Month: February 2017

Battery Rockets
When I post Slow Mo Guys videos, it often comes with a warning not to try this at home. For their latest video, that deserves an extra-special mention: seriously, don’t try this. In this video, Dan and Gav explode lithium-ion batteries. In the process, they discover a safety feature – namely vents on one face of the battery. Because runaway thermal reactions (a.k.a. explosions) are a possibility with this type of battery system, consumer-grade batteries are designed to try and prevent extreme damage. One of these outwardly visible safety features are these four vents that release gas when when the battery is too hot. By venting the gas, manufacturers keep the battery from exploding and sending hot chemicals and shrapnel in all directions. Instead the venting gas turns the entire battery into a miniature rocket. (Video and image credit: The Slow Mo Guys)
February 28, 2017
Wrinkling Winds
If you’ve ever sat out on a lake and just watched the water’s surface, you’ve probably noticed how complex and variable it looks. There may be waves that rock your kayak but there are smaller variations, too, like little ripples or even tiny wrinkles that appear on the surface. Much of this activity comes from wind blowing across the water. When the wind exceeds a critical speed, waves form. They generally travel in lines that are aligned perpendicular to the wind (lower right). But what happens when the wind is below the critical speed?
A recent study looked at just this question. By blowing air across the surface of different liquids and observing variations in the surface height as small as 2 micrometers, the researchers were able to measure tiny wrinkles on the water’s surface (lower left) when the wind speed was small. The size and shape of the wrinkles actually corresponds to structures in the turbulent air flow over the water! For fluids like water, there’s a smooth transition from wrinkles to waves as the wind speed increases, so both may be visible at the same time. For higher viscosity fluids, the switch from one to the other is more abrupt. (Image credits: water – M. Soveran; figure – A. Paquier et al. w/ annotations added in blue; research credit: A. Paquier et al.)
February 27, 2017
Aerodynamic Leidenfrost Effect
If you place a droplet on a surface much hotter than its boiling point, that droplet will skitter and float almost frictionlessly across the surface on a thin layer of its own vapor. This is what is known as the Leidenfrost effect. But you don’t have to heat a surface to get this behavior. There’s also an aerodynamic Leidenfrost effect, shown above, when the surface is moving. As the surface moves, it drags a layer of air along with it, and that layer of air is capable of keeping droplets aloft indefinitely. The thickness of the air layer depends on speed; the faster the plate moves, the thicker the air layer underneath droplets. The aerodynamic forces generated are large enough to drive a droplet up an incline against the force of gravity (bottom image). (Image credit: animation – M. Saito et al., source; chronophotograph – A. Gautheir et al., pdf)
February 24, 2017
An Octopus’ Handshake
Cephalopods, especially octopuses, are fascinating creatures. At sea level, an octopus can generate an impressive pressure differential of 1 to 2 atmospheres with each of its suckers. That incredible grip is possible thanks to fluid dynamics. An octopus’s sucker consists of two main parts: the ring-shaped infundibulum on the outer surface and the inner, cup-shaped acetabulum. When the infundibulum makes contact with a surface, it creates a water-tight seal. The octopus then contracts radial muscles along the acetabulum. This expands the inner chamber. The water trapped in the acetabulum now has to take up a greater volume, causing the pressure to drop and creating suction. To let go, the octopus simply relaxes the radial muscles or contracts circular ones to reduce the chamber volume and release the suction. (Video credit: Deep Look)
February 23, 2017
Vortex Impact
When a vortex ring impacts a solid wall (or a mirrored vortex ring), it expands and quickly breaks up. The animations above show something a little different: what happens when a vortex ring hits a water-air interface. As seen in the side view (top image), the vortex starts to expand, but its shear at the interface generates a stream of smaller vortices that disrupt the larger vortex. (They even look like a little string of Kelvin-Helmholtz vortices!) When viewed from above (bottom image), the vortex ring impact and breakdown look even more complicated. Mushroom-like structures get spat out the sides as those secondary vortices form, and the entire structure quickly breaks up into utter turbulence. There’s some remarkable visual similarities between this situation and some we’ve seen before, like a sphere meeting a wall and drop hitting a pool. (Image credit: A. Benusiglio et al., source)
February 22, 2017
Breaking Soon
Australian photographer Warren Keelan captures spectacular photos of waves just before and during the moment they break. Fluid dynamics is defined by motion – specifically the motion of substances that do not hold a single form – but one thing I love about wave photography is how crisp and solid water appears when frozen in time. In a way, it feels like a reminder that, even though we classify matter into different states, ultimately those states have a lot in common. (Image credit: W. Keelan; via Colossal)
February 21, 2017
Venusian Waves
Despite its proximity, Venus remains largely mysterious, thanks to its cloudy atmosphere and incredible harsh conditions. A recent study using data from the Japanese satellite Akatsuki revealed an enormous bow-shaped wave in the Venusian atmosphere. The wave appeared at an altitude of about 65 km and stretched more than 10,000 km long, across both the northern and southern hemispheres. Although surface winds on Venus are believed to be small due to its incredibly slow rotation, winds higher in the atmosphere are much faster – so it was strange to observe this wave sitting essentially stationary for five days of observation.
