In so much of fluid dynamics, size does not matter. We see the same patterns mirrored across nature from a fuel injection nozzle to galactic clusters. And no one plays with that sense of scale better than artist Roman De Giuli, whose microscale practical effects give the impression of flying above glittering alien coastlines. Ink and paint squeeze around craggy islands, leaving perfect streamlines to mark their passage. Fractal fingers expand like river deltas seeking the path to the sea. Enjoy more of De Giuli’s work on his website and Instagram. (Image and video credit: R. De Giuli; via Colossal)
Pour water out of a bottle, and you’ll see a jet with a shape that resembles chain links. Sometimes known as a “liquid chain,” this phenomenon occurs when water pours through a non-circular hole. It’s quite a complex behavior, as shown in this recent study of the nonlinear effect. Even so, the authors found that the amplitude and wavelength of the chain’s sections are tied directly to the shape of the opening. Current models of the effect don’t account for the viscosity of the liquid, though, so future experiments will have to explore how fluids other than water behave. (Image and research credit: D. Jordan et al.; via APS Physics; submitted by Kam-Yung Soh)
A comparison of an oscillating jet’s shape and metal chains. Each view is rotated 45 degrees from the one before.
Vibrate a liquid pool vertically, and it will form a pattern of standing waves known as Faraday waves. Here, researchers confine those waves to a narrow ring similar in size to the wave. The confinement causes a type of secondary flow — a streaming flow — beneath the water surface. As a result, the wave pattern rotates around the ring. The applications of this rotation are pretty neat. As the team demonstrates, it can drive complex fluid networks and even create a pump! (Image and video credit: J. Guan et al.)
Over the past few years, we’ve seen lots of research in walking droplets, especially as hydrodynamic quantum analogs. But did you know you can replicate this set-up at home and play with it yourself? This video gives an overview of the equipment you’ll need and a simple procedure to follow to get it up and running. From there, your imagination is the limit! (Image and video credit: R. Valani)
Two vertical fibers, with a gap left between them, form a playground for flow in this Gallery of Fluid Motion video. If the fiber spacing is small enough, the flow will form a stable liquid sheet that runs the full length of the fibers. With a little more distance, though, the fluid forms intermittent bridges, whose spacing depends on flow rate. And when the fibers are not perfectly vertical, even more complex flows are possible. I love how a seemingly simple situation begets such complexity! (Image and video credit: C. Gabbard and J. Bostwick)
These “flowers” blossom as two injected chemicals react in the narrow space between two transparent plates. The chemical reaction produces a darker ring that develops a streaky outer edge due to competition between convection and chemical diffusion.
To show how gravity affects the instability, the researchers repeated the experiment on a parabolic flight. In microgravity conditions, no instability formed. That’s exactly what we’d expect if convection (i.e. flow due to density differences) is a major cause. No gravity = no convection. In contrast, under hypergravity conditions, the instability was initially spotty before developing streaks. (Image and video credit: Y. Stergiou et al.)
When two liquid jets collide, they form a falling liquid sheet. Here researchers explore how that sheet breaks up when the liquids involved contain polymers. The intact areas of the sheet show as dark red or almost black. The edges of the sheet appear in brighter red and yellow, outlining the holes that form and grow during breakup. The type of breakup observed depends on the concentration of polymer in the liquid. (Image credit: C. Galvin et al.)
A smoke-like haze obscures colorful bouquets in these photographs from artist Robert Peek. To achieve the effect, Peek submerges his subjects underwater with white dye that sinks due to its greater density. The wakes traced by the dye are impressively laminar, so the dye must drift rather slowly past each petal. The overall effect is beautifully dream-like. You can find more of Peek’s work on Behance and Instagram. (Image credit: R. Peek; via Colossal)
Avulsions — sudden changes in the course of a river — are a river’s equivalent of an earthquake, and they can be similarly devastating for those in the river’s path. In a recent study, authors combed through 50 years’ worth of satellite data to catalog over 100 avulsions and categorize them into three regimes. About a quarter of the observed avulsions took place in the river delta’s fan, where the river spreads out once it exits a canyon or valley. These avulsions, they found, occur when rivers lose confinement and sediment can build up.
This animation of satellite images shows the sudden avulsion — a dramatic change in the river’s course — that took place on the Kosi River in 2008.
Among the other observations, the team linked avulsion location to the river’s flow properties. Most of these remaining avulsions took place in the river’s backwater region, where the river begins to slow down before its outlet. The last category of avulsion took place far upstream of the backwater region on rivers with high sediment flows. During flood conditions, erosion can travel far upstream on these rivers, causing avulsions in unexpected places. Changes in sediment load due to human activities, like deforestation, could even cause rivers to change from the backwater regime to the high-sediment load one. (Image credit: top – R. Simmon/USGS, bottom – S. Brooke et al.; research credit: S. Brooke et al.; via AGU Eos; submitted by Kam-Yung Soh)
Air and blue-dyed glycerin squeezed between two glass plates form curvy, finger-like protrusions. This is a close-up of the Saffman-Taylor instability, a pattern created when a less viscous fluid — here, air — is injected into a more viscous one. If you reverse the situation and inject glycerin into air, you’ll get no viscous fingers, just a stable, expanding circle. Although you sometimes come across this instability in daily life — like in a cracked smartphone screen — the major motivation for studying this phenomenon historically has been oil and gas extraction. (Image credit: T. Pohlman et al.)
