In the art of Akiko Nakayama, colors branch and split in a tree-like pattern. In studying the process, researchers found the physics intersected art, soft matter mechanics, and statistical physics. In dendritic painting, the process starts with an underlying layer of acrylic paint, diluted with water. Atop this wet layer, you place a drop of acrylic ink mixed with isopropyl alcohol.
The combination of both layers is key. The alcohol-acrylic drop on a Newtonian substrate will show spreading, driven by Marangoni forces, but no branching. It’s the slightly shear-thinning nature of the diluted acrylic paint substrate that allows dendrites to form. As the overlying drop expands, it shears the underlayer, changing its viscosity and allowing the branches to form. You can see video of the process here. (Image credit: A. Nakayama; research credit: S. Chan and E. Fried; via Physics World)
Drop a ball that’s partially filled with water and it may or may not bounce. Why the difference? It all comes down to where the water is before impact. The more distributed the water is along the walls, the less likely a container will bounce. Researchers found they could control the bounce by spinning the bottles before they dropped. Centrifugal force flings the water all over the walls of the spinning bottle, and, when impact happens, the water concentrates into a central jet. For the spinning bottles, that jet is wide, messy, and swirling; it breaks up quickly, expending energy that could otherwise go into a bounce. In effect, the spinning bottle’s jet forms quickly enough to “stomp” the rebound. (Video and image credit: A. Martinez et al.; research credit: K. Andrade et al.)
Soap films are a great system for visualizing fluid flows. Researchers use them to look at flags, fish schooling and drafting, and even wind turbines. In this work, researchers explore the soap film’s reaction to lasers. When surfactant concentrations in the soap film are low, laser pulses create shock waves (above) in the film that resemble those seen in aerodynamics. The laser raises the temperature at its point of impact, lowering the local surface tension. That temperature difference triggers a Marangoni flow that draws the heated fluid outward. The low surfactant concentration gives the soap film relatively high elasticity, and that allows the shock waves to form.
In contrast, a soap film with a high concentration of surfactants has relatively little elasticity. In these films (below), the laser creates a mark that stays visible on the flowing soap film. This “engraving” technique could be used to visualize flow in the soap film without using tracer particles. (Image and research credit: Y. Zhao and H. Xu)
When surfactant concentrations are high, a laser pulse “engraves” spots onto a flowing soap film. Shown in terms of interference (left) and Schlieren (right) imaging.
Water striders spend their lives at the air-water boundary, skittering along this interfacial world. But what happens when falling rain destroys their flat existence? That’s the question that motivated today’s research study, which looks water striders subjected to artificial rain.
Although the water drops themselves are far heavier than the insects, the water doesn’t strike hard enough to injure the insects. Neither a direct impact nor the forces from a neighboring impact, the researchers found, were enough to pose a problem for the water strider’s exoskeleton. Instead, they’re more likely to get flung or submerged, as follows:
The initial impact of a raindrop creates a large crater. Depending on the position of the insect relative to the point of impact, this may fling the insect away or pull it down into the cavity.
When the drop hits, it creates a big crater in the water’s surface. Insects to the outside of the splash get flung outward, while those closer to the point of impact ride the crater wall downward. As the crater collapses, it forms a thick jet that pushes nearby water striders up with it.
As the initial cavity collapses, it creates a large jet that can push the strider into the air.
As that initial jet collapses, it forms a second crater, which — being smaller and narrower — collapses much faster than the first one. That action, researchers found, often submerges a water strider caught in the crater.
The first jet’s collapse creates a second crater, and it’s this one that tends to trap and submerge the water strider underwater.
Fortunately for the insect, their water-repellent nature means they’re covered in a thin bubble of air that lets them survive several minutes underwater. That’s time enough for the water strider to rescue itself. (Image credit: top – H. Wang, animations – D. Watson et al.; research credit: D. Watson et al.; via APS Physics)
In nature, some powerful tornadoes form additional tornadoes within their shear layer. These subvortices revolve around the main tornado, causing massive destruction in their wake. In the laboratory, researchers create a similar multi-tornado system with a spinning disk at the bottom of a shallow, cylindrical layer of water. Depending on how fast the disk spins, different numbers of subvortices form around the main vortex.
