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.)
Category: Research
Sliding on Fibers
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)
Calming the Waves
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)
Exoplanet Heating
WASP-96B is a tidally-locked exoplanet 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. 
Swarm of Surfers
Self-propelled objects can form fascinating patterns. Here, researchers investigate how small plastic “surfers” move on a vibrating fluid. Each surfer is heavier in its stern than its bow. When the fluid vibrates, the surfer creates waves that are asymmetric — deeper in the stern than at the bow. For single surfers, this imbalance propels the surfer in the direction of its bow. But with more than one surfer, other patterns form.
The video demonstrates five of the seven patterns pairs of surfers exhibit. The team looked at groups of surfers all the way up to eight members. Among pairs, the researchers found seven distinctive patterns, including orbiting groups, tailgaters, and promenading pairs. Larger groups, they found, had similar collective behaviors. They hope their surfers will be an easily accessible platform for exploring active matter. (Image and research credit: I. Ho et al.; via APS Physics)
Controlling Finger Formation
When gas is injected into thin, liquid-filled gaps, the liquid-gas interface can destabilize, forming distinctive finger-like shapes. In laboratories, this mechanism is typically investigated in the gap between two transparent plates, a setup known as a Hele-Shaw cell. In the past, researchers looking to control the instability have explored how surface tension, viscosity, and the elasticity of the gap itself affect the flows. But a new set of studies look at the compressibility of the gas being injected.
The team found that viscous fingers formed later the higher the gas’s compressibility. That provides a potential control knob for people trying to exploit the mechanism, especially geologists. For geologists trying to extract oil, viscous fingering is detrimental, but, on the flip side, viscous fingers are desirable when injecting carbon dioxide for sequestration. With these results, users can tweak their injection characteristics to match their goals. (Image credit: C. Cuttle et al.; research credit: C. Cuttle et al. and L. Morrow et al.; via APS Physics)
Ice Damages With Liquid Veins
Water expands when it freezes, a fact that’s often blamed for ice-cracked roads. But expansion isn’t what gives ice its destructive power. In fact, liquids that contract when freezing also break up materials like pavement and concrete. A recent study pinpoints veins between ice crystals as the source of this infrastructure-cracking power.
Ice doesn’t like to stick on most surfaces, so when it forms, there’s often a narrow gap between the ice and a solid surface. That gap fills with water, and that water, it turns out, doesn’t just sit there. Instead, grooves between ice crystals act like tiny straws that are frigid on the icy end and warmer on the end connected to water. As ice forms on the cold end, it creates a negative pressure gradient that draws liquid up the groove. This ‘cryosuction’ keeps pumping water into the ice, where it freezes and further expands the icy zone, as seen in the image below.
Under a microscope, fluorescent particles show water (right side) getting pulled into an ice groove (left). If the ice is made up of a single crystal, this growth rate is very slow. But most ice is polycrystalline — made up of many crystals, all separated by these liquid-filled grooves. That, researchers found, is a recipe for fast growth and quickly-expanding ice capable of breaking concrete and other structures. (Image credits: pothole – I. Taylor, experiment – D. Gerber et al.; research credit: D. Gerber et al.; via APS Physics)
Granular Gaps
Push air into a gap filled with a viscous fluid, and you’ll get the branching, dendritic pattern of a Saffman-Taylor instability. Here, researchers use a similar set-up: injection into a narrow gap between transparent planes to explore something quite different. In this experiment, the gap was initially filled with a mixture of air and tiny hydrophobic glass beads. When the team injected a viscous mixture of water and glycerol, new patterns emerged. At low injection rates, a single finger structure formed. But at high injection rates, a whole spoke-like pattern formed. (Image and research credit: D. Zhang et al.; via Physics Today)
The Jumping Jump
Turn on your kitchen sink, and the falling jet may form a circle of shallow flow where it strikes the sink. This fast-moving region of flow, surrounded by a wall of water, is a hydraulic jump. A recent study delves into a previously-missed phenomenon of this flow: intermittent disruption and reappearance.
An oscillating hydraulic jump, viewed from below. The team found that, within a narrow range of jet and surface sizes, a hydraulic jump will periodically appear and disappear. The effect comes from the hydraulic jump itself; waves from the jump propagate outward, hit the edge of the circular plate, and reflect inward. When the incoming and outgoing waves interfere, it floods the jump zone, making it disappear briefly. (Image credit: sink – Nik, jump – A. Goerlinger et al.; research credit: A. Goerlinger et al.; via APS Physics)
Turbulent Thermal Convection
In the winter, warm air rises from our floor vents or radiators, creating a complex, invisible flow in the background of our lives. Buoyancy lifts warmer air upward while cooler, denser air sinks back down. This thermal convection is everywhere: in our buildings, the ocean, the sky overhead — even in the visible layer of our sun.
In nature, these systems are so large and complex that fully measuring or simulating them remains impossible. Instead, researchers focus on a simplified system — a Rayleigh-Bénard cell — that’s essentially an idealized version of a pot on a stovetop. The lower surface of the cell is heated — like the bottom of a pan on the burner — while the upper surface of the fluid cools. Even this idealized system is a challenge, though, and neither lab-scale versions nor simulations can reach the same conditions that we find in nature.
To bridge the gap, scientists rely on mathematical models — theories built on our best understanding of the physics — and physical analogies to similar systems — like flow over a flat plate — that are “easier” to measure. For a thorough overview of recent work in the area, check out this review in Physics Today. (Image credit: A. Blass; research credit: D. Lohse and O. Shishkina in Physics Today)
