Stingrays swim over rippled sands in this award-winning image from photographer Gregory Sherman. Above this peaceful vista, waves break and a dramatic skyscape looms. It’s the regular back-and-forth of the waves that shapes the ripples of sand. The rays can also use the sand for cover; note the remnants of sand atop a few of these rays, and then see how they hide themselves. See more winners from the Underwater Photographer of the Year awards in this gallery. (Image credit: G. Sherman/UPY 2023; via Colossal)
Month: April 2023
Stellar-Wind-Shaped Nebula
Stars about 100 times more massive than our sun live fast and die young. They burn through their hydrogen supply quickly, then start fusing heavier elements. As they do, their strong stellar winds start blowing off the exterior layers of the star. That’s the story behind WR 40, the star at the center of Nebula RCW 58. The nebula itself is made up of material blown off the star, carved into turbulent filaments by stellar winds. (Image credit: M. Selby and M. Hanson; via APOD)
Bouncing Bullets Off Water
With the right shot, it’s possible to skip a bullet off water, as shown in this video from the Slow Mo Guys. The angle of the bullet relative to the water needs to be quite shallow, as this sets the bullet up for the hydrodynamic lift needed to skip. Physically, the mechanism for skipping a bullet is similar to rock-skipping. The bullet’s impact creates a cavity that the bullet rides. With the right conditions, the cavity orients the bullet upward, creating the lift needed to skip. (Video and image credit: The Slow Mo Guys)
Soap Film Ruptures
Soap film ruptures are well understood for your typical bubble solution, but what happens when tiny particles get added to the soap film? That’s the question in this recent study. Researchers added 660-nanometer particles, in varying amounts, to their soap films to see how it affected rupture. When they broke the films just after formation (top image), they found results that were quite similar to the usual, particle-free case. But when the films sat for awhile before breaking spontaneously (bottom image), the rupture caused wrinkling and folding similar to a piece of fabric. The researchers hypothesize that aging allowed the soap film to thin until the film and the particles were similar in size. Then, when the film ruptured, the particles affected how it broke up. (Image and research credit: P. Shah et al.)
After aging and thinning, a colloidal film ruptures spontaneously, forming fabric-like wrinkles. 
Spiral Ice Cracks
This odd puddle was found in Arizona after a night with low temperatures around -8 degrees Celsius (18 degrees Fahrenheit). Unlike the concentric rings sometimes seen on ice, this puddle formed one spiraling crack. It’s hard to know exactly what factors played into this formation since it was only found after the fact, but one possibility is that the puddle was initially frozen in a continuous sheet. Then, as the temperature cooled overnight, the ice contracted, forming a crack. As the ice kept cooling and contracting inward, the crack grew, spiraling toward the center of the puddle. (Image credit: M. Hendrickson; via EPOD; submitted by Kam-Yung Soh)
A Glimpse of Earth’s Interior
Lava spurts from the Fagradalsfjall volcano in Iceland in this award-winning photo by Riten Dharia. It’s always bizarre to see molten rock flowing in fountains and rivers because it’s so unlike our daily experiences. Some deeply buried areas of the Earth, including the outer part of the core, are often described as liquid rock, which brings to mind lava. But that’s not, in fact, what those regions are like. If you were to visit Earth’s outer core in some super-submersible, you would not find a sea of lava. Instead, you would find yourself surrounded by what seemed to be solid rock. That’s not to say that the outer core is solid — just that it flows on geological timescales that are far longer than any human’s lifetime! (Image credit: R. Dharia; via Gizmodo)
Curved Cracks
When mixtures of particles and fluids dry, they typically leave a pattern of straight cracks. Here researchers explore what happens when the drying film contains bacteria from the family E. coli. Instead of straight cracks, the films form curved ones. With bacteria that rotate or tumble, the crack pattern is spiral-like. With bacteria that swim, the remaining pattern consists of circular cracks. Thus, the motility of the bacteria affects how cracks form and spread. (Image and research credit: Z. Liu et al.)
Nacreous Clouds
Iridescent clouds shine bright over this Finnish sunset. These colorful clouds are nacreous clouds, also known as mother-of-pearl clouds. Formed from ice crystals during frigid conditions in the lower stratosphere, these clouds are most visible before dawn and after sunset, when their high altitude catches sunlight while the lower atmosphere doesn’t. These rare clouds form mostly in high latitudes during winter. While they appear similar to other iridescent clouds that occur all over the world, nacreous clouds are far brighter and more vivid. (Image credit: D. Lehtonen; via APOD)
Superradiance in Fluids
A group of excited atoms can collectively emit more photons than they could individually in a phenomenon known as superradiance. Now researchers have shown that vibrating fluids can produce superradiance as well.
Two different wave fields used in the experiment, each with a different distance between the circular cavities. Similar to other hydrodynamic quantum analogs, the researchers vertically vibrated a pool of liquid at a frequency that produced Faraday waves. Beneath the pool, they placed two circular wells, varying the distance between them to observe how their wave fields interacted. With a large enough vibration, the two circular wells emitted droplets (top image), and the number of droplets they produced was higher than expected for two independent wells, indicating superradiance. The results suggest that it may be possible to build even more hydrodynamic analogs of quantum systems than previously thought! (Image and research credit: V. Frumkin et al.; via APS Physics)
Placental Fluid Dynamics
The placenta, critical as it is to human life and development, is likely the least-studied organ in the body. Reasons for that abound, from the ethics of studying pregnant people to the historical marginalization of female bodies in medical studies. But what little we do know shows that the placenta is quite incredible.
Shaped somewhat like a flattened cake, the placenta contains a tangle of fetal blood vessels — an estimated 550 kilometers’ worth — bathed in maternal blood. The enormous surface area — nearly 13 meters squared — enables the exchange of oxygen, glucose, and urea through diffusion. These exchanges don’t take place in still conditions, though; blood is always flowing through the vessel network. This means that each exchange depends on both the speed of diffusion and the speed of the flow, a relationship that’s captured with the dimensionless Damköhler number.
Illustration of the intertwined blood vessels of the placenta. Some exchanges, like carbon monoxide and glucose, are diffusion-limited, meaning that increased blood flow cannot speed up the process (though additional blood vessel surface area could). In contrast, carbon dioxide and urea are flow-limited exchanges. Fascinatingly, oxygen is close to being both diffusion- and flow-limited, suggesting that the organ has optimized for this exchange. Since pregnancy also involves a large increase in maternal blood volume and changes in lung capacity to help provide oxygen, it seems like the pregnant body heavily emphasizes delivering oxygen to the developing fetus. (Image credit: newborn – J. Borba, placenta – iStock/Sakurra; via Physics World; submitted by Kam-Yung Soh)
