Granular materials like sand are sometimes very fluid-like in their behaviors. The high-speed video above shows a ball bearing being dropped into packed sand. Many features of the splash are fluid-like; the initial impact creates a spreading crownlike splash, followed by a strong upward jet that eventually collapses back into the medium. At the same time, many of the impact characteristics are decidedly non-fluidic. Sand has no surface tension, so both the crown and the jet readily break up into small particles. The granular jet is very narrow and energetic, reaching heights greater than the impacter’s drop height. Interestingly, the column begins collapsing on its lower end before the jet even reaches its highest peak. This may be due to the lower energy of the sand particles that were ejected later in the crater formation process. (Video credit: J. Verschuur, B. van Capelleveen, R. Lammerink and T. Nguyen)
Category: Research
The Science of Champagne
Champagne owes much of its allure to its tiny bubbles. Unlike other wines, champagne undergoes a secondary fermentation in the bottle, during which the yeasts in the wine consume sugars and produce carbon dioxide, which dissolves into the wine. When opened, the carbon dioxide can begin to escape. Bubbles form in the glass around imperfections, either due to intentional etching of the glass or impurities left behind by cleaning. Once formed, trails of bubbles rise to the surface, swelling as more dissolved carbon dioxide is absorbed into each bubble. The bubbles then cluster near the surface of the champagne, occasionally popping and creating a flower-like distortion of the surrounding bubbles. The gases within the bubbles contains higher concentrations of aromatic chemicals than the surrounding wine, and the bursting of each bubble propels tiny droplets of these aromatics upwards, carrying the scent of the champagne to the drinker. For more beautiful champagne photos, I recommend this LuxeryCulture article; for more on the science of champagne, see Chemistry World’s coverage. Happy 2014! (Image credits: G. Liger-Belair et al.)
Huddling Penguins and Traffic Jams
Male emperor penguins have the unenviable task of incubating their eggs in temperatures as cold as -50 deg Celsius and winds of up to 200 km/h. To stay warm, the penguins form huddles of up to thousands of individuals. Observations in the wild show that these huddles move in a stop-and-go fashion, with changes propagating through the penguins like waves. Researchers adapted a model used for heavy traffic flow to describe the penguins’ motion. They found that motions like those found in observed penguin huddles could be initiated by slight movements of any penguin in the model huddle, regardless of its position; in other words, the huddle has no leader. They also found that the wave that travels through the penguins can align the huddle to uniform density or help two huddles merge. To learn more, check out the researchers’ video or their paper. (Video credit: D. Zitterbart et al./New Scientist; via J. Ouellette)
Collapsing Soap Bubbles
The colors of a soap film are directly related to their thickness. If a film becomes thin enough (~10 nanometers), it appears black. (Here’s why.) This video shows the thinning of a vertical soap film. Normally, this is a linear process, with gravity pulling the fluid downward and progressively thinning the film from top to bottom at a constant rate. At 0:20 a cold rod slowly contacts the film, adding a thermal driver for the film’s thinning. Two large counter-rotating convection cells form underneath the rod, with weaker secondary vortices in the lower corners of the film. This drastically increases mixing in the film. Gradually small black spots, indicating very thin areas of the film, form and advect. Eventually these spots stretch, forming long tails. The thinning of the film kicks up to an exponential rate until the film becomes uniformly thin. (Video credit: M. Winkler et al.)
Pitcher Plant Fluid Dynamics
Carnivorous pitcher plants owe much of their efficacy to the viscoelasticity of their digestive fluid. A viscoelastic fluid’s resistance to deformation has two components: the usual viscous component that resists shearing and an elastic component, often derived from the presence of polymers, that resists stretching – kind of like a liquid rubber band. It’s the latter effect that’s important when it comes to the pitcher plant trapping insects. When a fly or ant falls into the liquid within the plant, it will flail and try to swim, thereby straining the fluid. In part © of the image above, you can see how long fluid filaments stretch as the fly moves; this is because the digestive fluid’s extensional viscosity, the elastic component, is 10,000 times larger than its shear viscosity, the usual viscous component, for motions like the fly’s. This viscoelastic fluid is so effective at trapping insects that, as seen in part (b) above, it has to be diluted by more than 95% before insects can escape it! (Image credit: L. Gaume and Y. Forterre)
Mushrooms Make Their Own Breeze
Mushrooms don’t rely on a stray breeze to spread their spores; they generate their own air currents instead. The key is evaporation. The mushroom cap contains large amounts of water, and, as this water evaporates, it cools the mushroom and the air next to it. This cool air is denser than the surrounding air, and so tends to spread out and convect. At the same time, though, the water vapor that evaporated from the mushroom is less dense than nearby air, which helps it rise. This combination of spreading and rising air carries spores away from the mushroom cap and, as seen in the video above, can combine to form beautiful and complex currents that spread the spores. (Video credit: E. Dressaire et al.)
Solar Wind
Fluid dynamics appear at all kinds of scales. The animation above shows two comets, Encke and ISON, on their recent approach toward the sun. The darker wisps emanating from the right side of the image are part of the solar wind, a plasma stream continuously emitted by the sun’s upper atmosphere. Although the solar wind is very rarefied by terrestrial standards, its density is sufficient to whip the comets’ tails of gas and dust from side-to-side. Scientists use images like these to learn more about the structure of the solar wind based on its interaction with the comets. For more great images of ISON’s journey around the sun, check out NASA Goddard. (Image credit: K. Battams/NASA/STEREO/CIOC; submitted by John C)
Put the Lid Down When You Flush
Hospital-acquired infections are a serious health problem. One potential source of contamination is through the spread of pathogen-bearing droplets emanating from toilet flushes. The video above includes high-speed flow visualization of the large and small droplets that get atomized during the flush of a standard hospital toilet. Both are problematic for the spread of pathogens; the large droplets settle quickly and contaminate nearby surfaces, but the small droplets can remain suspended in the air for an hour or more. Even more distressing is the finding that conventional cleaning products lower surface tension within the toilet, aggravating the problem by allowing even more small droplets to escape. To learn more, see the Bourouiba research group’s website. (Video credit: Bourouiba research group)
Bubbles Through Constrictions
Surface tension usually constrains bubbles to the smallest area for a given volume – a sphere – but sometimes other forces generate more complicated geometries. The images above show bubbles flowing through microfluidic channels filled with a highly viscous carrier fluid. The bubble size and packing affects the shapes they assume, but so does the geometry of the channel. The narrow constrictions accelerate the flow, elongating the bubbles, whereas the wider channel regions slow the carrier fluid and squish the bubbles together. (Image credit: M. Sauzade and T. Cubaud (Stony Brook University))
Pathlines vs. Streaklines
When considering fluid motion, there are many ways to describe trajectories through the flow. One is the pathline, the trajectory followed by an individual fluid particle. Imagine releasing a rubber duck down a stream. Following the duck’s position over time would give you a pathline. Now imagine that instead of releasing a single rubber duck you release lots of them – say one every half-second from the exact same starting spot. You would end up with a line of rubber ducks stretching downstream, each of them sharing the same origin but with a different starting time. This is called a streakline. Would the streakline of rubber ducks follow the same trajectory as the lone duck? Not if the flow is time-varying! In fact, for unsteady flows, pathlines and streaklines can give completely different pictures of a flow, as illustrated in the video above. Knowing and understanding the difference between these types of trajectories is extremely important when it comes interpreting flow visualizations in unsteady flows because some visualization methods produce pathlines and others produce streaklines. (Video credit: V. Miller and M. Mungal)
