Convection cells like these are all around us — in the clouds, on the Sun, and in our pans — but we rarely get to watch them in action. Convection results from densities differing in different areas of a fluid. Under gravity’s influence, having a dense fluid over a lighter one is unstable; the dense fluid will always sink and the lighter one will rise. When that motion has to take place across a large surface area, we often end up with cells like the ones seen here.
Convection cells in an alcohol-paint mixture.
What drives the density differences in the fluid? That depends. Often there’s a temperature difference that drives warmer fluid to rise and cool fluid to sink. But that’s not always the source of convection. Evaporating a volatile chemical — like alcohol — out of a mixture can also create the density differences needed for convection. That may be the source of the convection we see here in a mixture of paint and alcohol. (Video and image credit: W. Zhu; via Nikon Small World in Motion)
Around the world, salt playas are criss-crossed with meter-sized polygons formed by ridges of salt. The origins of these structures — and the reason for their consistency across different regions of the world — have been unclear, but a new study shows that salt polygons form due to convection happening in the soil underground.
Through a combination of numerical modeling, simulation, lab-scale experiment, and field work, the team revealed the mechanism underlying salt polygons. Areas that form polygons have much greater rates of evaporation than precipitation, and, as water evaporates, these areas draw groundwater from nearby. Salt gets carried with this groundwater.
With strong evaporation, the lake bed forms a highly-concentrated layer of salty water near the surface. Convection cells form, with some regions drawing less saline water upward, while denser, saltier water sinks in other areas. The subsurface convection lines up exactly with the surface structures. The interior regions of polygons are areas where less salty water rises, and salt instead concentrates along the edges of polygons, where saltier water sinks below the surface while evaporation draws solid salt to the surface.
This snapshot shows a numerical simulation of the subsurface convection and surface evaporation that lead to salt polygon formation. Low salinity areas are yellow, while high salinity ones are black. At the surface, blue regions have the maximum upward flow and red regions have the maximum downward flow. The dark, highly saline fingers under the surface align to the red areas on the surface, indicating areas where salty water is sinking.
It’s a beautiful result that matches the size, shape, and development time observed for salt polygons in the real world. The team even excavated below salt polygons in Death Valley to confirm that the salt content below ground matched their model’s patterns. Since salt playas are a major source for dust and aerosols that affect climate, their work will be an important factor in future climate modelling. (Image credit: feature – T. Nevidoma, simulation – J. Lasser et al.; research credit: J. Lasser et al.; via APS Physics; submitted by Kam-Yung Soh)
Though tough to make out from the surface, our oceans are often covered by cell-shaped clouds stretching thousands of kilometers. This satellite image shows off two such types of marine stratocumulus cloud. Open-celled clouds appear as thin wisps of vapor around an empty middle; in these clouds, cool air sinks through the center while warm air rises along the edges. Open-celled clouds are good rain producers.
On the flip side, closed-cell clouds have a vapor-filled center and breaks in the cloud cover along each cell’s edge. These clouds don’t produce much rain, but they do lift warm, moist air through their middles and let cool air sink along their edges. Closed-cell clouds tend to last much longer than their open-celled counterparts; they can stick around for half a day, whereas open-celled clouds break up in only a couple hours. (Image credit: J. Stevens; via NASA Earth Observatory)
Drops of ethanol on a heated surface contract and self-propel as they evaporate. My first thought upon seeing this was of Leidenfrost drops, but the surface is not nearly hot enough for that effect. Instead, it’s significantly below ethanol’s boiling point. Looking at the drops in infrared reveals beautiful, shifting patterns of convection cells on the drop. The patterns are driven by the temperature difference along the drop; at the bottom, the drop is warmest, and at its apex, it is coldest. Those differences in temperature create differences in surface tension, which drives a surface flow that breaks the drop’s symmetry. The asymmetry, the authors suggest, is responsible for the drop’s propulsion. (Image and video credit: N. Kim et al.)