When the scientists mapped the location of wave relative to the surface, they found it was sitting over the Aphrodite Terra highlands, suggesting that this structure is a gravity wave generated by winds interacting with the topography. Similar, albeit smaller, gravity waves are often observed on Earth near mountains. The finding raises questions about our understanding of Venusian atmospheric dynamics and exactly how disturbances from surface winds could create enormous structures so high in the atmosphere. (Image credit: T. Fukuhara et al.; h/t to SciShow Space)
February 20, 2017
Creating Clouds
What you see here is the formation of clouds and rain – but it’s not quite what you’re used to seeing outside. This is an experiment using a mixture of sulfur hexafluoride and helium to create clouds in a laboratory. Everything is contained in a cell between two transparent plates. Liquid sulfur hexafluoride takes up about half of the cell, and when the lower plate is heated, that liquid begins evaporating and rising in the bright regions. When it reaches the cooled top plate, the liquid condenses into droplets inside the dimples on the plate, eventually growing large enough to fall back as rain. The dark wisps you see are areas where cold sulfur hexafluoride is sinking, much like in the water clouds we are used to. Setups like this one allow scientists to study the effects of turbulence on cloud physics and the formation of droplets. (Image credit: E. Bodenschatz et al., source)
Boston-area folks! I’ll be taking part in the Improbable Research show Saturday evening at 8 pm at the Sheraton Boston. Come hear about the Boston Molasses Flood and other bizarre research!
February 17, 2017
Leidenfrost Atop a Fluid
Leidenfrost droplets typically hover on a thin layer of vapor above a surface that is much hotter than the boiling point of the liquid. Such drops move almost frictionlessly across these surfaces and can even propel themselves. The question of how hot is hot enough to produce the Leidenfrost effect is still being debated, but recent research suggests that the answer may depend strongly on surface roughness.
To test the role of surface roughness, one group tested drops of ethanol atop a heated pool of silicone oil, as pictured above. Ethanol’s boiling point is 78 degrees Celsius, and the researchers found they could hold the ethanol drop in a Leidenfrost state by heating the pool to 79 degrees Celsius – only 1 degree above ethanol’s boiling point! Thanks to surface tension, a liquid surface is essentially molecularly smooth. The fact that solid surfaces require much higher temperatures before the Leidenfrost effect is observed indicates that even the slightest roughness can have a large impact on the Leidenfrost temperature. (Image credit: F. Cavagnon; research credit: L. Maquet et al., pdf)
Heads-up for Boston-area folks! I’ll be taking part this Saturday evening in the Improbable Research show at the AAAS conference. The show is free and open to the public but fills up quickly, so be sure to come early for a seat.
February 16, 2017
Leapfrogging Vortices
Two vortex rings travelling along the same line can repeatedly leapfrog one another. During my recent visit to the University of Chicago, PhD student Robert Morton of the Irvine Lab demonstrated this leapfrogging in the same apparatus they use to study knotted vortices. Leapfrogging works because of the mutual interaction of the flow fields of the two vortex rings. Their influence on one another causes the front vortex ring to slow down and widen while the trailing vortex narrows and speeds up. Once the vortices have switched places, the process repeats. In a real fluid, viscosity eventually breaks things down and causes the vortex rings to merge, but in simulation, inviscid vortex rings can leapfrog indefinitely. Our friend Physics Girl even showed that half-vortex-rings can leapfrog. (Image credit: N. Sharp; thanks to R. Morton for the demo)
February 15, 2017