This year’s Nikon Small World in Motion competition was won by fluid dynamics! The first place video shows droplets on a superhydrophobic surface coalescing. The droplets are a mixture of water and ethanol. Their initial merger creates a ripple of waves that’s followed by a ghostly vortex ring that jets into the interior. Previous research on coalescence during impact shows jets driven by surface tension but the jet here doesn’t appear to be confined to the surface. (Image and video credit: K. Rabbi and X. Yan; via Nature; submitted by Kam-Yung Soh)
In industry, drying droplets often have many components: a liquid solvent, solid nanoparticles, and dissolved polymers. The concentration of that last component — the polymers — can have a big effect on the way the droplet dries, as seen in the video above.
Without polymers, the droplet dries similarly to a coffee ring stain. But at moderate concentration, we see something very different. The droplet forms an eye in the middle, similar to a hurricane’s, and the edges of the droplet sprout mushroom-shaped plumes that grow and merge with one another along the edge. With even larger polymer concentrations, the mushrooms sweep their way inward, leaving a feathery stain behind. (Video, image, and research credit: J. Zhao et al.)
When a skillet is hot enough, water droplets will skitter across the surface almost frictionlessly thanks to the Leidenfrost effect. The incredibly high temperature of the surface relative the the liquid’s boiling point causes part of the drop to vaporize, enveloping the remainder of the liquid in a protective vapor cocoon.
We see this effect for more than just solid surfaces, though. This video demonstrates how pouring liquid nitrogen on a pool of water creates plenty of Leidenfrost weirdness as well. It looks as though the initial pour freezes some condensation to dust or other particles, which then stream outwards on a cloud of vapor. Larger droplets of liquid nitrogen actually manage to hold together on the pool’s surface. Their vapor keeps them from touching the water, but that flow also jostles them, creating a ring of ripples around the jiggling drop. (Video and image credit: Science Marshal)
Paint, soap, bleach, oil, and oat milk combine to create the gorgeous colorscapes of Thomas Blanchard’s short film “Colors”. Watch as droplets burst and waves of color flow past. It’s a lovely break from whatever you’re dealing with at the moment, and at less than 3 minutes long, you can spare the time! (Image and video credit: T. Blanchard)
Natural oils provide critical nutrients to filter feeders like zooplankton and barnacles. These creatures capture oil droplets on bristle-like appendages such as cilia and setae. But this droplet-catching turns into a disadvantage during petroleum spills, when capturing and ingesting oil can be lethal. A recent study looks at the fluid dynamics of oil droplet capture for these tiny creatures.
The authors found that filter feeders capture a range of droplets regardless of size and oil viscosity. But not all droplets stay attached long enough to get consumed, and the larger a droplet is, the lower the flow velocity necessary to detach it from the animal. That suggests a method of limiting uptake of spilled petroleum into the marine food chain: use surfactants to break up the oil into droplets large enough that they’ll detach from filter feeders before getting eaten. (Image credit: D. Pelusi; research credit: F. Letendre et al.; submitted by Christopher C.)
Every day human activity pumps aerosol particles into the atmosphere, potentially altering our weather patterns. But tracking the effects of those emissions is difficult with so many variables changing at once. It’s easier to see how such particles affect weather patterns somewhere like the Sandwich Islands, where we can observe the effects of a single, known source like a volcano.
That’s what we see in this false-color satellite image. Mount Michael has a permanent lava lake in its central crater, and so often releases sulfur dioxide and other gases. As those gases rise and mix with the passing atmosphere, they can create bright, persistent cloud trails like the one seen here. The brightening comes from the additional small cloud droplets that form around the extra particles emitted from the volcano.
As a bonus, this image includes some extra fluid dynamical goodness. Check out the wave clouds and von Karman vortices in the wake of the neighboring islands! (Image credit: J. Stevens; via NASA Earth Observatory)
In 1977, one passenger with the flu infected 38 people onboard a flight with malfunctioning ventilation. In this video, Dianna digs into the physics of respiratory disease transmission and just why ventilation is so key to preventing it.
There are three primary modes of transmission for respiratory diseases like influence or SARS-CoV-2: 1) touching an infected surface and then oneself, i.e., self-inoculation; 2) inhaling virus-filled droplets larger than 5 nm; and 3) inhaling virus-filled droplets smaller than 5 nm. That size cut-off may seem a little arbitrary, but it’s how scientists distinguish between droplets that fall quickly to the ground and ones that can persist on buoyant air currents.
That airborne persistence is one of the reasons ventilation — in other words, replacing the air — is so important. So many people on that 1977 flight got sick because there was no system removing the infected air and bringing in fresh air. For more on the fluid dynamics disease transmission, check out these posts. Curious about those bacterial bubble bursts? I’ve covered that, too. (Video and image credit: Physics Girl)
In nutrient-poor soils, carnivorous plants like the cape sundew supplement their diets by eating insects. To entice their prey, the cape sundew secretes droplets of sugary water. But unwary insects who land to feed soon find themselves unable to pull away from this viscoelastic liquid. Complex molecules in the fluid grant it elasticity, so when insects pull against it, the liquid stretches and pulls back instead of breaking up. Other carnivorous plants, like the pitcher plant, use similar non-Newtonian tricks to trap insects. (Video and image credit: Deep Look)
High-speed photography gives us an alternate glimpse of reality. Here it provides an all-new perspective on making espresso. Surface tension plays a starring role, first in pulling together the film that forms over the exit, then in creating the drips and drops that follow. The break-up of espresso into individual droplets is an example of the Plateau-Rayleigh instability, where surface tension drives any wobble in the falling jet to pinch off. For more slow-motion espresso, you can also check out this behind-the-scenes video. (Video and image credit: J. Hoffmann; submitted by Jerrod H.)
This glowing, molten liquid captured by the Slow Mo Guys is thermite. The chemical reaction behind thermite is highly exothermic, hence its intense glow. There’s some great fluid dynamics hiding in this video. First, there’s the dripping thermite (Image 1), which breaks up into droplets via the Plateau-Rayleigh instability before shattering when it hits the ground.
Then there are the sequences (Images 2 and 3) of thermite dripping into water. The heat of the reacting thermite vaporizes a layer of water around it, creating a bubble that completely envelops the thermite. In other words, the falling thermite is supercavitating! That layer of air significantly reduces drag on the thermite and it insulates the thermite from the cooler temperature of the water. (Video and image credit: The Slow Mo Guys)
