With the right lighting and eye, billowing streaks of dye and paint can become the ethereal wisps of silk. Artist Susi Sie explores this dichotomy in “Silk,” a short, fluidic film made entirely in macro. Sometimes there’s astounding beauty in the complexity of a fluid filmed up close. (Video and image credit: S. Sie)
Satellite imagery has revolutionized remote sensing and our ability to observe the world around us. But peering past the surface of water has always been next to impossible. We might be able to see the extent of a coral reef from a photo, but thanks to the interplay of light and water, the details are too blurry to identify what species we’re looking at.
To solve this issue, researchers decided to work backwards, taking everything we understand about the physics of light – refraction, reflections, and so on – and using it to remove the distortions. The result is NASA’s FluidCam, an instrument capable of of taking a video of shallow waters less than 10 m deep, processing it, and producing images with sub-centimeter accuracy showing what lies beneath. Tests in American Samoa revealed details fine enough that scientists were able to identify multiple coral species as well as many of the species of fish inhabiting the reef.
With coral reefs changing quickly, this technology may be invaluable for monitoring coral health without actively disrupting these delicate systems. (Image credit: N. Usry; research credit: V. Chirayath and A. Li; via OceanBites; submitted by Kam-Yung Soh)
Whether in groups or as individuals, humpback whales are canny hunters. They herd prey together by encircling them and releasing bubbles that form a “net” that bars escape. Then, the whales lunge through the center with open mouths, gathering prey. Scientists have long wondered whether humpbacks’ unusually long pectoral fins played any role in their hunting. New drone observations of whales feeding (see video below) are beginning to provide some hints.
The scientific teams observed multiple individual whales feeding under the same circumstances and found that the whales used their fins quite differently. Both used them as additional barriers to prevent prey from escaping, but one whale favored a horizontal fin position that created currents that helped sweep prey into its mouth. The other whale used a more vertical fin position that, while hydrodynamically unfavorable, exposed its bright underside, which seemed to startle prey into fleeing into its darker, more inviting mouth. (Image credit: K. Kosma; video credit: M. Kosma; research credit: M. Kosma et al.; via Science)
Embers fly through the Kincade wildfire leaving streaks of light that reveal the strong winds helping drive the fire. This unintentional flow visualization mirrors techniques used by researchers to understand how flows are moving. The shutter of the camera remains open for a fixed time, so the length of each streak tells us about the speed of the flow. Longer streaks occur where embers moved faster.
Here we see the longest streaks in the upper left side of the image, which tells us that the wind was moving faster there than it did at lower heights, like near the photographer in the picture. That’s in keeping with what we would expect. In general, winds move faster above the ground than they do near the surface. That speed difference is one of the reasons wildfires are so difficult to contain; a single ember caught by high winds is easily carried to unburnt areas, allowing the fire to spread more quickly than if it had to burn along the ground. (Image credit: J. Edelson/Getty Images; via Wired)
Scientists have observed distinctive differences in the way soap bubbles freeze depending on their environment. If a bubble is surrounded by room temperature air but placed on a cold surface (top), it freezes from the bottom up, with a clear freeze front that slowly creeps upward.
In contrast, bubbles in an isothermal environment – one where it’s equally cold everywhere – freeze with a snow-globe-like effect of ice crystals (bottom). This freezing mode is actually triggered by a Marangoni flow. As the thin bottom layer of the soap bubble begins to freeze, it releases latent heat. That local heating changes the surface tension enough to generate an upward flow. You can see the plumes form right as the bubble touches the surface. Those plumes lift up tiny ice crystals, which continue to grow, ultimately forming the snowy crystals we see take over the surface. (Image and research credit: S. Ahmadi et al.; submitted by Kam-Yung Soh)
Over the past couple decades, microfluidic devices have become a staple of medical and biological diagnostics and analysis. Tests that once required large and specialized equipment can now be completed closer to a patient, using only a few drops of sample fluid. Running multiple tests on a single chip can become difficult, though, since flow through the device tends to dissolve and mix the dried reagents used for tests. But a new method cleverly uses fluidic forces to keep reagents separated without the need for complicated microfluidic structures.
