Qinghai Lake sits in western China, where a warmer and wetter climate has been raising the lake’s water level in recent years. These two satellite images, from 2010 and 2022, show the effects of those changes. Sand spits that once separated the smaller Shadao Lake from the surrounding lake have worn away and sunk, rejoining the two bodies of water.
Why is the area’s warmer climate also wetter? The lake has risen due to increased precipitation and river run-off feeding it, but it’s also seen less evaporation. So far the area’s temperature increases have been most notable in winter months, when the lake is covered in ice. In contrast, the summers have been wetter, which means more cloudy days and less chance for evaporation. (Image credit: A. Nussbaum, modified by N. Sharp; via NASA Earth Observatory)
At the bottom of ponds, nematodes and other creatures swim in a world of mud. They squirm their way through a sediment of dirt particles suspended in water. Mud, of course, is notoriously impossible to see through, so to understand these creatures’ movements, scientists turn instead to biorobotics. Here, a team uses a magnetic head attached to an elastic tail to mimic these tiny creatures.
To drive the robot’s motion, they use an oscillating magnetic field, which forces the magnetic head to rotate. Combined with the elastic tail and the drag caused by surrounding materials, this causes the robot to swim in a fashion similar to its biological inspirations.
A biomimetic robot swims through immersed grains. The robot’s magnetic head is forced with an oscillating magnetic field. It swims through an underwater bed of hydrogel beads, with diameters smaller than that of the robot’s head.
To mimic the muddy environment of a pond’s bottom, scientists used a bed of hydrogel beads immersed in water. Looking at the experimental video above, you’ll see no sign of the beads. That’s because the hydrogel beads have nearly the same index of refraction as water. Once you pour water in, they seem to disappear. That allows the researchers to focus instead on the robot’s motion. In other experiments, they added dye to the beads so that they could see how they moved around the robot.
They found that the robot’s motion fluidizes the grains around it. Effectively, the robot’s motion creates an area with fewer grains and more water for it to move through. Once it’s passed, however, more grains settle in, and the bed returns to a denser packing. (Image credit: nematode – P. Garcelon, experiment – A. Biswas et al.; research credit: A. Biswas et al.)
Moving across sand is quite challenging for bipedal creatures like us, but other animals have their ways. Photographer Paul Lennart Schmid caught this snake on the move, with impressions of its passage still in the sand. X-ray observations of snakes moving in sand show that they swim through the granular medium. Snakes are quite efficient in their swimming, moving most of their body through the tunnel created by their head, thereby reducing their overall effort. (Image credit: P. Schmid; via Nature TTL POTY)
Liquid crystals, bottles of pills, and hoppers of grains can all involve disk-shaped particles. To better understand how disks pack together, researchers studied how disks in a box orient themselves after shaking. They used MRI to observe the disks’ interior packing.
These reconstructions show the packing found in the experiment. The disks are color-coded by orientation; horizontal disks are redder and vertical ones are bluer. Initially, the packing has many horizontal disks (left), but after shaking, the disks get more compacted (right). The disks form short stacks that are randomly oriented. This increases the overall density but the random orientations reduce the total alignment of disks.
The team found that shaking increases the disks’ density, but that increase does not come from disks orienting in the same direction. Instead, the disks form short stacks of similarly-oriented disks. The stacks themselves took on many different orientations, which reduced the system’s overall alignment in orientation. (Image credit: coins – M. Blan, packing – Y. Ding et al.; research credit: Y. Ding et al.; via APS Physics)
In snowy mountainous regions, avalanches are a dangerous and destructive problem. Researchers studying the mechanisms of these flows have a suggestion: plant more trees. A group of researchers found that a “forest” of regularly spaced pillars slowed avalanches by as much as two-thirds. On an empty slope, the avalanche picked up speed as its thickness grew. But with regularly-spaced pillars the slower flow rate became almost completely independent of avalanche thickness.
The researchers with their avalanche set-up, which releases glass beads through a forest of pillars.
