Shear-thickening non-Newtonian fluids like oobleck become more viscous as force is applied to them. This behavior causes them to form finger-like structures when vibrated, makes it good liquid armor, and even enables people to run across a pool of it without sinking. Now undergraduates at Case Western Reserve University have found a new use for such fluids: pothole filling. They have created a pothole patch that consists of a waterproof bag filled with a dry solution that, when mixed with water, creates a non-Newtonian fluid capable of flowing to take the shape of the pothole but resisting a car tire like a solid. They cover the patch with a layer of black fabric so that drivers don’t avoid the patch. See the video above for a demonstration and ScienceNOW for more. (submitted by aggieastronaut)
Month: April 2012
Vibrating Oil
This high-speed video shows the behavior of oil on a vibrating surface. As the amplitude of the vibration is altered various behaviors can be observed. Initially small waves appear on the surface of the oil, then the surface erupts into a mass of jets and ejected droplets, reminiscent of a vibrated interfaces within a prism or vibration-induced atomization. When the amplitude is reduced after about half a minute, we see Faraday waves across the surface, as well as tiny droplets that bounce and skitter across the surface. They are kept from coalescing by a thin layer of air trapped between the droplet and the oil pool below. Because of the vibration, the air layer is continuously refreshed, keeping the droplet aloft until its kinetic energy is large enough that it impacts the surface of the oil and gets swallowed up.
Jumping Water Droplets
Superhydrophobic surfaces resist wetting from water, but it turns out they can also trigger interesting behaviors in the tiny droplets condensing on the surface. High-speed video reveals that when two condensate droplets coalesce, the energy released by surface tension causes the new droplet to jump off the surface. The phenomenon is the same as one observed in some types of mushroom–when a condensate droplet touches a wetted spore, the spore is ejected from the mushroom. (Video credit: J. Boreyko)
Frozen Fluid Illusion
This video creates the illusion of a jet of water frozen in mid-air. The effect is achieved by vibrating the water at the frequency of the speaker, then filming at a frame rate identical to the vibrational frequency. Thus the water pulses at the exact rate that the camera captures images, making the water appear stationary even though it is moving. (submitted by Simon H)
Dancing Plasma
Two dark areas of plasma, cooler than the surrounding fluid, dance and intertwine above the sun’s surface. Plasma, a rarefied gas made up of ions, is an electrically conductive fluid, shaped here by the magnetic field of the sun. Note how the strands pass material back and forth along the magnetic field lines. This timelapse video, captured by NASA’s Solar Dynamics Observatory, takes place over the course of a day and is captured in the extreme ultraviolet range.
Rogue Wave Recreated
For years, mariners have reported occurrences of rogue waves–sudden, isolated waves many times larger than the surrounding surface waves. Until 1995, when a rogue wave was first measured, debate raged as to whether such waves even existed. Scientists have since agreed that nonlinear models of wave interaction are the most likely source of the amplification necessary to create rogue waves. Since the Navier-Stokes equations that govern hydrodynamics are so difficult to solve, scientists have looked to simpler nonlinear wave equations, like the nonlinear Schroedinger equation that governs optics, to generate rogue-wave-like behavior. While the equation gives insight into how a given wave system will evolve, it is still necessary to determine what initial conditions can lead to the formation of a rogue wave. All manner of random conditions exist in the ocean, but to recreate the behavior in a simplified system, we must know which initial conditions are the right ones. Akhmediev et al presented a theoretical perspective on the initial conditions that might lead to rogue wave amplification, and now, for the first time, researchers have been able to create a rogue wave in a wave tank. That little blip that sinks the Lego pirate ship is a great accomplishment toward understanding a phenomenon whose very existence was in question less than twenty years ago. (Video credit: A Chabchoub, N Hoffmann, and N Akhmediev; via Gizmodo; for more, see APS Viewpoints and Akhmediev et al)
Volcanic Clouds
The volcano Tungurahua erupts in a cloud of ash while molten lava flows down the mountain’s sides. Overhead a wispy lenticular cloud has formed where moist air flowing over the volcano dropped below its dew point. Volcanic eruptions have been known to produce shock waves and vortex rings as well as their distinctive turbulent plumes. (submitted by A. Jones III)
Brine Shrimp Swimming
For small creatures, swimming is dominated by viscosity. Here researchers use particle image velocimetry (PIV) to explore the flow field around brine shrimp. Its motion is divided into two vorticity-generating phases–the wide power stroke where the shrimp generates most of its forward motion and the recovery stroke where the shrimp returns its starting position while generating as little motion and drag as it can. (Video credit: B. Johnson, D. Garrity, L. Dasi)
Granular Eruptions
Granular flows, which are made up of loose particles like sand, often display remarkably fluid-like behavior. Here researchers explore the behavior of granular flows when a solid impacts them at high speed. The sand, unlike a fluid, does not have surface tension, yet we still observe many of the same behaviors. Like a fluid, the sand splashes and creates cavities and jets as it deforms around the fallen object. The sand even “erupts” as submerged pockets of air make their way back to the surface.
Tornadogenesis
Tornadogenesis–the formation of tornadoes–remains a topic of active research as there is relatively little direct experimental data, owing to the difficulty of prediction as well as measurement. Initially, a variation of wind speed at different altitudes in the atmosphere causes shearing, which can lead to the formation of a horizontal column of rotating air–a vortex line similar to a roll cloud. Beneath a developing storm, the updraft of warm local air can pull this vortex line upwards, creating vertical rotation in the cloud, thereby birthing a supercell. Supercells do not always spawn tornadoes, and the exact causes that result in tornadic or nontornadic supercells are not fully understood. However, the formation of tornadoes within the supercell seems dependent on the downdraft of cool air within the storm as well as stretching of the vortex line, which increases its rate of rotation. For more information, check out this explanatory video and some of the talks by Paul Markowski. (Thanks to mindscrib, aggieastronaut and others for their submissions related to this topic! Photo credits: P. Markowski and D. Zaras)
