Mezcal is a traditional Mexican liquor distilled from agave. (The more commonly known tequila is actually a special type of mezcal.) As a part of the production process, distillers pour a stream of mezcal into a bowl, creating a flotilla of small bubbles called pearls. Strange as it sounds, these pearls let the distiller judge the alcohol content of the liquor! When the ratio of alcohol and water in the mixture is just right, the bubbles will have a longer lifetime before they coalesce. If there’s too little or too much alcohol, the bubbles won’t last as long. The effect depends on both the viscosity and the surface tension of the liquor, but it’s the odd way that viscosity changes in water/alcohol mixtures that creates this Goldilocks behavior. It’s a fascinating demonstration of how traditional techniques often have true scientific underpinnings. (Video credit: M. Wilhelmus et al.)
Tag: fluid dynamics
Ocean Mixing
Movement in Earth’s oceans is driven by a complicated interplay of many factors like temperature, salinity, and Earth’s rotation. Above are results from a numerical simulation of the top 100 meters of ocean contained within a 1 km x 1 km box. The colors indicate surface temperature. Two major processes create the motion we see. The first is convection, in which water at the surface releases heat to the atmosphere and cools, causing it to then sink due to its greater density. Warmer water rises to replace it. This process happens quickly and dominates the early part of the simulation where we see the puffy convection cells shown on the left animation.
A slower process is in effect as well. Because of variations in the water temperature, the density of the fluid at a given depth is not constant. We can already see that at the water surface, where the temperature (and thus density) is varying significantly. Those variations in density at the same depth combined with gravity’s tendency to shift fluids create what is known as a baroclinic instability. Put simply, this instability will cause warmer water to slide horizontally past colder water. The result is the large, spinning eddy motion seen in the animation on the right. To see how the whole system develops, check out the full video below. (Image/video credit: J. Callies)
Viscous Fingers
Viscous fingers form between air and titanium dioxide sol-gel in this photograph. The two fluids are trapped in a thin gap between glass plates – a set-up known as a Hele-Shaw cell. The dendritic fingers we see form when the less viscous air pushes into the more viscous sol-gel. This is an example of the Saffman-Taylor instability. The psychedelic colors are a result of thin-film interference and the way light interacts with very thin materials. The same effect is responsible for the colors on soap bubbles. (Image credit: C. Trease)
Fluids Round-Up
Time for another fluids round-up! Here are some of the best fluids-related links I’ve seen around:
– Above The Brain Scoop tells us about beetles that spend their whole lives underwater. They carry a little bubble of air with them in order to breathe!
– Microfluidics are helping reveal how cancer cells metastasize and spread through the bloodstream.
– It’s official! NASA’s going to build X-planes again.
– See how snake venom kills by changing the fluid properties of a victim’s blood. (via Gizmodo)
– Metallic foams can stop bullets and radiation, spawning many potential future uses here on Earth or in space.
– Why nature prefers hexagons, especially in honeycomb, bubbles, and foam.
– Earth has beautiful auroras, but if you could look at Jupiter with x-ray vision, you’d see something even more spectacular – a non-stop aurora that brightens on a regular schedule.
– SciShow asks where the water goes in Minnesota’s Devil’s Kettle Falls. Conservation of mass says it has to go somewhere!
And, in case you missed it, you can check out the latest FYFD video and learn more about the Brazil Nut effect over at Gizmodo.
(Video credit: The Brain Scoop)
Roll Clouds
The roll cloud, or Morning Glory cloud, is a rare phenomenon that looks rather like a horizontal tornado. In reality, it is part of a soliton wave traveling through the atmosphere. At its leading edge, moist air is forced upward, causing water vapor to condense, and, at the trailing edge, air moves downward, dissipating the cloud. These clouds are most frequently observed in Australia near the Gulf of Carpentaria, where local geography and sea breezes promote their growth during springtime. The clouds do appear elsewhere on occasion; the photos above show rolls clouds in Calgary, Alberta and coastal Uruguay, respectively. (Image credits: G. E. Nyland, D. M. Eberl; see also: Z. Ouazzani)
Boiling on Mars
Today’s Mars is cold and dry, with a thin and insubstantial atmosphere. One of the challenges facing planetary scientists is unraveling the processes behind the complex terrain we can observe on the surface. Without flowing water, how do we explain these features? A new experiment suggests that the answer lies in boiling.
