3D printers are a neat apparatus for exploring flow instabilities. If too much material is extruded compared to the speed of the printer head, coiling takes place. But under-extrusion creates patterns, too. Here, researchers show how under-extrusion can create a stable lace-like pattern. Once dried, the material can stretch, but only in certain directions, a bit like many textiles. (Video and image credit: L. Dreier et al.)
Roman De Giuli’s “High Flow” is vibrant and energetic. Colorful paints and inks flow across the page, creating complex patterns. I love the blossoming flows, feathery fronds, and spreading Marangoni effects. De Giuli’s films never disappoint! (Video and image credit: R. De Giuli)
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.)
Ice stupas are conical artificial glaciers built with snow cannons; they’re used to store water for spring irrigation. Here, researchers explore a miniaturized lab-grown version made from atomized water droplets. The growing drop breaks and spills, forming frozen fingers in all directions. Further drops flow and freeze as rivulets atop the stupa — or they destabilize and rotate toward another finger, leaving behind a wrinkling shape. Although the formation works very differently (and the scales are completely different) these tiny ice stupas remind me of volcanic flows. (Image credit: D. Papa 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.
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)
Imagine a layer of particles sitting at the interface between oil and water. Known as a granular raft, these particles can interact in interesting ways with other objects. Here, researchers experiment with allowing different shapes to fall through the raft. At slow speeds, the raft deforms to coat the object, even if it has a complex shape (top images). At higher insertion speeds, however, the granular raft can break up around the object. The lower sequence of images show a cylinder interacting with the raft. Moving from left to right, each image shows a larger cylinder diameter and an increasingly complex break-up of the raft. (Image credit: C. Gabbard et al.)
In “Alive,” filmmaker Christopher Dormoy explores acrylic paints and the variety of ways in which the medium can be used. From a fluids perspective, there’s dripping, viscous flow, turbulent eddies, billowing plumes, and “accidental painting” due to density-driven instabilities. It’s a fun tour of fluid phenomena in art. What examples do you spot? (Video and image credit: C. Dormoy)
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)
Signs of a ship’s passage can persist long after it’s gone. The churn of its propellers and the oil leaked from its engines leave a mark on the water’s surface that, in some cases, is visible even from orbit. But the frothy wake of a ship is no easy place to measure; there are simply too many bubbles. To reveal the physics behind that froth, these researchers turned to direct numerical simulation, a type of computational fluid dynamics that calculates the full details of a flow, typically using a supercomputer to do so.
In their poster, the blue field of wavy lines shows turbulence under the water’s surface. For (relative) simplicity, the turbulence is statistically uniform — as opposed to matching a particular ship’s wake. The interface between air and water is shown in red. The water surface is complex and undulating, spotted with bubbles trapped below the water and droplets flying through the air. Simulations like these help scientists focus on the detailed mechanisms that connect the turbulent water to the complex air-water surface. Once those are understood, researchers can develop models that approximate the physics for more specific situations, like the passage of a cargo ship. (Image credit: A. Calado and E. Balaras)
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.)