Vibrate a pool of water in air and the interface will form a distinctive pattern of waves called the Faraday instability. But what happens when you vibrate the interface between two fluids that can mix? That’s the question at the heart of this video. The researchers consider the situation both in simulation and experiment, showing how what begins as a smooth interface quickly becomes a thick turbulent mixture. Since the thickness of that mixing layer can be predicted theoretically, this set-up could be useful in industrial applications where mixing is needed. (Video, image, and research credit: G. Louis et al.)
When a vortex ring forms, it’s often from fluid forced through a round outlet, whether that’s someone’s mouth, a pipe, or a dolphin’s blowhole. But vortex rings can come from other shapes, too. This video shows us several examples, including slots and square outlets. The vortex rings blown from a square outlet are messier but still recognizable. The slot-shaped outlets produce even neater results, including twin vortex rings that move parallel to one another! (Image, video, and research credit: B. Steinfurth et al.)
Every day I stand in front of my refrigerator and listen to the water dispenser pouring water into my glass. The skinny, fast-moving jet of water plunges into the pool, creating a flurry of bubbles. Those bubbles come from air the water jet pulls in with it, and the sound the water makes (minus the fridge’s noises) comes from those bubbles. A short, laminar jet will make fewer bubbles and, therefore, be quieter than a a jet that falls farther before hitting the water.
The reason? That tall jet falls for long enough that its walls start to wobble or even break up completely into separate droplets. Compared to a smooth jet, these wobbly or broken-up jets pull in more air and create more bubbles. That makes them louder. Researchers even suggest that listening to these bubbles can give a noninvasive method for finding how much fresh oxygen is in the water. (Image credit: R. Piedra; research credit: M. Boudina et al.; via APS Physics)
Small creatures like springtails and spiders can jump off the air-water interface using surface tension. But larger creatures can water-jump, too, using drag. Here, researchers study drag-based water jumping with a simple elastic hoop. Initially, two sides of the hoop are pulled closer by a string, deforming the hoop. Then, with the hoop sitting upright on the air-water interface, a laser burns the string, releasing the energy stored in the hoop. The hoop’s bottom pushes into the water, generating drag. That resistance provides a reaction force strong enough to launch the hoop.
Compared to the hoop’s jumps off land, it’s slower to take-off from water, and it’s less efficient at jumping. Lighter hoops, however, jump better off water than heavier ones — a wrinkle that isn’t seen in ground jumpers. That suggests that weight reduction is more important for aquatic jumpers than for their terrestrial counterparts. (Image and research credit: H. Jeong et al.)
Water evaporates faster in dry conditions than in humid ones, but the same isn’t true of paint. Instead, paint’s drying time is largely independent of the day’s humidity. That’s because of paint’s long chains of polymers. As water in the paint evaporates, these polymers are drawn to the surface, forming a viscoelastic layer that hinders evaporation and keeps the drying rate independent up to about 80 percent humidity.
Illustration depicting evaporation of water (left) and evaporation of a polymer solution (right). As water evaporates from the polymer solution, it draws polymers to the surface, where they form a layer that hinders evaporation and makes its rate independent of humidity.
The polymer layer explains why evaporation isn’t affected by humidity at longer times, but researchers also saw humidity-independent evaporation early in their experiments. Under a microscope, they discovered a thin gel layer (top image) covering the air-polymer interface. They propose that this fast-forming layer further hinders evaporation. Their findings may be significant for virus-laden respiratory droplets, which also contain polymers. (Image and research credit: M. Huisman et al.; see also J. Salmon et al.; via APS Physics)
A particle suspended in a liquid typically jitters haphazardly about as it’s struck randomly by nearby liquid molecules. But when a temperature gradient is applied to the liquid, that random motion instead becomes directional. In a recent study, researchers directly mapped the motions underlying this thermophoresis.
