This combined video shows the fall of a heated centimeter-sized steel sphere through water. From left to right, the sphere is at 25 degrees C (left), 110 degrees C (middle), and 180 degrees C, demonstrating how the Leidenfrost effect–which vaporizes the water in immediate contact with the sphere–can substantially reduce the drag on a submerged object. In the middle video, the vaporization of the water around the sphere is sporadic and incomplete, only slightly reducing the sphere’s drag relative to the room temperature case. The much hotter sphere on the right, however, has a complete layer of vapor surrounding it, allowing it to travel through a gas rather than the denser liquid. (Video credit: I. Vakarelski and S. Thoroddsen; from a review by D. Quere)
Videos
Inksplosion
Artist Pery Burge utilizes surface tension driven flows created with inks and water for much of her work. As mesmerizing as this is in still-life, it is more lovely still to see it develop and evolve in motion. The explosive outward motion of the ink is driven by the addition of a liquid with a lower surface tension than the ink/water mixtures. This is known as the Marangoni effect. You can observe it yourself using a plate of milk and food coloring into which you drop a tiny bit of dish soap. (The experiment works best with milk with some fat content.) Or, like the artist herself, you can experiment with other fluids you have on-hand! For more of Bruge’s work, see her website. (Video credit: Pery Bruge)
Mixing Physics
One of the most commonly observed fluid instabilities is the Rayleigh-Taylor instability, which occurs between fluids of differing densities. It’s most often seen when a denser fluid sits over a lower density fluid. In the video above, this is demonstrated experimentally: a lower density green fluid mixes in with the clear, higher density fluid. This is the classical case in which each initial region of fluid is uniform in density prior to the removal of the barrier. But what happens when each zone has its own variation in density? This is the second case. Before the barrier is removed, each region of the tank has a varying–or stratified–fluid density. In this case, the unmixed fluids are stably stratified, meaning that the fluid density increases with depth. At the barrier interface, the two separate fluids are still unstably stratified–with the denser fluid on top–so when the barrier is removed, the Rayleigh-Taylor instability still drives their mixing. Because of the stable stratification within the original unmixed fluids, the mixing region after the barrier’s removal is more limited. (Video credit: M. D. Wykes and S. B. Dalziel; via PhysicsCentral by APS)
Sloshing in a Bouncing Sphere
The sloshing of liquids inside solids is usually presented as a difficulty to overcome, as with the transport of tanks, the motion of fuel in satellites, or even the problem of walking with a full cup of coffee. But liquids also make a very effective damper, as in the case of a bouncing ball partially filled with liquid. Here we see high-speed video of the liquid’s motion inside the ball as it bounces and rebounds. Part of the ball’s kinetic energy at rebound is transferred into the fluid jet, reducing that available for the ball to transfer into potential energy. (Video credit: BYU Splash Lab)
Countertop Fliers
In this video, researcher Leif Ristroph and his colleagues have used a clever way to simulate flapping flight, not by actuating their fliers but by oscillating the flow. The flow is driven by a speaker, which causes the air above it to move up and down. Using straws to simulate the honeycomb flow conditioners often used in wind tunnels helps smooth flow. The end result is a great table-top set-up for testing and refining miniature flier designs. The best fliers stay aloft thanks to asymmetry in the streamwise direction; when the air moves upward, the flier catches the air, maximizing drag so that it is carried upward. When the flow reverses, however, the shape of the flier is more streamlined, so the drag is reduced, helping the flier stay aloft. (Video credit: Science Friday/Leif Ristroph et al.)
The Water Bridge
This short film offers an artistic look at the phenomenon of the water bridge. When subjected to a large voltage difference, such as the 30 kV used in the film, flow can be induced between water in two separated beakers. This creates a water bridge seemingly floating on air. There are two main forces opposing the bridge: gravity, which causes it to sag, and capillary action, which tries to thin the bridge to the point where it will break into droplets. These forces are countered by polarization forces induced at the liquid interface due to the electrical field separating the water’s positive and negative charges. This separation of charges creates normal stresses along the water surface, which counteracts the gravitational and capillary forces on the bridge. The artist has done a beautiful job of capturing the unsteadiness and delicacy of the phenomenon. (Video credit: Lariontsev Nick)
The Kaye Effect
The Kaye effect is an instability particular to a falling stream of non-Newtonian fluids with shear-thinning properties. When these fluids are deformed, their viscosity decreases; this, for example, is why ketchup flows out of a bottle more easily once it’s moving. Like most fluids, the falling shampoo creates a heap on the surface. The Kaye effect is kicked off when the incoming jet creates enough shear on part of the heap that the local viscosity decreases, causing the streamer–or outgoing jet–to slip off the side of the heap. As the incoming jet continues, a dimple forms in the heap where the streamer originates. As the dimple deepens, the streamer will rise until it strikes the incoming jet. This perturbation to the system collapses the streamer and ends the Kaye effect. This video also has a good explanation of the physics, along with demonstrations of a stable form of the Kaye effect in which the streamer cascades down an incline. (Video credit: Minute Laboratory; inspired by infplusplus)
Saffman-Taylor Demo
In this video, a thin film of viscous glycerin sits between two glass plates. As the plates are forced apart, air gets entrained from either side, causing finger-like instabilities to form between the two fluids. This is a result of the Saffman-Taylor mechanism. The final dendritic pattern depends on the fluid viscosities, surface tension, and any non-uniformities in the apparatus. (Video credit and submission by M. Goodman)
Flame Thrower Physics
This high-speed video–which we do not recommend recreating yourself–features burning gasoline flying through the air. In addition to the sheer entertainment value, there are some neat physics. In the first segment, when they kick a tray of gasoline, one can see lovely fiery vortices forming around the backside of the tray as it’s launched. This is the start of the tray’s wake. In the latter half of the video, they launch the flaming gasoline from a bucket. Notice how the flames are in the wake while liquid gasoline streams out ahead without burning. This is because it is primarily gaseous petrol that is flammable. As the liquid fuel breaks up into droplets heated by the burning gasoline vapors nearby, the rest of the fuel changes to a vapor state and catches flame. (Video credit: The Slow Mo Guys; submitted by Will T)
Champagne Science
Today many a glass of champagne will be raised in honor of the end of one year and the beginning of a new. This French wine, known for its bubbly effervescence, is full of fascinating physics. During secondary fermentation of champagne, yeast in the wine consume sugars and excrete carbon dioxide gas, which dissolves in the liquid. Since the bottle containing the wine is corked, this increases the pressure inside the bottle, and this pressure is released when the cork is popped. Once champagne is in the glass, the dissolved carbon dioxide will form bubbles on flaws in the glass, which may be due to dust, scratches, or even intentional marks from manufacturing. These bubbles rise to the surface, expanding as they do so because the hydrodynamic pressure of the surrounding wine decreases with decreasing depth. At the surface, the bubbles burst, creating tiny crowns that collapse into Worthington jets, which can propel droplets upward to be felt by the drinker. For more on the physics of champagne, check out Gerard Liger-Belair’s book Uncorked: The Science of Champagne and/or Patrick Hunt’s analysis. Happy New Year! (Video credit: AFP/Gerard Liger-Belair)