The regular hexagonal structure of honeycomb may owe more to fluid dynamics than the careful engineering of the bees that build it. Observations indicate that honeycomb cells start out circular and become hexagonal as the bees continue building. Both experiments and models show that an array of circular cells can transform into hexagons due to surface tension driving flow at the junctions where the three cell walls meet. But for the wax to flow, it has to be warm–about 45 degrees Celsius compared to the hive’s ambient temperature of 25 degrees. The researchers suggest that the worker bees constructing the comb knead and heat the wax with their bodies until it’s able to flow and form the hexagons. (Photo credit: G. Mackintosh; via Nature and B. L. Karihaloo et al.)
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Evaporating Drops
When still drops evaporate from a surface, they do so in several phases, as illustrated in the video above. Initially, the drop forms a spherical cap. At this point the velocity within the droplet is so small that it is difficult to resolve, but particles within the drop move outward toward the contact line. As the drop evaporates, they form a circle of sediment – the familiar coffee ring. As the drop flattens, radial velocity increases, drawing more and more particles to the coffee ring. Eventually the drop pulls away from the ring, leaving surface tension and evaporation to compete in driving the internal flow. During this phase, some parts of the contact line try to re-establish the flow pattern that made the first ring; this leaves behind circular segments broken up by the increasing instabilities in the contact line. In the final stage, surface tension smooths some of the irregularities and drives an inward velocity that leaves behind radial sediment segments. (Video credit: G. Hernandez-Cruz et al.)
Vortex Street in the Clouds
Most objects are not particularly aerodynamic or streamlined. When air flows over such bluff bodies, they can shed regular vortices from one side and then the other. This periodic shedding creates a von Karman vortex street, like this one stretching out from Isla Socorro off western Mexico. From the wind’s perspective, the volcanic island forms a blunt disruption to the otherwise smooth ocean. This vortex shedding is seen at smaller scales, as well, in the wind tunnel, in soap films, and in water tunnels. If you’ve ever been outside on a windy day and heard the electrical lines “singing” in the wind, that’s the same phenomena, too. With the right crosswind, radial bicycle spokes will buzz for the same reason as well! (Photo credit: MODIS/NASA Earth Observatory)
Breaking Waves
Most beach-goers have probably wondered just what makes the waves coming in to shore rear up and break. The secret lies in the depths–or rather the lack thereof–beneath the waves. Far from shore, the wave’s length scale is small compared to the ocean depth, and the ocean’s bottom is effectively infinitely far away to all parts of the wave. But, as the wave rolls toward shore, the depth decreases and the ocean bottom begins to influence the wave. In the trough, the ocean bottom slows the wave. Meanwhile, the crest of the wave carries forward, rising until its height reaches 80% of the water depth, at which point it will tip over and break.(Video credit: BBC)
Falcon vs. Raven
Earth Unplugged has posted some great high-speed footage of a peregrine falcon and a raven in flight. Notice how both birds draw their wings inward and back on the upstroke. By doing so, they decrease their drag and thus the energy necessary for flapping. On the downstroke, they extend their wings fully and increase their angle of attack, creating not only lift but thrust. The falcon boasts an incredibly streamlined shape, not only along its body but also along its wings. In contrast, the raven has broader wings with large primary feathers that fan out near the tips. Splaying these large feathers out decreases the strength of the bird’s wingtip vortices, thereby reducing downwash and increasing lift, much the same way winglets do on planes. That extra lift and control the big primaries provide is important for the raven’s acrobatic skill. (Video credit: Earth Unplugged; via io9)
Elastic Walls and Viscous Fingers
The Saffman-Taylor instability, characterized by the branchlike fingers formed when a less viscous fluid is injected into a more viscous one, is typically demonstrated between two rigid walls, as in part (a) of the figure above. But what happens if one of the rigid walls forming the Hele-Shaw cell is replaced with an elastic wall? This is the case for (b) and (c) in the figure. The flexibility of the wall causes the expansion of the air-fluid interface to slow down relative to the rigid wall case and causes the interface to move toward a narrowing fluid-filled gap (as opposed to a constant thickness one). Both of these effects reduce the viscous instability mechanism that drives the fingering instability. With a high enough mass flow rate as in ©, there is still some instability in the interface, but it is dramatically reduced. (Photo credit: D. Pihler-Puzovic et al.)
Granular Gases
Vibrating particles or granular materials can produce many fluid-like behaviors. In this video, researchers demonstrate how a granular gas made up of particles of two sizes behaves at different conditions. By tweaking the amplitude of the vibration, they alter how the particles cluster in a divided container. At large vibrational amplitudes, the particles behave much like a gas–energetic and spread out. At lower amplitudes, though, the particle density and the number of particle collisions increases. Each collision dissipates some of a particle’s energy; more collisions means less energy available to escape. As a result, the particles cluster, forming an attractor that draws in additional particles over time. (Video credit: R. Mikkelson et al.)
Rebounding Off Dry Ice
Droplet rebound is frequently associated with superhydrophobic surfaces but can also be generated by very large temperature differences. For very hot substrates, a thin layer of the drop vaporizes on contact via the Leidenfrost effect and helps a drop rebound by preventing it from wetting the surface. This video shows almost the opposite: a water droplet hitting solid carbon dioxide (-79 degrees C). Upon contact, the solid carbon dioxide sublimates, creating a thin layer of gas that separates the droplet from the surface. You can also see the vortex ring that accompanies the drop’s impact. Water vapor near the carbon dioxide surface has condensed into tiny airborne droplets that act as tracer particles that reveal the vortex’s formation and the rebounding droplet’s wake. (Video credit: C. Antonini et al.; Research paper)
Navigating the Interface
Walking on water may be the stuff of legend at human scales, but it’s a fact of everyday life for many smaller species. Waxy, hydrophobic coatings typically make such insects’ points of contact (feet, legs, etc.) water-repellent, and their light weight can be supported by surface tension. Navigating the interface between air and water is more complicated, though, and these creatures have evolved several mechanisms to help. Some, like water striders, use appendages they insert below the surface for propulsion. At 0:49 in the montage above, you can see flow visualization of the vortices generated by a stroke. Other insects release a chemical in their wake that lowers the local surface tension and drives them away via the Marangoni effect. For more, see here and especially this Physics Today article. (Video credit: D. Hu and J. Bush)
Fluids Round-up – 11 August 2013
Time for another fluids round-up! Here are your links:
- Back in January 1919, a five-story-high metal tank full of molasses broke and released a wave of viscous non-Newtonian fluid through Boston’s North End. Scientific American examines the physics of the Great Molasses Flood, including how to swim in molasses. If you can imagine what it’s like to swim in molasses, you’ll know something of the struggle microbes experience to move through any fluid. They also discusses some of the strange ways tiny creatures swim.
- In sandy desert environments, helicopter blades can light up the night with so-called helicopter halos. The effect is similar to what causes sparks from a grinding wheel. Learn more about this Kopp-Etchells effect.
- Check out this ominous footage of a tornadic cell passing through Colorado last week.
- If you want more of a science-y look to your drinkware, you should check out the Periodic TableWare collection over on Kickstarter.
- Finally, wingsuits really take the idea of gliding flight to some crazy extremes. Check out this video of in-flight footage. Watch for the guy’s wingtip vortices at 3:16 (screencap above)! (submitted by Jason C)
(Photo credit: Squirrel)