Category: Phenomena

Collective Catfish Convection
Gather many birds, fish, or humans together and you often get collective motion that’s remarkably fluid-like in appearance. This video shows a group of juvenile striped eel catfish, an (eventually) venomous species that uses strength in numbers for protection while young. Their movement is rather mesmerizing, and if you watch individual catfish, you’ll see a sort of convective motion inside the blob. There’s a general downward trend near the front of the school and a rising one on the backside. Perhaps they’re taking turns feeding near the bottom of the pack? (Image and video credit: Abyss Dive Center; via Colossal)
February 4, 2020
Breaking the Euler Equations
Mathematicians like to break things. Or, more exactly, they like to know when the equations we use to describe physics break down. One popular target in fluid mechanics are the Euler equations, which describe the motion of frictionless, incompressible flows. Mathematicians have been on the hunt for centuries for situations where these equations predict singularities, points where the velocity or vorticity of a fluid change infinitely quickly. Since that can’t happen in reality (at least as far as we understand it), these singularities indicate weaknesses in our mathematical description and may help uncover fundamental flaws in our understanding.
Despite centuries of effort, the Euler equations withstood mathematical assault… until recently. Since 2013, a series of mathematicians have been successfully chipping away at the Euler equations’ seeming perfection with a series of scenarios that seem to lead to singularities. One is similar to stirring a cup of tea, except that you stir the upper part of the cup in one direction and the bottom half in the opposite. As the flow develops, a singularity occurs where the secondary flows of these two stirring motions collide. For more, check out these two articles over at Quanta. (Image credit: L. Fotios; see also Quanta Magazine 1, 2)
January 27, 2020
When the Mediterranean Dried Up
Geological evidence shows that millions of years ago, the Mediterranean Sea nearly dried out. In fluid mechanics, we’d describe this problem using one of our fundamental equations: conservation of mass, also known as continuity.
Imagine a volume containing the entire Mediterranean. To describe the amount of sea water in that volume, you need to keep track of two major quantities: how much water is flowing into the volume and how much is leaving it. For the prehistoric (as well as today’s) Mediterranean, the sources feeding the sea are 1) an inflow from the Atlantic through the Strait of Gibraltar; 2) inflows from rivers; and 3) rainfall. Water is lost primarily to evaporation.
As explained in the video, the Mediterranean’s dry spell was heralded by tectonic changes that sealed the Strait of Gibraltar, depriving it of its largest source of inflow. At the same time, warmer temperatures and less rainfall reduced influx from rivers and the atmosphere while increasing evaporation rates. The result? Water levels in the Mediterranean dropped by hundreds of meters, creating massive salt deposits, wiping out native marine life, and allowing mass migration by land-dwelling animals. Eventually, though, the Strait re-opened, creating what might have been a massive flood. (Video and image credits: PBS Eons)
January 24, 2020
Captured by Waves
Acoustic levitation and optical tweezers both use waves — of sound and light, respectively — to trap and control particles. Water waves also have the power to move and capture objects, as shown in this award-winning poster from the 2019 Gallery of Fluid Motion. The central image shows a submerged disk, its position controlled by the arc-shaped wavemaker at work on the water’s surface. The complicated pattern of reflection and refraction of the waves we see on the surface draws the disk to a focal point and holds it there.
On the bottom right, a composite image shows the same effect in action on a submerged triangular disk driven by a straight wavemaker. As the waves pass over the object, they’re refracted, and that change in wave motion creates a flow that pulls the object along until it settles at the wave’s focus. (Image and research credit: A. Sherif and L. Ristroph)
January 23, 2020
Bouncing Off Defects
The splash of a drop impacting a surface depends on many factors — among them droplet speed and size, air pressure, and surface characteristics. In this award-winning video from the 2019 Gallery of Fluid Motion, we see how the geometry of a superhydrophobic surface can alter a splash.
When a drop falls on a protruding superhydrophobic surface, like the apex of a cone, it can be pierced from the inside, completely changing how the droplet rebounds and breaks up. The variations the video walks us through are all relatively simple, but resulting splashes may surprise you nevertheless. (Image and video credit: The Lutetium Project)
January 21, 2020
Superman’s Hair Gel
I love a good tongue-in-cheek physical analysis of superheroes. This estimate of the drag force experienced by Superman’s hair when outracing a plane or speeding bullet was done by Cornell students. According to their calculations, Superman’s hair (or his hair gel) must withstand nearly 80,000 Newtons of force. That’s a bit less than the typical force experienced by a restrained passenger in a car crash at highway speeds.
In grad school, my labmates and I held a spirited debate about the difference in drag Superman would experience when flying at hypersonic speeds depending on whether he had one or both arms extended in front of him. Sadly, we never found the chance to test our hypotheses in the wind tunnel. (Image and video credit: R. Geltman et al.)
January 20, 2020
Spinning Ink Out of Markers
I have to say I’m grateful that my classmates in school never discovered the mess-generating superpower of felt-tipped markers. As the Slow Mo Guys demonstrate here, when you spin or fling these markers, ink will stream out of them. That’s due, in part, to the air vents present near the tips. Markers (and other pens) have those to equalize the pressure between the outside and the ink reservoir; otherwise, the ink won’t flow to the felt tip as it should. Is anyone else surprised by the sheer volume of liquid ink apparently contained in these pens? (Image and video credit: The Slow Mo Guys)
January 8, 2020
Siberia’s Rivers
Each winter the Kolyma River in Siberia freezes to a depth of several meters. But by June the river thaws and discharges its annual 136 cubic kilometers of water into the Arctic. The dark color of the river comes from the sediment and organic material it carries. The Kolyma is the world’s largest river underlain with continuous permafrost. Parts of the river system’s permafrost date back to the Pleistocene more than 12,000 years ago. Since much of its organic matter comes from its permafrost, researchers expect the amount of organic material in the Kolyma’s discharge to increase as the permafrost degrades in our warming climate. (Image credit: NASA Earth Observatory)
January 3, 2020
Behind the Bubbly
Carbonation and the fizzy bubbles that come with it are surprisingly popular among humans. Through fermentation or artificial introduction, carbon dioxide gas gets dissolved into a liquid under high pressure. Then, when the pressure is released to atmospheric levels, that gas comes out of solution, forming tiny bubbles that eventually grow large enough to rise buoyantly to the surface. There they will either pop – releasing carbon dioxide gas and aromatics – or form a layer of foam – like in beer – that insulates the liquid and makes it harder to spill. (Image credit: D. Cook; see also R. Zenit and J. Rodríguez–Rodríguez; via Jennifer O.)
December 31, 2019
Waltzing Defects
Liquid crystals are a peculiar state of matter with both liquid and crystalline properties. In this video, a microfluidic device breaks water into droplets surrounded by a shell of liquid crystal. Because the molecular structure of the liquid crystals is helical and cannot pack neatly in a spherical shell, there are visible defects in the liquid crystal shells. Given time, those defects can merge as the liquid crystal shell thickens. (Image and video credit: The Lutetium Project)
December 26, 2019