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.)
Tag: fluid dynamics
Inhibiting Marine Lightning
Thunderstorms over the ocean have substantially less lightning than a similar storm over land. Scientists wondered whether this difference could be due to lower cloud bases over the ocean or differences in the cloud droplets’ nuclei. But a new study instead implicates coarse sea spray as the deciding factor. By tracking the full lifetime of storm systems through remote sensing, the team found that fine aerosols can increase lightning activity over both land and ocean. But adding coarse sea salt from sea spray reduced lightning by 90% regardless of fine aerosols. With sea salt in the mix, clouds seem to develop fewer but larger condensation droplets, providing less opportunity for the electrification necessary to generate lightning. (Image credit: Z. Tasi; research credit: Z. Pan et al.)
Seeing the Flow
Experimentalists often need a sense for the overall flow before they can decide where to measure in greater detail. For such situations, flow visualization techniques are a powerful tool since they provide quick ways to see and compare flows.
Here, researchers paint a viscous oil atop their flying wing model and observe how the oil moves once the air flow starts up. This oil flow visualization shows the large-scale shifts in how air flows over the craft as the angle of attack increases. The disadvantage is that these techniques often give only a qualitative sense of the flow. But they can allow experimentalists to test many different conditions to decide which specific cases they should examine quantitatively. (Image and video credit: V. Kumar et al.)
Rain-Driven Prey Capture
Pitcher plants often entice their insect victims with sweet nectar before trapping them in inescapable viscoelastic goo. But some species go even further. Nepenthes gracilis, a species native to Southeast Asia uses its leafy springboard to lure its prey. Once an ant crawls to the underside of the leaf, a falling rain drop will spell its doom. When drops hit the leaf, it deflects down and jerks up, thanks to its shape and stiffness. The motion catapults insects into the pitcher, where digestive fluids await. While we’ve seen some fast-moving plants before, this is a rare example of a plant with an externally-driven speed mechanism. With it, the pitcher plant doesn’t have to wait or expend any metabolic effort to reset for the next insect. (Image credit: GFC Collection/Alamy; research credit: A. Lenz and U. Bauer; via New Scientist)
Reefs Along New Caledonia
Brown reefs edge a turquoise lagoon in this astronaut snapshot of the New Caledonian coastline. Reefs like these form a natural barrier that protects coastlines from storms by breaking up waves (seen here as those white edges) before they reach the shore. The lagoon is streaked with lines of tan where sediment flows from the uplands into the water. Similarly, the color variations from green to blue in water hint at changes in depth, organic content, and more. (Image credit: NASA; via NASA Earth Observatory)
Stunning Waves
Photographer Lloyd Meudell captures breathtaking images of ocean waves off his home shores of New South Wales. The waveforms and lighting combine to create infinite variety in shape and texture. Some waves look like towering mountain landscapes; some look like glass sculptures. Every one of them draws you into the ocean’s power. (Image credit: L. Meudell; via Bored Panda)
Absorbing Sound with Moth Wings
Manmade soundproofing tends to be porous and bulky or very limited in the range of frequencies it can handle. In contrast, moths are natural absorbers of ultrasound, having evolved to avoid reflecting those frequencies back to the bats hunting them. Researchers took the structures from a moth wing and applied them to an aluminum disk to see how the coating performed. They found that the moth wing’s structures reduced sound reflection by as much as 87% at the lowest frequency tested (20kHz, still beyond human hearing.) As researchers explore how the individual structures of the wing perform, they hope to adapt the moth’s prowess to soundproof within the human range of hearing. (Image and research credit: T. Neil et al.; via Physics World)
A Levitated Boil
When acoustically levitated, objects tend to clump together and move like a single, large solid. But researchers found more fluid-like states for their levitated particles when the particles were smaller. At low acoustic power, the particles behave like a liquid and shift primarily within a plane. But as the acoustic power increases, the granular liquid begins to “boil” and transition into a gaseous state, with particles moving in all directions. It’s amazing how often these metaphors (e.g., treating a group of particles as a “liquid”) hold true when observing different physical systems! (Image and video credit: B. Wu et al.)
Aligning by Bubble Array
Assembling structures from small components is often difficult. Techniques like optical tweezers are limited to very small objects, and magnetic techniques only work with certain materials. Here, researchers use acoustical forces on bubbles to move and align centimeter-sized objects.
When a single bubble oscillates in an ultrasonic field, its changing size creates pressure variations around it. When an acoustic wave scatters off one bubble and impacts another, it sets up a small attractive force between the bubbles, known as the secondary Bjerknes force. For individual bubble pairs, this force is extremely small and unable to affect much. But using arrays of bubbles — one array on a fixed object and another on a floating object — researchers amplified the attraction and showed that the resulting forces could manipulate and align their components. (Image credit: top – J. Thomas, others – R. Goyal et al.; research credit: R. Goyal et al.; via APS Physics)
Perturbations
At first glance, today’s video appears to have little to do with fluid dynamics since it’s a demonstration of interactions between magnets. But for those who’ve delved into the mathematics of fluid dynamics — especially subjects like perturbation theory — there’s a lot to appreciate here. In the video, we see systems of magnets constructed and then manipulated, often by moving a single magnet and watching how the rest respond. Visually, this demonstrates how disturbances move in complex, interconnected damped systems. The auditory component — definitely turn the sound on for this video — is an extra layer of fluids-related goodness that also shows how reconfiguring a system changes its resonant frequencies. (Image and video credit: Magnetic Tricks and Magnetic Games; via Colossal)
