To the human eye, the burst of a soap bubble appears complete and instantaneous, but high-speed video reveals the directionality of the process. Surface tension is responsible for the spherical shape of the bubble, and, when the bubble is pierced, surface tension is broken, causing the soap film that was the bubble to contract like elastic that’s been stretched and released. Droplets of liquid fly out from the edges of the sheet until it atomizes completely.
Previously we saw how vibration could atomize a water droplet, breaking it into a spray of finer droplets. Here astronaut Don Pettit shows us what the process looks like in microgravity using some speakers and large water droplets. At low frequencies the water displays large wavelength capillary waves and vertical vibrations. Higher frequencies—like the earthbound experiment on much smaller droplets—cause fine droplets to eject from the main drop when surface tension can no longer overcome their kinetic energy. (submitted by aggieastronaut, jshoer and Jason C)
Surface tension arises from intermolecular forces along the interface of a fluid, but despite its molecular origins, it can have some substantial macroscopic effects. Here researchers demonstrate how surface tension can hold up metal coins that would otherwise sink. Moreover, when multiple coins are set on the surface of the water, surface tension draws them together into a closely packed array because it reduces the surface energy by creating a single large well instead of many small ones. This is the same reason that your Cheerios tend to clump together on the surface of your milk when you’re eating breakfast! (Video credit: Lawrence Berkeley National Lab)
Sometimes bursting one bubble just leads to more bubbles. This high-speed video shows how popping a bubble sitting on a fluid surface can lead to a ring of daughter bubbles. When the surface of the bubble is ruptured, filaments of the liquid that made up the surface are drawn back toward the pool by surface tension, trapping small pockets of the air that had been inside the bubble. A dimple forms on the surface and rebounds as a jet that lacks the kinetic energy to eject droplets. Watch as the jet returns to the interface, and you will notice the tiny bubbles around it. At 56 ms, one of the daughter bubbles on the left bursts. See Nature for more. (Video credit: J. Bird et al)
Superhydrophobic surfaces resist wetting from water, but it turns out they can also trigger interesting behaviors in the tiny droplets condensing on the surface. High-speed video reveals that when two condensate droplets coalesce, the energy released by surface tension causes the new droplet to jump off the surface. The phenomenon is the same as one observed in some types of mushroom—when a condensate droplet touches a wetted spore, the spore is ejected from the mushroom. (Video credit: J Boreyko)
(Source: pratt.duke.edu)
In this video, a miniature tornado-like vortex is created inside a soap bubble. Here’s how it works: after the first bubble is formed and the smoke-filled bubble is attached to the outside, he blows into the main bubble, creating a weak angular velocity, before breaking the interface between the two bubbles. As the smoke mixes in the main bubble, note how it is already spinning slowly due to the free vortex he created. Then, when the top of the bubble is popped, surface tension pulls the bubble’s surface inward. Because the bubble radius is decreasing, conservation of angular momentum causes the angular velocity of the fluid inside to increase, pulling the smoke into a tight vortex, much like a spinning ice skater who pulls her arms inward.
Stuck here on Earth, it’s hard to know sometimes how greatly gravity affects the behavior of fluids. Fortunately, astronaut Don Pettit enjoys spending his free time on the International Space Station playing with physics. In his latest video, he shows some awesome examples of what is possible with a thin film of water—not a soap film like we make here on Earth—in microgravity. He demonstrates vibrational modes, droplet collision and coalescence, and some fascinating examples of Marangoni convection.
This high-speed video shows a soap film formed across two rings and its deformation and breakup as the two rings are pulled apart. As the rings get further apart, surface tension deforms the soap film until the distance is too great to continue sustaining that shape. The film breaks into two—a sheet of soap film in each ring—and a little satellite bubble. Note the similarities in breakup between this soap film and a thin liquid column or water from a faucet.
The splashes from droplets impacting jets create truly mesmerizing liquid sculptures. Corrie White is one of the masters of this type of high-speed macro photography. Her work captures the instantaneous battles between viscosity, surface tension, and inertia. The fantastic structure seen here through the falling droplets is created by a series of drops timed so that the later ones strike the Worthington jet produced by the initial drop’s impact. (Photo credit: Corrie White)
A little polymer goes a long way when it comes to changing a fluid’s behavior. Normally, a falling jet of fluid will develop waviness and be driven by surface tension and the Plateau-Rayleigh instability to break up into a stream of droplets. We see this at our water faucets all the time. But when traces of a polymer are dissolved in water, the behavior is much different. The viscoelasticity of the polymer chains creates a force that opposes the thinning effects caused by surface tension. So, instead of thinning to the point of breaking into droplets, a drop is able to climb back up the jet until it reaches a critical mass where it reverses direction, accelerates downward due to gravity and eventually breaks off the jet. Then the whole process begins again with a new terminal drop. (Video credit: C. Clasen et al)
(Source: web.mit.edu)
Celebrating the physics of all that flows. 
