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)
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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.)
Holiday Fluids: What is Fire?
Snowy holidays and long, dark nights are a great time to sit by the fire or enjoy some candlelight. We’ve talked before about how buoyancy affects a flame’s shape, how atomization mixes liquid fuel and oxidizers, how flames propagate, how internal combustion works and how instabilities can end combustion. But in all that we haven’t addressed what fire actually is! Combustion is a chemical process–a reaction between a hydrocarbon fuel and oxygen, but the flame we’re accustomed to seeing is a combination of blue light produced by the complete reaction and incandescent red/orange/yellow light from glowing soot particles produced when there is insufficient oxygen for the reaction. If you have time after the Minute Physics version, this video from Ben Ames has a wonderful explanation of flames. Of course, if you just prefer your holiday fun with more explosive high-speed videos, you’re going to want to see this Christmas tree made from detonation cord (see 2:40 for the start of the best part). This wraps up our holiday-themed fluid dynamics series. Happy holidays from FYFD! (Video credit: Minute Physics)
Holiday Fluids: Cocoa Convection
If you make a proper cup of hot chocolate this holiday, watch carefully and you just may catch some Rayleigh-Benard convection like the video above. (Note, video playback is 3x.) The canonical Rayleigh-Benard problem is one in which fluid is heated from below and cooled from above. For the cup of hot chocolate, the cooling comes from the colder, ambient air at the cocoa’s surface. Because cooler fluid is denser than warmer fluid, the cocoa near the surface will tend to sink down, allowing warmer cocoa to rise. As that warm cocoa reaches the surface, it too will cool and sink back down, continuing the cycle. The effect relies on buoyancy and, by extension, gravity; on the International Space Station, for example, astronauts would not observe such convection. The distinctive shape of the cells depends on the boundaries of the cup. This post is part of our weeklong holiday-themed fluid dynamics series. (Video credit: Armuotas)
Holiday Fluids: Santa’s Aerodynamics
Today we have some holiday-themed fluid dynamics: visualization of flow around Santa’s sleigh! This is a flowing soap film visualization at a low speed (author Nick Moore has some other speeds as well). Santa’s sleigh is what aerodynamicists call a bluff body–a shape that is not streamlined or aerodynamic–and sheds a complicated wake of vortices. Like any object moving through a fluid, Santa’s sleigh generates drag forces made up of several components. There is viscous drag, which comes from friction between the sleigh’s surface and the fluid, and form drag (or pressure drag), which comes from the shape of the sleigh. That wake full of complicated vortices significantly increases the sleigh’s pressure drag, requiring Rudolph and the other reindeer to provide more thrust to counter the sleigh’s drag. Speaking thereof, the visualization does not take into account the aerodynamics of the reindeer, who, in addition to providing the sleigh’s thrust, would also affect the flowfield upstream of the sleigh. This post is part of this week’s holiday-themed post series. (Video credit: N. Moore)
Holiday Fluids: Snowflakes
Just about everyone wishes for a White Christmas, but even when that happens, it’s rare to get a good look at the beauty of individual snowflakes. Alexey Kljatov’s macro photography of snowflakes is simply stunning and highlights the incredible variety of forms snowflakes take. A snowflake forms when a water droplet freezes onto dust or other particles and grows as more water vapor freezes onto the initial crystal. The symmetry of the snowflakes, as with any crystal, comes from the internal order of its water molecules. The shape and features that form vary due to the local temperature and humidity level while vapor is freezing onto the crystal. Check out this handy graph showing which shapes form for various situations. Since snowflakes can encounter wildly different conditions on their path to the ground, it’s rare or next-to-impossible to find any two alike. Join us all this week at FYFD as we look at holiday-themed fluid dynamics. (Photo credit: A. Kljatov)
Holiday Fluids
BYU Splash Lab–those breakers of bottles, skippers of rocks, spinners of eggs, students of soap films, masters of splashes, and all-around cool fluid dynamicists–have some fluids-themed, high-speed holiday greetings. Likewike, here at FYFD we’ll be spending the next week celebrating the physics and fluid dynamics of the winter holiday season! In the meantime, you can whet your appetite by brushing up on your cookie dunking techniques, watching how icicles form, and enjoying a good beverage. Stay tuned and happy holidays from FYFD! (Video credit: BYU Splash Lab/BYU News)
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)
Reader Question: What is Surface Tension?
Last week reader thesnazz asked:
Is there a difference between surface tension and viscosity, or are they two manifestations of the same process and/or principles? If you know a given fluid’s surface tension, can you predict its viscosity, and vice versa?
I’m tackling this one in parts, and you can click here to read about viscosity.
Surface tension’s intermolecular origins are a bit clearer than those of viscosity. Essentially, within the interior of a water drop, you can imagine water molecules all hanging out with other water molecules. They tug on one another, but because they are surrounded on all sides by other water molecules, the net force of all these interactions on any molecule is zero. Not so at the surface of the drop. The surface is also called an interface; it’s a place where the fluid ends and something else–another fluid or perhaps a solid–begins. For a water molecule at that interface, the forces exerted by neighboring molecules are not balanced to zero. Instead, the imbalance causes the water molecules to be tugged inward. We call this effect surface tension.
Because surface tension is an interfacial effect, it is not completely dependent on the fluid alone. For example, a drop of water sitting on a solid surface can take a variety of shapes depending on the properties of the solid (see also hydrophobicity) and the surrounding air as well as those of the water. This is only one of many manifestations of surface tension. Wikipedia has a pretty good overview of some others, if you’d like to learn more. Like viscosity, surface tension is usually measured rather than calculated from first principles.
In the end, both surface tension and viscosity have molecular origins, but they are two very different and independent properties. Viscosity is inherent to a fluid, whereas surface tension depends on the fluid and its neighboring substance. Both quantities are more easily measured than calculated. Thanks again to thesnazz for a great question! As always, you can ask questions or submit post ideas here on Tumblr or via Twitter or email. (Image credit: Wikimedia)
Reader Question: What is Viscosity?
Reader thesnazz asks:
Is there a difference between surface tension and viscosity, or are they two manifestations of the same process and/or principles? If you know a given fluid’s surface tension, can you predict its viscosity, and vice versa?
This is a good question! To answer it, let’s think about where surface tension and viscosity come from. Like many concepts in fluid dynamics, these quantities describe for a whole fluid the properties that arise from interactions between molecules.
To prevent this becoming overly long, I’m going to tackle this over a couple posts. Today, I’ll talk about viscosity.
One way to describe a fluid’s viscosity is as a measure of its resistance to deformation. Another way to think of it is how easily momentum is transmitted from one part of the fluid to another. The diagram above is the classic representation. A layer of fluid is sandwiched between two flat plates. If the top plate moves, friction requires that the fluid particles in contact with the plate get dragged along. This shears the fluid just below that and drags it along, but not quite as much. Those fluid particles do the same to their neighbors and so on down to the stationary second plate, where the fluid is at rest.
Viscosity is the property that determines how much those neighboring fluid particles move; the more viscous the fluid, the more the neighboring bits of fluid resist getting pulled along. This is a property that’s inherent to a fluid. It comes from how the molecules of the fluid interact with one another, but there are no simple expressions to calculate the viscosity of a liquid or a gas from the individual interactions of its molecules. Instead we experimentally measure viscosity values and use empirical formulas to approximate how viscosity changes with temperature and other effects. (Image credit: Wikimedia)
