There are many ways to levitate a droplet – heating, vibration, and acoustic levitation all come to mind – but this video demonstrates a simpler method: a moving wall. Depositing a drop on a moving wall keeps it aloft with a thin, constantly replenished layer of air. The thickness of this lubricating air film is directly measurable from interference fringes created by light reflecting off the surface of the drop. Incredibly, the air layer is only a few microns thick, but the resulting pressure in the air film is high enough to levitate millimeter-sized droplets! (Video credit: M. Saito et al.; via @AlvaroGuM)
Videos
Bounce or Freeze?
Icing is a major problem for aircraft. When ice builds up on the leading edge of a wing it creates major disruptions in flow around the wing and can lead to a loss of flight control. One of the important factors in predicting and controlling ice building up is knowing when and where water droplets will freeze. The video above shows how surface conditions on the wing affect how an impacting droplet freezes. On a subzero hydrophilic surface, a falling droplet spreads and freezes over a wide area, which would hasten ice buildup. A hydrophobic surface is slightly better, with the droplet freezing over a smaller area, whereas a superhydrophobic surface shows no ice buildup. Unfortunately, at present superhydrophobic surfaces and surface treatments are extremely delicate, making them unsuitable for use on aircraft leading edges. (Video credit: G. Finlay)
“Cymatic Sun”
“Cymatic Sun” from artist Lachlan Turczan uses vibrating fluids to generate mesmerizing and surreal visuals. At some points distinct Faraday waves are visible on the surface. At other times, there is simply a blur of motion and refracted light. Check out my “fluids as art” tag for many more great examples of fluid dynamics and art merging. (Video credit and submission: L. Turczan)
Shooting Droplets with Lasers
Last week we saw what happens when a solid projectile hits a water droplet; today’s video shows the impact of a laser pulse on a droplet. Several things happen here, but at very different speeds. When the laser impacts, it vaporizes part of the droplet within nanoseconds. A shock wave spreads from the point of impact and a cloud of mist sprays out. This also generates pressure on the impact face of the droplet, but it takes milliseconds–millions of nanoseconds–for the droplet to start moving and deforming. The subsequent explosion of the drop depends both on the laser energy and focus, which determine the size of the impulse imparted to the droplet. The motivation for the work is extreme ultraviolet lithography–a technique used for manufacturing next-generation semiconductor integrated circuits–which uses lasers to vaporize microscopic droplets during the manufacturing process. (Video credit: A. Klein et al.)
Zesty Fireballs
Zesting the skin of a citrus fruit like oranges releases a spray of tiny oil droplets. Citrus oil has several volatile components, meaning that it evaporates quickly at room temperature. It is also a liquid with a relatively low flash point, meaning that only modest temperatures (~40-60 degrees Celsius) are needed to generate enough vapor to ignite a vapor/air mixture. With volatile and flammable liquid fuels, a spray of droplets is an ideal platform for combustion because the essentially spherical droplets have a high surface area from which they can evaporate and provide vaporous fuel. (Video credit: ChefSteps)
Bouncing with Liquids and Grains
Bouncing a ball partially filled with a liquid can create chaotic results when the motion of the ball, fluid, and vibration plate couple. The behavior of a grain-filled ball is a bit different, though. Large grains will tend to bounce with the same frequency as the ball, even across a range of vibration conditions. A ball filled with smaller grains displays a variety of responses depending on the vibration conditions. Among these is a localized wave-like form called an oscillon which oscillates with a period different from but coupled to that of the vibration plate. All these different behaviors inside the bouncing sphere have noticeable effects on its outward motion, too. The chaotic activity of the fluid inside a bouncing ball makes it unstable, and, if not confined, it will bounce itself off the vibration platform. The grain-filled ball, on the other hand, remains bouncing on the platform even after being perturbed. This seems to be a result of the energy dissipation provided by the many inelastic collisions inside the ball as it bounces. (Video credit: F. Pacheco-Vazquez et al.)
Turbine Blade Separation
[original media no longer available]
Maintaining consistent air flow along the contours of an object is key to aerodynamic efficiency. When air flow separates or forms a recirculation zone, the drag increases and efficiency drops. On wind turbine blades, flow often separates on the root end of the blade near its attachment point. This behavior is apparent in the video above at 0:34. The tufts in the foreground on the turning blade flap and flutter with no clear pattern because the air flow has separated from the surface. In the subsequent clip, a line of vortex generators has been attached near the leading edge of the blade. These structures–also commonly seen on airplanes–trail vortices behind them, mixing the flow and generating a turbulent boundary layer which is better able to resist flow separation. The effect on the flow is clear from the tufts, most of which now point in a consistent direction with little to no fluttering, indicating that the air flow has remained attached. (Video credit: Smart Blade Gmbh/Technische Universität Berlin)
(Source: /)Bardarbunga Eruption
I thought I was done with volcanoes for this week, but DJI’s aerial footage from Iceland’s Bardarbunga eruption is too fantastic not to share. The eruption is over a month old now and more than 25,000 earthquakes have been registered in Iceland since this eruption began. The lava field covers more than 46 square kilometers, and experts remain unsure how long the eruption will continue. The lava itself is a basalt, which is lower in viscosity than more silica-rich lava. This lower viscosity means that the gases dissolved in the rising magma can escape more easily, like carbon dioxide fizzing out of a soda. If the lava’s viscosity were higher, those dissolved gases would generate a more explosive eruption as they try to escape. (Video credit: DJI; via Wired)
Freediving
The freediving del Rosario brothers have created a real treat with this underwater film. There are no computer-generated special effects, just some clever tricks with camera angles, perspective, and buoyancy. The end result is slightly surrealistic and captures some of the fluid beauty of the ocean. And don’t miss the excellent bubble ring vortices. (Video credit: The Ocean Brothers; via Gizmodo; submitted by jshoer)
City Winds Simulated
Anyone who has spent much time in an urban environment is familiar with the gusty turbulence that can be generated by steady winds interacting with tall buildings. To the atmospheric boundary layer–the first few hundred meters of atmosphere just above the ground–cities, forests, and other terrain changes act like sudden patches of roughness that disturb the flow and generate turbulence. The video above shows a numerical simulation of flow over an urban environment. The incoming flow off the ocean is relatively calm due to the smoothness of the water. But the roughness of an artificial island just off the coast acts like a trip, creating a new and more turbulent boundary layer within the atmospheric boundary layer. It’s this growing internal boundary layer whose turbulence we see visualized in greens and reds. (Video credit: H. Knoop et al.)