In their experiment, the team placed a 7-micron sphere in water laced with 500-nanometer fluorescent tracers. Using a laser, they optically trapped the sphere, pinning it in place. Then, with a second laser, they heated the water on one side of the sphere and observed, under a microscope, what happened. After a few seconds, the tracers began moving toward the hot region, creating a slip flow along the surface of the sphere. Had the sphere been able to move freely, they found, the flow would have been strong enough to move it. (Image and research credit: T. Tsuji et al.; via APS Physics)
Air pushes into a thin gap filled with water and granular particles in the labyrinth-like image above. The encroaching air pushes grains like a bulldozer’s blade, building up a compacted wall. The invasion continues until the pressure of the air is countered by the combined capillary and frictional forces of the wet grains. Researchers built an analytical model that explains how these frictional fingers form and grow. Unlike Saffman-Taylor fingering patterns, which depend on long-range viscous forces, these patterns depend entirely on short-range forces from surface tension and friction. (Image and research credit: E. Flekkøy et al.)
Water drops slide down spiderwebs, along the spines of desert plants, and across the armored exterior of horned lizards. Thin, grooved surfaces like these pop up frequently in nature when organisms need to direct water. A recent study of droplets sliding on fibers suggests why.
A drop sliding down a fiber is constantly shrinking, leaving a little of itself behind as a thin film that coats the fiber. The thicker a fiber is, the slower the drop moves along it. Similarly, if you bundle multiple fibers together, a drop will travel slower along the thicker bundle. But, to the researchers’ surprise, droplets actually travel faster on bundles than they do along single fibers of the same overall diameter. The key to this result seems to be the tiny grooves between fibers in a bundle. Water fills these areas, creating a “rail” along which the droplets slide more efficiently.
The team hope to put their new insights to use on a water harvester that could help capture precious moisture in arid environments, much like those desert-dwelling plants and lizards do. (Image and research credit: M. Leonard et al.; via Physics World)
Wave action can be a major source of erosion along riverbanks and shorelines. But in a recent study, scientists were able to perfectly absorb incoming waves to create a downstream region with calm, wave-free waters.
Experimental data shows that waves approaching from the left interact with the resonant chambers and get perfectly absorbed, leaving the water on the right side still.
The group began with a narrow channel that waves could move down. They added two small, side-by-side cavities perpendicular to the channel; as waves travel down the channel, they resonate with the cavities, which reflect and transmit their own waves back into the channel. With the right tuning to the size and spacing of the cavities, the team was able to make the cavities’ waves perfectly cancel the channel’s waves. The group demonstrated this absorption theoretically, numerically, and experimentally.
Currently, they’ve only managed perfect absorption with a single wave frequency, but an array of cavities should be able to absorb a range of incoming waves. The authors hope their work will one day help protect coastal structures and prevent erosion by countering incoming waves. (Image and research credit: L-P. Euvé et al.; via APS Physics)
WASP-96B is a tidally-lockedexoplanet between the size of Saturn and Jupiter. This hot, massive planet lies close to its star, orbiting in less than three-and-a-half Earth days. A recent study shows that planets like these can have very different weather, depending on what depth their atmosphere absorbs heat at.
Using numerical simulations, researchers took a detailed look at the possible atmospheric dynamics on this planet. When the atmosphere absorbed heat at a shallow depth — near the outer layers of the planet — a coupled vortex pair formed (left, below). These vortices promenaded westward and completed a circuit around the planet every 11-15 days.
Shallow heating on a hot Jupiter produces a pair of coupled vortices (left), but deeper heating in the atmosphere generates four more-chaotic vortices (right).
In contrast, deeper heating produced a more-chaotic pattern of four vortices (right, above) that each lasted 3 to 15 days before disappearing, replaced by a new vortex. This atmosphere, they found, was very turbulent, with smaller-scale vortices as well.
Since each weather pattern is visually distinct and carries its own brightness signature, the authors predict that additional observations of WASP-96b with the current generation of telescopes will show which type of heating dominates on the exoplanet. (Image and research credit: J. Skinner et al.; via APS Physics)
Snapshots from a simulation of a deep-heated hot Jupiter. Each image shows the planet on a different consecutive day.
