Krill and other tiny marine zooplankton make daily migrations to and from the ocean surface. Previously, models of ocean mixing ignored these migrations; these animals are tiny, researchers argued, so any effects they could have would be too small to matter. But zooplankton make these migrations in huge swarms, and studies of a laboratory analog of their migrations (using brine shrimp rather than krill) reveal that, when moving en masse, these tiny swimmers create turbulent jets and eddies far larger than an individual. Their collective motion is enough to mix salty water layers 1000 times faster than molecular diffusion alone! Learn more in the latest FYFD video, embedded below. (Image and video credit: N. Sharp; research credit: I. Houghton et al.; h/t to Kam-Yung Soh)
Month: April 2018
“Flowers and Colors”
Many children have done the simple experiment of placing a cut flower in dyed water and watching as it changed color. The latest video from Beauty of Science relies on some related physics. Since the color of flowers typically depends on acidity, immersing a flower in dilute acid will change its color from pinks and purples to yellows and greens. Watching this transformation, we can learn about how fluids get transported through flowers.
Like the leaves on a tree, flowers are covered in tiny cells called stomata that can open and close. In the daytime, stomata are typically open to allow carbon dioxide to diffuse into the plant. (At the same time, water pulled up from the roots is evaporating out the stomata, as seen previously.) Once immersed in acid, the open stomata are no longer bringing in carbon dioxide; instead, the acid is diffusing in and slowly spreading through the petals. In the timelapse video, some areas of the petal change faster than others. This could indicate more open stomata in the regions that change first or even that some areas inside the petal transport water (and acid) better than others. (Video and image credit: Beauty of Science; see also Making Of)
Bubble Trains in a Microchannel
Trains of bubbles flowing through a microchannel get distorted by periodic expansions and constrictions. In these images, flow is from left to right, and the narrow point of the channel is about 250 microns across. In narrow regions, the front of the bubble tends to move faster, while in wider areas, the back of the bubble speeds up. This causes the distinctive shape changes we see. Microfluidic channels with these exaggerated shifts in geometry allow researchers to study the physics behind liquids and gases seeping through the interstitial gaps of a porous media, like when water and gases move through rock and soil. (Image and research credit: M. Sauzade and T. Cubaud)
The Disintegrating Splash
A drop of blue-dyed glycerine impacts a thin film of isopropanol, creating a spectacular splash and breakup. The drop’s impact flings a layer of the isopropanol into the air, where air currents make the thin sheet buckle inward and break into a spray of droplets. Meanwhile, the liquid from the drop forms a thick, blue crown that rises and expands outward. When tiny droplets of the isopropanol hit the splash crown, their lower surface tension causes the blue glycerine to pull away, due to the Marangoni effect. This opens up holes in the crown, which grow quickly, until the entire sheet breaks apart. (Image and research credit: A. Aljedaani et al., source)
Snowmelt
Much of the rain that falls on Earth began as snow high in the atmosphere. As it falls through warmer layers of air, the snowflakes melt and form water droplets. The details of this melting process have been difficult to capture experimentally, but a new computational model may provide insight. The basic process has a couple stages. As snow begins to melt, surface tension draws the water into concave areas nearby. When those regions fill up, the water flows out and merges with neighboring liquid, forming water droplets around a melting ice core.
Although this same sequence was observed for many types of snow, scientists also observed some important differences between rimed and unrimed snowflakes. Rime forms when supercooled water droplets freeze onto the surface of a snowflake. Lightly rimed snow still looks light and fluffy, like the animation above, but heavily rimed snow forms denser and more spherical chunks. Because there are lots of porous gaps in heavily rimed snow, water tends to gather there during initial melting. Rimed snow was also more likely to form one large water droplet rather than breaking into multiple droplets like snow with less rime. For more, check out NASA’s video and the Bad Astronomy write-up. (Image credit: NASA, source; research credit: J. Leinonen and A. von Lerber; via Bad Astronomy; submitted by Kam Yung-Soh)
Skyglow
Timelapse can be a beautiful way to highlight slow-moving flows like those in the sky. But it can also be valuable in showing differences in speed, as in the latest SKYGLOW Project video, “Colorado Serenade”, which shows the Colorado River and the skies overhead simultaneously. Timelapse highlights the difference in time scales between the fast-moving river and slower-moving clouds.
This mirrors an important phenomenon in fluid dynamics known as “separation of scales”. In a flow, there are often multiple effects at play and they may occur on different time (or length) scales. Which matters most in a given situation will depend on those scales. Consider a rocket engine. Combustion inside the engine ignites fuel and oxidizer, releasing heat. At the same time, the flow in the engine is key to mixing that fuel and oxidizer together so that all of the fuel and oxidizer ignites before it is sent downstream into the rocket nozzle. There are two important time scales here: the time it takes for the flow to mix fuel and oxidizer together and the time it takes for the combustive chemical reaction to take place. In an ideal world, engineers can balance those two time scales to maximize efficiency. But in the (admittedly less ideal) real world, this is not always possible. (Video and image credit: H. Mehmedinovic/SKYGLOW)
Happy 2000 Posts!
