent a signal with a series of wavelets rather than sinusoids. They’re also widely used for data compression. (Image credits: M. Fischer/Aguasonic Acoustics; via DailyMail)
Liquid Sculptures
Water droplet art celebrates the infinite forms created from the impact of drops with a pool and rebounding jets. It’s a still life captured from split second interactions between inertia, momentum, and surface tension. These examples from photographer Markus Reugels are among some of the most complex shapes I’ve seen captured. Be sure to check out his website for more beautiful examples of liquids frozen in time. (Photo credits: Markus Reugels; via Photigy)
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.)
Cracks in Sea Ice
Arctic sea ice often appears as a single extensive sheet when, in reality, it is made up of many smaller sections of ice shifting and grinding against one another under the influence of winds and ocean currents. This can cause cracks–known as leads–to open up between sections of the ice. This animation, constructed from infrared satellite images, shows the growth of several cracks, leading to extensive break-up of the ice sheet from late-January through March. The fracturing was driven by a high-pressure system that parked over the region, bringing warmer temperatures and southwesterly winds that fueled the Beaufort Gyre, a large-scale, wind-driven, clockwise circulation in the sea that helped pull the ice apart. For more, see NASA EO’s explanation. (Video credit: NASA Earth Observatory)
