Fuck Yeah Fluid Dynamics

Celebrating the physics of all that flows. Ask a question, submit a post idea or send an email. You can also follow FYFD on Twitter and Google+. FYFD is written by Nicole Sharp, PhD.

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Posts tagged "plateau rayleigh instability"

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)

Taylor and Culick predicted a constant velocity for the rim of an opening hole in a soap film of uniform thickness. Unfortunately, it is difficult to experimentally produce a soap film of uniform thickness. It is much easier to create films of uniform thickness with liquid crystals in their smectic-A phase, in which the molecules are ordered in layers along a single direction. When smectic-A bubbles burst, however, it bears little resemblance to a soap bubble. Smectic-A bubbles burst spontaneously during oscillations, the holes in the film growing until a network of filaments is left behind. The filaments themselves will rapidly break up into droplets due to the Plateau-Rayleigh instability.  (Photo credit: R. Stannarius et al.)

What is the shape of a falling raindrop? Surface tension keeps only the smallest drops spherical as they fall; larger drops will tend to flatten. The very largest drops stretch and inflate with air as they fall, as shown in the image above. This shape is known as a bag and consists of a thin shell of water with a thicker rim at the bottom. As the bag grows, its shell thins until it ruptures, just like a soap bubble. The rim left behind destabilizes due to the surface-tension-driven Plateau-Rayleigh instability and eventually breaks up into smaller droplets. This bag instability limits the size of raindrops and breaks large drops into a multitude of smaller ones. The initial size of the drop in the image was 12 mm, falling with a velocity of 7.5 m/s. The interval between each image is 1 ms. (Photo credit: E. Reyssat et al.)

Have you ever wondered what happens inside a jet of fluid as it breaks into droplets? Such events are not commonly or readily measured. This video uses a double emulsion—in which immiscible fluids are encapsulated into a multi-layer droplet—to demonstrate interior fluid flow during the Plateau-Rayleigh instability. The innermost drops and the fluid encapsulating them have a low surface tension between them, thanks to the addition of a surfactant to the inner drops. As a result, the inner drops are easily deformed by motion in the fluid surrounding them. Flow on the left side of the jet is clearly parabolic, similar to pipe flow. Closer to the pinch-off, the inner droplets shift to vertical lines, indicating that the interior flow’s velocity is constant across the jet. After pinch-off, the inner droplets return to a spherical shape because they are no longer being deformed by fluid movement around them. The coiling of the inner drops inside the bigger one is due to the electrical charges in the surfactant used. (Video credit: L. L. A. Adams  and D. A. Weitz)

A falling column of liquid, like the water from your faucet, will tend to break up into a series of droplets due to the Plateau-Rayleigh instability. This instability is driven by surface tension. Small variations in the radius of the column occur naturally. Where the radius shrinks, the pressure due to surface tension increases, causing liquid to flow away, which shrinks the column’s radius even further. Eventually the column pinches off and breaks into droplets. What’s especially neat is that the size of the final droplets can be predicted based on the column’s initial radius and the wavelength of its disturbances. (Video credit: BYU Splash Lab)

When a stream of liquid falls, a surface tension effect called the Plateau-Rayleigh instability causes small variations in the jet’s radius to grow until the liquid breaks into droplets. For a kitchen faucet, this instability acts quickly, breaking the stream into drops within a few centimeters. But for more viscous fluids, like honey, jets can reach as many as ten meters in length before breaking up. New research shows that, while viscosity does not play a role in stretching and shaping the jet as it falls—that’s primarily gravity’s doing—it plays a key role in the way perturbations to the jet grow. Viscosity can delay or inhibit those small variations in the jet’s diameter, preventing their growth due to the Plateau-Rayleigh instability. In this respect, viscosity is a stabilizing influence on the flow. (Photo credit: Harsha K R; via Flow Visualization)

Artist Fabian Oefner captures these colorful portraits of fluid instability by dripping acrylic paints onto a metal rod, which is connected to a drill. When the drill is switched on, paint is flung away from the rod, creating these snapshots of centripetal force and surface tension. Note how droplets gather at the ends of the spiral arms like in a Plateau-Rayleigh or a rimming instability. For more, check out Oefner’s webpage, which includes a video showing how the images are made, or his previously featured work, “Millefiori”. (Photo credit: F. Oefner; submitted by Stephen D.)

A drop of red dye falls into a thin layer of milk, forming a crown splash. Notice the pale edges of the droplets at the rim of the crown; this is milk that has been entrained by the original drop. The rim and satellite droplets surrounding the splash are formed due to surface tension effects, chiefly the Plateau-Rayleigh instability—the same effect responsible for breaking a falling column of liquid into droplets like in a leaking faucet. The instability will have a most unstable wavelength that determines the number of satellite droplets formed. (Photo credit: W. van Hoeve et al., University of Twente)

In the midst of holiday travels, take a moment (particularly if you’re flying through Detroit) to enjoy the simple beauty of WET Design’s fountain in the McNamara Terminal. Laminar jets arc through the air almost like perfect crystalline columns of fluid. Watch closely and you’ll see a few wavy variations—like a Plateau-Rayleigh instability creeping in—but there will be no turbulence to distress passengers and passers-by. (Video credit: WET Design)