Surface conditions on Mars include atmospheric pressures low enough to be below the triple point of water* – the critical temperature and pressure where water vapor, liquid water, and ice can all exist simultaneously. This means that liquid water is unstable under Martian conditions; any water that seeped up to the surface would immediately begin to boil. That explosive boiling ejects sand particles, as seen in the animation above. The authors suggest that this hybrid process of wet percolation combined with vaporous ejection of sediment may better explain the Martian surface features we observe. (Image credit: M. Masse et al., source: Supplementary Movie 3; via Gizmodo; submitted by Paul vdB)
* The evidence we’ve seen so far on Mars points to briny water flowing near the surface. Although brines have lower freezing temperatures than pure water, the authors’ argument holds for them, as well. The boiling is simply not as vigorous.
Silent Flying
As nocturnal hunters, owls are aerodynamically optimized for stealthy flying. This clip from BBC Earth demonstrates just how quiet a barn owl is in flight compared to a pigeon or a peregrine falcon. The owl’s large wingspan relative to its body size gives it enough lift that it does not have to flap often, allowing it to glide instead, but this is far from its only stealthy adaptation. Owl feathers feature a serrated leading edge that helps break flow over the wing into smaller, quieter vortices. Their fringe-like trailing edge breaks flow up even further and acts to damp noise from airflow. The downy feathers of the owl’s body also help muffle any noise from the bird’s movement, allowing the barn owl to fly almost silently. (Video credit: BBC Earth; via Gizmodo)
The Bubble Nebula
This spectacular Hubble image shows the Bubble Nebula. The source of this nebula is the star seen toward the upper left side of the bubble. This massive, super-hot star has ceased to fuse hydrogen and is now fusing helium, powering its way to a likely end as a supernova. As it burns, the star emits a stellar wind of gas moving at over 6.4 million kilometers an hour. As the flow moves outward, it encounters colder dense gases that it pushes along as it expands; this is the blue bubble surface that we see. The asymmetry of the bubble with respect to its source star is caused by the variation in the surrounding gas’s density. The bubble’s front moves more slowly in areas with more gas, thus making the bubble appear lop-sided. (Image credit: NASA; via Gizmodo)
Striking Oobleck
Mixing cornstarch and water creates a fluid called oobleck that has some pretty bizarre properties. Oobleck is a shear-thickening, non-Newtonian fluid, which means its viscosity increases when you try to deform it with a shearing, or sliding, force. But as the Backyard Scientist demonstrates above, striking oobleck with a solid object produces some spectacular and very non-fluid-like results. The golf ball’s impact blows the oobleck into pieces that look more like solid chunks than liquid droplets. This solid-like behavior occurs because the impact jams the suspended cornstarch particles together, creating a solidification front that travels ahead of the golf ball. Imagine how a snow plow pushes a denser region of snow ahead of it as it drives; the cornstarch behaves similarly but only in a region near the impact. Once that impact force dissipates, the particles unjam and the mixture responds fluidly again. (Image credit: The Backyard Scientist, source; research credit: S. Waitukaitis and H. Jaeger, pdf)
Emulsion Impact
Emulsions – mixtures of two immiscible fluids – are quite common; the oil and vinegar combination used in many salad dressings is one. The image sequence above shows the first 800 microseconds of the impact of a similarly emulsified droplet. The outer drop, seen on the left, consists of a water/glycerin mixture, and inside the drop are 20 smaller perfluorohexane droplets. These smaller droplets are denser and tend to settle toward the bottom of the outer drop. When the compound droplet hits a solid surface, it spreads in a spectacular starburst pattern that depends on the number and location of interior droplets. You can see a similar impact in motion here. (Image credit: J. Zhang and E. Li; source: C. Josserand and S. Thoroddsen)
