Chemical Bouillon’s art often mixes chemistry and fluid dynamics. Here dense UV dyes falling through a less dense fluid form long strings with mushroom-like caps or tree-like branches. (For reference, gravity is pointing up relative to the video frame in most clips.) This behavior is related to the Rayleigh-Taylor instability that deforms interfaces and causes mixing between unstably stratified fluids. (Video credit: Chemical Bouillon)
Videos
Stirring Up
When a viscoelastic non-Newtonian fluid is stirred, it climbs up the stirring rod. This behavior is known as the Weissenberg effect and results from the polymers in the fluid getting tangled and bunched due to the stirring. You may have noticed this effect in the kitchen when beating egg whites. In this video, researchers explore the effect using rodless stirring. The first example in the video shows a viscous Newtonian fluid being stirred. The stirring action creates a concave shape in the glycerin-air interface, and dye injection shows a toroidal vortex formed over the stirrer. Fluid near the center of the vortex is pulled downward and circulates out to the sides. In contrast, the viscoelastic fluid bulges outward when stirred. Dye visualization reveals fluid being pulled up the center into the bulge. It then travels outward, forming a mushroom-cap-like shape before sinking down the outside. This is also a toroidal vortex, but it rotates opposite the direction of the Newtonian one. Exactly how the polymers create this change in flow behavior is a matter of active research. (Video credit: E. Soto et al.)
Going Supercritical
Supercritical fluids exist at temperatures and pressures above the critical point, in a region of the phase diagram where there is no clear boundary between the liquid and gaseous state. Supercritical fluids have some of the properties of each state: they can move as freely as a gas, but they are still capable of dissolving materials like a liquid does. They also have no surface tension because there is no interface between liquid and solid. These properties make supercritical fluids very useful in industrial applications, including decaffeination and chemical deposition. Interestingly, the temperatures and pressures on Venus are so high that scientists think the atmosphere at the surface is a supercritical fluid. (Video credit: SCFED Project)
The Upside-Down Jellyfish
The upside-down jellyfish, Cassiopea, rests its bell against the ocean floor and points its frilly oral arms up toward the sun for the benefit of the symbiotic algae living on it. In return, the algae provide some of the nutrients the jellyfish needs. The rest it obtains by filter feeding for zooplankton. The video above shows how a combination of flow visualization and simplified computational modeling can reveal the jellyfish’s methods for eating. A simple pulsing bell has limited fluid flow in the region of the jellyfish’s mouths, but the addition of a permeable layer (representative of the oral arms) significantly enhances mixing. (Video credit: T. Rodriguez et al.)
Granular Jets
Object impacts in water and other fluids often create cavities that generate jets when they collapse. But impacts on granular materials can produce similar results, forming a cavity, a splash corona, and, under the right circumstances, a jet. This Sixty Symbols video explores the effect of grain size (and thus weight) on the formation of such a rebound jet. Ultimately, the jet behavior is driven by air. When the granular material is poured, air gets trapped between the grains. The impact compresses the grains, forcing the previously trapped air up and out through the cavity created by the impact. Interestingly, once the air pressure is low enough, jet creation is suppressed, not unlike splash suppression in liquids. (Video credit: Sixty Symbols/Univ. of Nottingham)
Kelvin-Helmholtz in the Lab
The Kelvin-Helmholtz instability looks like a series of overturning ocean waves and occurs between layers of fluids undergoing shear. This video has a great lab demo of the phenomenon, including the set-up prior to execution. When the tank is tilted, the denser dyed salt water flows left while the fresh water flows to the right. These opposing flow directions shear the interface between the two fluids, which, once a certain velocity is surpassed, generates an instability in the interface. Initially, this disturbance is much too small to be seen, but it grows at an exponential rate. This is why nothing appears to happen for many seconds after the tilt before the interface suddenly deforms, overturns, and mixes. In actuality, the unstable perturbation is present almost immediately after the tilt, but it takes time for the tiny disturbance to grow. The Kelvin-Helmholtz instability is often seen in clouds, both on Earth and on other planets, and it is also responsible for the shape of ocean waves. (Video credit: M. Hallworth and G. Worster)
Colliding in Microgravity
On Earth, it’s easy for the effects of surface tension and capillary action to get masked by gravity’s effects. This makes microgravity experiments, like those performed with drop towers or onboard the ISS, excellent proving grounds for exploring fluid dynamics unhindered by gravity. The video above looks at how colliding jets of liquid water behave in microgravity. At low flow rates, opposed jets form droplets that bounce off one another. Increasing the flow rate first causes the droplets to coalesce and then makes the jets themselves coalesce. Similar effects are seen in obliquely positioned jets. Perhaps the most interesting clip, though, is at the end. It shows two jets separated by a very small angle. Under Earth gravity, the jets bounce off one another before breaking up. (The jets are likely separated by a thin film of air that gets entrained along the water surface.) In microgravity, though, the jets display much greater waviness and break down much quicker. This seems to indicate a significant gravitational effect to the Plateau-Rayleigh instability that governs the jet’s breakup into droplets. (Video credit: F. Sunol and R. Gonzalez-Cinca)
Droplets Surfing
The Leidenfrost effect can make water droplets skitter across a hot griddle or briefly protect a hand dunked in liquid nitrogen. When a liquid is exposed to a solid surface much, much hotter than its boiling point, the contact vaporizes part of the liquid, and, in the case of a droplet, forms a thin lubricating layer of vapor that the liquid drop can skate around on. Researchers have found that releasing these Leidenfrost droplets on textured surfaces creates self-propelling drops by directing the flow of vapor. In this video, one team demonstrates some of the neat tracks they’ve built for their drops. (Video credit: D. Soto et al.)
The March of Drops
I love science with a sense of humor. This video features a series of clips showing the behavior of droplets on what appears to be a superhydrophobic surface. In particular, there are some excellent examples of drops bouncing on an incline and droplets rebounding after impact. For droplets with enough momentum, impact flattens them like a pancake, with the rim sometimes forming a halo of droplets. If the momentum is high enough, these droplets can escape as satellite drops, but other times the rebound of the drop off the superhydrophobic surface is forceful enough to overcome the instability and draw the entire drop back off the surface. (Video credit: C. Antonini et al.)
Oily Foams
It is common in many industries to use oil as a defoamer to break up existing foams or prevent foams from forming. But with the right surfactants–additives that change the foam’s surface tension–it’s possible to make aqueous foams that are actually stabilized by the presence of oil. This video explores some of the ways that oil can interact with these kinds of foam, beginning with capillary action, which draws the oil up into the junctions between foam films. For more, see Piroird and Lorenceau. (Video credit and submission: K. Piroird)