In this poster, researchers show the transition from a 3-subvortex system to a 2-subvortex one. Starting at the 12 o’clock position and moving clockwise, we see 3 subvortices arranged in a triangle. A sudden change in the disk’s rotation speed destabilizes the system, causing the subvortices to break down and shift into a new 2-subvortex configuration. As this happens, material that was isolated in each subvortex (darker blue regions) is suddenly able to mix. That suggests that a real-world multiple vortex tornado might suddenly shed debris if it lost enough angular momentum. Back in the lab, though, the shift to a stable 2-subvortex system once again isolates material in individual subvortices and prevents it from mixing with the rest of the flow. (Image and research credit: G. Di Labbio et al. 1, 2)
When a drop of ethanol lands on a pool of water, surface tension forces draw it into a fast-spreading film. Evenly-spaced plumes form at the edges of the film, then the film stops spreading and instead retracts. All of this takes place in about 0.6 seconds. But, as the image above shows, there’s more that goes on beneath the surface. A vortex ring forms and spreads under the film, driven by the shear layer under the edge of the plumes. Here, the vortex ring is visible in the swirling particles near the water surface. (Image and research credit: A. Pant and B. Puthenveettil)
Just as prairie dogs bark to warn the colony of danger, many plants can signal their neighbors when they’re under attack. This thale cress releases calcium when caterpillars eat it; neighboring plants pick up the chemical signal and pass it along. To better understand how the signal gets passed, researchers genetically modified this plant to fluoresce when extra calcium is on the move. It’s incredible to watch the flow from one side of a leaf to another. (Image and research credit: Y. Aratani et al.; via Colossal)
Near absolute zero, as atoms slow down, some materials become a superfluid, a type of matter with zero viscosity. Superfluids do all kinds of strange things like generate fountains, leak from sealed containers, and form quantized vortices. Theorists also predicted that in a superfluid heat would slosh back and forth like a wave, even without any flow. They call this “second sound” and researchers have now detected it for the first time.
In a typical experiment, we’d use an infrared camera to see how heat moves in a substance, but at the frigid temperatures of superfluids, that’s not possible. Instead, the team developed a method that measured the temperature of their atomic gas using radio frequency. When their lithium-6 fermions were at a higher temperature, they resonated with a higher radio frequency. Using radio frequency to probe the substance, they were able to observe exactly when heat stopped diffusing like in normal matter and switched to the superfluid second sound state. Since superfluids may live at the heart of neutron stars, further experiments will help us understand these exotic forms of matter. (Image credit: J. Olivares/MIT; research credit: Z. Yan et al.; via MIT News and Gizmodo)
Engineers have developed a new 3D-printing technique that uses molten aluminum to quickly manufacture large-scale parts. This Liquid Metal Printing method deposits the metal into a bed of tiny glass beads, which hold the metal in place while it cools. In minutes, they can produce furniture-sized parts, but that speed comes at a cost in resolution; the printed parts are rough, but they have the strength to withstand further machining by bending, milling, etc. The process is also well-suited for reusing scrap metal. The team hopes their method will be a useful prototyping tool as well as a possible manufacturing technique in architecture and construction. (Image and video credit: MIT News; research credit: Z. Karsan et al.)
Beads of condensation on a cooling, oil-slicked surface have a dance all their own in this video. Large droplets gobble up their fellows as they follow serpentine paths; each new droplet donates its interfacial energy to feed the larger drop’s kinetic energy. Eventually, the big drops switch to a circular path, like an ouroboros, the tail-eating serpent of mythology. This transition happens due to the oil shifted by the dancing droplets. You can recreate the effect at home by rubbing a thin layer of oil over glass and setting it atop a hot mug of your favorite beverage. (Video and image credit: M. Lin et al.; research credit: M. Lin et al.)