Though it may look like the Eye of Sauron, this image is actually one of our best-ever glimpses of a sunspot. Captured by the Daniel K. Inouye Solar Telescope, this sunspot is larger than our entire planet, yet we can see details as small as 20km across. The dark central region of the image is the sunspot’s umbra, surrounded by the lighter, streakier penumbra. Along the edges of the image, you see a more typical pattern of bright convection cells. Compared to the rest of the sun’s surface, sunspots are cool — about 1,000 K cooler — due to their intense magnetic field flux inhibiting convection. (Image credit: NSO/AURA/NSF; via Bad Astronomer; submitted by Kam-Yung Soh)
As part of its shakedown, the new Inouye Solar Telescope has captured the surface of the sun in stunning new detail. Seen here are some of the sun’s turbulent convection cells, each about the size of the state of Texas. Hot plasma rises in the center of each cell, cools, and then sinks near the dark edges. Also visible within these dark borders are bright spots thought to mark magnetic fields capable of channeling energy out into the corona. Researchers hope the new telescope will help them uncover the physics behind these processes. (Image and video credit: Inouye Solar Telescope)
Editor’s note: Like several other telescopes located in Hawai’i, the Inouye Solar Telescope was built against the wishes of many native Hawaiians. Although FYFD supports scientific progress, it is my personal belief that scientific advances should not come at the expense of indigenous populations. I strongly urge my scientific colleagues to listen to and work alongside those with concerns about future facilities.
One of my favorite aspects of fluid dynamics is the way that the same patterns and phenomena appear over and over again – sometimes in the most unexpected places. That’s the theme of my new article in American Scientist, which focuses on the connections between our daily lives and the stars:
“Solar energy arises from nuclear fusion reactions in the core, but that energy is buried hundreds of thousands of kilometers beneath the surface, and most of the Sun’s overlying gas is nearly opaque; it hinders light from passing through, like a blanket thrown over a flashlight. Clearly the Sun does shine—but how? For the answer, you can simply go to your kitchen, fill a kettle, and flip on a burner.” #
We typically think of convection in terms of temperature differences, but the real driver is density. In the animations above, cream sitting atop a liqueur is undergoing solutal convection – no temperature difference needed! The alcohol in the liqueur mixes with the cream to form a lighter mixture that rises to the surface. The lower surface tension of the alcohol is also good at breaking up the cream, forming little cells. As the alcohol in those cells evaporates, the cream gets heavier and sinks down into the liqueur, where it can pick up more alcohol, rise back to the surface, and begin the cycle again. (Image credit: J. Monahan et al., source)
Glycerol is a sweet, highly viscous fluid that’s very good at absorbing moisture from the ambient air. That’s why a drop of pure glycerol in laboratory conditions quickly develops convection cells – even when upside-down, as shown above. This is not the picture of Bénard-Marangoni convection we’re used to. There’s no temperature or density change involved; in fact, there’s no buoyancy involved at all! This convection is driven entirely by surface tension. As glycerol at the surface absorbs moisture, its surface tension decreases. This generates flow from the center of a cell toward its exterior, where the surface tension is higher. Conservation of mass, also known as continuity, requires that fresh, undiluted glycerol get pulled up in the wake of this flow. It, too, absorbs moisture and the process continues. (Image credit: S. Shin et al., pdf)
Looking at convective cells, it’s easy to think that they are still and unmoving. But when you add particles, their inner flow becomes obvious. Warm, light fluid moves up through the center of each cell, skims along the surface, and then sinks at the edges of the cell after losing its heat at the cooling surface. Below, the fluid moves back toward the cell center, getting warmer as it’s heated by the lower surface. Once it reaches the middle of the cell, it’s light enough to rise up and start the process again. Convective cells like these are typical in cooking – watch for them forming in your miso soup or hot chocolate – but they can also be found on the sun and even in situations without heating! (Image credit: G. Kelemen, source)