The basic concept is outlined in the illustration above. You’re looking down on a microfluidic channel that’s long and very thin. A shallow groove down the middle serves as a barrier by pinning the contact line of the incoming fluid. So when the sample fluid flows in through the inlet on the left, it will only fill the top half of the cell. When it reaches the far right side, it turns the corner and flows to the left, encountering the first of the dried reagents it must dissolve for the device’s tests. The fluid will fill the lower channel quickly and then come to rest while the reagents dissolve.
With both sides of the channel full of liquid, the shallow barrier can no longer hold, and the fluid will take up the full width of the channel, with two well-dispersed – but separated – regions of reagents. Once that’s happened, a valve – represented by the pale blue line near the right side of the illustration – releases the fluid into the next section of the chip, allowing the analysis to proceed. (Image credit: Nature; research credit: O. Gökçe et al.; submitted by Kam-Yung Soh)
Where cold and warm air meet, our atmosphere churns with energy. From the turbulence of supercell thunderclouds to the immense electrical discharge of lightning, there’s much that’s breathtaking about stormy skies. Photographer Dustin Farrell explores them, with a special emphasis on lightning, in his short film, “Transient 2″.
As seen in high-speed video, lightning strikes begin with tree-like leaders that split and spread, searching out the path of least resistance. Once that line from cloud to ground is discovered, electrons flow along a plasma channel that arcs from sky to earth. The estimated temperatures in the core of this plasma reach 50,000 Kelvin, far hotter than the Sun’s surface. It’s this heating that generates the blue-white glow of a lightning bolt. The heating also expands the air nearby explosively, producing the shock wave we hear as a crash of thunder. (Images and video credit: D. Farrell et al.; via Colossal)
Clouds spiral behind the islands of Tenerife and Gran Canaria in this nighttime satellite imagery. Although it’s not entirely unusual to see these von Karman vortex street clouds in the wakes of islands, this is the first time I’ve seen them at night. They form when winds off the ocean are forced up and around rocky islands. Like air moving past a cylinder, the flow forms a swirling vortex off one side of the island, which separates and moves downstream while another forms on the island’s opposite side. When the resulting flow mixes with a cloud layer, we can see the pattern from space. (Image credit: J. Stevens; via NASA Earth Observatory)
Moisture is cotton candy’s natural enemy. The spun sugar dissolves incredibly quickly under the influence of even a couple drops of water. Why that’s so is clearer when looking at a single fiber. Inside the droplet there’s a gradient in the sugar concentration. The more sugary water sinks, and the sugar fiber dissolves more quickly in the upper part of the droplet, where the less sugary water can more easily take up new sugar.
Once the fiber breaks, capillary forces draw the droplet upward, giving it a fresh section of fiber to dissolve. In a web of fibers, this process can pull droplets apart and together as they quickly eat through the spun sugar. (Image and video credit: S. Dorbolo et al.; submitted by Alexis D.)
Reader Matt G asks:
[What’s] going on here?
Why’s the pattern square? Just a special case of waves traveling in different directions, and this photo happened to catch some at right angles to one another?
You’re not far off, Matt! This is an example of cross sea, where wave trains moving in different directions meet. Like most ocean waves, these waves originated from wind moving over the water. As the wind blows, it transfers energy to the water, disturbing what would otherwise be a smooth surface and setting up a series of waves. Oftentimes, these waves can outlast the wind that generates them and travel over long distances of open water as a swell.
Cross seas occur when two of these wave systems collide at oblique angles. They’re most obvious in shallow waters like those seen here, where the depth makes their criss-cross pattern clearer. Another name for them is square waves, and although the pattern isn’t a perfect square, it’s usually fairly close. If the waves aren’t separated by a large angle, they’re more likely to merge than to create this sort of pattern.
Neat as cross seas look, they’re quite dangerous, both to ships and swimmers. Ships are built to tackle waves head-on and don’t fare well when they’re forced to take waves from the side. For swimmers, the danger is a little different. Cross seas create intense vorticity under the surface and can generate stronger than usual riptides that sweep the unwary out to sea. (Image credit: M. Griffon)
