A good soccer player can kick the ball from the corner of the field into the goal thanks to the Magnus effect. But if you’ve ever tried to play soccer with a smooth ball, you may have noticed that sometimes the ball bends the wrong way! This is the reverse Magnus effect and it’s caused when the boundary layers on either side of the ball switch from turbulent to laminar flow at different times. Dianna Cowern explains (with a little help from yours truly) in the video above. Want to learn more about how roughness affects boundary layers? Check out our companion video on FYFD’s YouTube channel. (Video credit: D. Cowern/Physics Girl)
Category: Phenomena
Climbing Up the Walls
You may have noticed when baking that fluids don’t always behave as expected when you agitate them. If you put a spinning rod into a fluid, we’d expect the rod to fling fluid away, creating a little vortex that stirs everything around. And for a typical (Newtonian) fluid, this is what we see. The fluid’s viscosity tries to resist deforming the fluid, but the momentum imparted by the rod wins out. With a viscoelastic fluid, on the other hand, the story is much different. As before, the spinning of the rod deforms the fluid. But the viscoelastic fluid contains long chains of polymers. As those polymers get stretched by the deformation, they generate their own forces, including forces parallel to the rod. Instead of being flung outward, the viscoelastic fluid starts climbing up the rod, with the stretchy elasticity of the polymers helping pull more fluid up and up. (Image credit: Ewoldt Research Group, source)
Bursting Into Droplets
Our atmosphere is full of aerosols – extremely tiny particles and droplets of salt, dust, pollutants, and other substances. Wind’s effects alone cannot account for the sizes and quantities of aerosols we measure. Another potential source is the bursting of bubbles; more specifically, the bubbles that form at the oceans’ surface. Frothy, crashing waves often capture pockets of air. When these bubbles burst, the thin film of their surface ruptures into long filaments that break into tiny droplets. Such droplets can be small enough to get carried on the breeze, eventually evaporating and leaving the particulates that were once in the water to ride the winds. (Image credit: H. Lhuissier & E. Villermaux; see also: Y. Couder)
Vortex Ring Roll-Up
Vortex rings are endlessly fascinating, and they appear throughout nature from dolphins to volcanoes and from splashes to falling drops. One way to form them is to inject a jet into a stationary fluid. Viscosity between the fast-moving jet and the quiescent surrounding fluid slows down fluid at the jet’s edge. That slower fluid slips to the rear, only to get sucked into the faster -moving flow and pushed forward again. The result is a spinning toroid, or ring. A similar method generates vortex rings by pushing a fluid out a round orifice. In this case, interaction between the fluid and the wall provides some of the force necessary to form the vortex ring. (Image credit: Irvine Lab, source)
“Bubble Circus”
The “Bubble Circus” is a delightful outreach device equipped for all manner of physics demos, as seen in the video above. Many of its exercises explore surface tension, a force observed at the interface of a fluid. Surface tension is what provides bubbles with their surface-minimizing spherical shape. That same property determines the minimal distance between the four vertices of a pyramid (0:54). Changing the surface tension causes fluid at the interface to move. At 1:16 adding a lower surface tension fluid makes the water and black pepper pull away; the same physics drives the boat away at 2:09. For more on the Bubble Circus, see here. (Video credit: A. Echasseriau et al.; via J. Ouellette)
Bioluminescent Plankton
The blue-outlined dolphins you see above get their glow from microorganisms called dinoflagellates. They are a type of bioluminescent plankton, shown in the lower image, that can be found in oceans around the world. Their glow comes from combining two chemicals: luciferase and luciferin. The dinoflagellates suspended in the ocean do this when they are disturbed–specifically, when the water around them transmits a shear stress above a certain threshold. Typically, this is caused by something larger–a potential predator–moving past, although it can also be stimulated by breaking waves. The higher the shear stress, the more intense the glow, but the dinoflagellates only use their bioluminescence sparingly. If you apply shear stress and keep applying it, their glow fades away without reactivating. After all, they can only produce so much chemical fuel. (Image credit: BBC from Attenborough’s Life That Glows; h/t to Gizmodo; research credit: E. Maldonado and M. Latz)
Pearls of Mezcal
Mezcal is a traditional Mexican liquor distilled from agave. (The more commonly known tequila is actually a special type of mezcal.) As a part of the production process, distillers pour a stream of mezcal into a bowl, creating a flotilla of small bubbles called pearls. Strange as it sounds, these pearls let the distiller judge the alcohol content of the liquor! When the ratio of alcohol and water in the mixture is just right, the bubbles will have a longer lifetime before they coalesce. If there’s too little or too much alcohol, the bubbles won’t last as long. The effect depends on both the viscosity and the surface tension of the liquor, but it’s the odd way that viscosity changes in water/alcohol mixtures that creates this Goldilocks behavior. It’s a fascinating demonstration of how traditional techniques often have true scientific underpinnings. (Video credit: M. Wilhelmus et al.)
Roll Clouds
The roll cloud, or Morning Glory cloud, is a rare phenomenon that looks rather like a horizontal tornado. In reality, it is part of a soliton wave traveling through the atmosphere. At its leading edge, moist air is forced upward, causing water vapor to condense, and, at the trailing edge, air moves downward, dissipating the cloud. These clouds are most frequently observed in Australia near the Gulf of Carpentaria, where local geography and sea breezes promote their growth during springtime. The clouds do appear elsewhere on occasion; the photos above show rolls clouds in Calgary, Alberta and coastal Uruguay, respectively. (Image credits: G. E. Nyland, D. M. Eberl; see also: Z. Ouazzani)
Silent Flying
As nocturnal hunters, owls are aerodynamically optimized for stealthy flying. This clip from BBC Earth demonstrates just how quiet a barn owl is in flight compared to a pigeon or a peregrine falcon. The owl’s large wingspan relative to its body size gives it enough lift that it does not have to flap often, allowing it to glide instead, but this is far from its only stealthy adaptation. Owl feathers feature a serrated leading edge that helps break flow over the wing into smaller, quieter vortices. Their fringe-like trailing edge breaks flow up even further and acts to damp noise from airflow. The downy feathers of the owl’s body also help muffle any noise from the bird’s movement, allowing the barn owl to fly almost silently. (Video credit: BBC Earth; via Gizmodo)
The Bubble Nebula
This spectacular Hubble image shows the Bubble Nebula. The source of this nebula is the star seen toward the upper left side of the bubble. This massive, super-hot star has ceased to fuse hydrogen and is now fusing helium, powering its way to a likely end as a supernova. As it burns, the star emits a stellar wind of gas moving at over 6.4 million kilometers an hour. As the flow moves outward, it encounters colder dense gases that it pushes along as it expands; this is the blue bubble surface that we see. The asymmetry of the bubble with respect to its source star is caused by the variation in the surrounding gas’s density. The bubble’s front moves more slowly in areas with more gas, thus making the bubble appear lop-sided. (Image credit: NASA; via Gizmodo)