Happy Friday and happy 2000th FYFD post! To celebrate, I played with surface tension and the Marangoni effect to make some art. For a run-down on the physics, check out this previous post on water calligraphy. Two thousand posts feels like a major milestone. Not everyone realizes this, but FYFD is a one-woman operation, so 2000 posts is a whole lot of research, image editing, and writing. For fun, I’m including here eight completely random FYFD entries, representing less than one-half of one percent of my total archives:
1. Why did Chinese junks put holes in their rudders?
2. Making droplets in an ultrasonic humidifier
3. Floating on a granular raft
4. Air-trapping fur keeps otters warm
5. The physics of the knuckleball
6. What makes badminton so fast?
7. Playing with fluorescein
8. How frost formsWant to keep up the random walk? Use https://fyfluiddynamics.com/random to find random entries, or if you prefer your browsing to be more directed, check out the visual archive or the themed series page.
As always, a special thanks to those who help support FYFD through Patreon subscriptions – I couldn’t keep writing and making videos without your help! And thank you to all of you who read and share FYFD. Whether you’ve been following along for a week or for the last eight years, your enthusiasm keeps me motivated! Thank you!
(Image credits: 2k animation – N. Sharp; Chinese junk ship – Premier Ship Models; ultrasonic humidifier – S. J. Kim et al.; granular raft – E. Jambon-Puillet and S. Protiere; 3D-printed “fur” – F. Frankel; knuckleball – L. Kang; shuttlecock – Science Friday; fluorescein – Shanks FX; freezing droplets – J. Boreyko et al.)
Riding Across Water
Humans may not be fast enough to run across water, but we’ve found other ways to conquer the waves. It’s even possible (though definitely not recommended) to ride across stretches of water on a dirt bike. To do so, you have to keep the bike (hydro)planing, and to understand what that means, let’s take a moment to talk about boats.
At low speeds, boats stay afloat based on buoyancy, a force that depends on how much water they displace. But when moving at high speeds, modern speedboats lift mostly out of the water and skim the surface instead. At this point, it’s hydrodynamic lift that keeps the boat above the surface and we say that the boat is planing. Calculating that hydrodynamic lift is fairly complicated and depends on many factors – for those who are interested, check out some of David Savitsky’s papers – but, generally speaking, going faster gives you more lift.
This brings us back to the dirt bike. There’s nothing particularly hydrodynamic about a dirt bike. It’s not shaped to provide hydrodynamic lift, but it does come with a high power-to-weight ratio. It’s this ability to create pure speed, and a rider’s keen sense for holding the bike at the right angle, that enables pros to cross open water. Needless to say, this is the kind of stunt that could end really badly, so don’t try it yourself. (Image credits: C. Alessandrelli, source; EnduroTripster, source; via Digg; submitted by 1307phaezr)
Martian Mantle Convection
Over geological timescales – on the order of millions of years – even hard substances like rock can flow like a fluid. Heat from the Earth’s core drives convection inside our mantle, and that fluid motion ultimately drives the plate tectonics we experience here at the surface. But most other planetary bodies, including those with mantle convection similar to ours, don’t have a surface that shifts like our tectonic plates. Mars and Venus, for example, have solid, unmoving surfaces. The images above provide a peek at what goes on beneath. The upper image shows a simulation of mantle convection inside Mars over millions of years. The lower image is a timelapse of dye convecting through a layer of glucose syrup being heated from below. Notice how both examples show evidence of convective cells and plumes that help circulate warm fluid up and colder fluid downward. (Image credit: Mars simulation – C. Hüttig et al, source; N. Tosi et al., source; submitted by Nicola T.)
Wave Clouds
Stripe-like wave clouds can often form downstream of mountains. This satellite image shows such clouds in the South Pacific where rocky mountains jut 600 meters (2,000 ft) above the sea. This disrupts air flowing east by forcing it to move up and over the island topography. The air does not simply settle back down on the other side, though. It must come back into equilibrium with its surroundings in terms of density and temperature. While doing so it will travel up and down along a wavy path. As it reaches the crest of the wave, humid air cooling condenses and forms a cloud. At troughs, the air warms and the condensation disappears. This creates the stripey cloud pattern in the mountain’s wake, which fades out as the atmospheric gravity waves die out. (Image credit: NASA/J. Schmaltz; via NASA Earth Observatory)