For now, the researchers suggest placing trees every 3 meters on steep, avalanche-prone slopes — a technique that, admittedly, only works for slopes below the treeline. In their next round of experiments, the researchers plan to see how a randomly arranged forest affects an avalanche. (Image credit: top – N. Cool, apparatus – Université Paris-Saclay/FAST; research credit: B. Texier et al.; via Physics World)
Sometimes landscapes have a beauty that’s hard to see from the ground. This astronaut’s photo shows a dune field in the sand seas of Saudi Arabia. Vast linear dunes line up along the direction of prevailing winds. Atop these dunes are more complex formations, star dunes, that are built up in the wake of changing winds. Built from three or more intersecting arms, the star dunes are steeper than the linear dunes they sit atop. Such complex dune fields — with multiple types of dunes — form in areas with especially abundant sands. (Image credit: NASA; via NASA Earth Observatory)
The clear plastic disks use to study clogging appear rather plain — at least until you look at them through polarizers. Then the disks light up with a web of lines that reveal the unseen forces between the particles. In this video, researchers use this trick to explore how spontaneous clogs occur. If particles jam together into an arch, that bridge can be strong enough to hold the weight of all the particles above it, bringing the flow to a halt. Some arches aren’t strong enough to hold for long; they can break in moments. Other more stable arches persist. By watching the flow through polarizers and carefully tracking the ebb and flow of the forces between particles, researchers can predict which clogs will have staying power. (Video credit: B. McMillan et al.)
Getting around on sandy slopes is no easy feat. On steep inclines, even small disturbances will cause an avalanche. The predatory antlion takes advantage of this fact by building a conical pit that makes ants that walk in slide down into its waiting jaws. But a new study shows that it’s more than just pressure that determines when an object slides down the slope.
To simulate hapless ants sliding into an antlion’s pit, researchers used plexiglass disks with four smaller disks that act as legs on the granular slope. By varying the distance between these points of contact, researchers found that stance also affects when a slide starts. The closer together the contacts are, the more likely the disk would slide. In contrast, spreading the points of contact increased stability, meaning that adopting a wider stance could keep an animal, human, or robot from sliding as easily. (Image credit: NEOM; research credit: M. Piñeirua et al.; via APS Physics)
The ancient Greeks first recognized static electricity, but the mechanisms behind it remain somewhat mysterious. In particular, it’s unclear how two pieces of the same material can build a charge between them simply by touching. Yet we regularly see examples of this when volcanic ash creates enough charge to discharge lightning. A new study sheds light on the question by studying the impact of a single grain of silica on a silica disk.
The researchers used acoustic levitation to hold their silica particle in place. By turning the acoustic waves off, they could bounce the grain off the disk, then catch the particle again with the acoustic field. After a bounce, they swept an electrical field across the particle and observed its oscillations to determine how much charge the particle held. When necessary, they could also discharge the particle.
This animation demonstrates the three phases of an experiment. In the first (left), the acoustic field is shut off, allowing the silica grain to fall and strike the disk. Then the field is turned back on to “catch” the particle. In the second phase (middle), the researchers use a sweeping electrical field to determine the charge built up on the grain. In the third phase (right), they periodically discharge the built-up charge on the particle.
What they found was that charge on the particle grew with the number of impacts. They also saw that they could reverse the polarity of the charge with careful cleaning and baking of their objects. Their conclusion is that adsorption of water from the surrounding air is what enables the build-up of static charge on identical materials. (Image credit: volcano – M. Szeglat, experiment – G. Grosjean and S. Waitukaitis; research credit: G. Grosjean and S. Waitukaitis; via APS Physics)
Photographer Mikko Lagerstedt specializes in Nordic landscapes, like the windswept snow seen here. I love the way he’s captured the snow that gets picked up and blown by the wind. Notice the hazy layer of snow hovering over the foreground. This snow is saltating, just as sand does in the desert. When flakes get picked up by the wind, they follow a ballistic trajectory, much like a cannonball in a high-school physics class. As the snow crashes back down, its impact knocks up more flakes, and the process continues. Repeat enough times, and you’ll see this hazy layer of blowing snow blanketing a snowscape. (Image credit: M. Lagerstedt; via Colossal)
