When a water balloon pops in microgravity, waves propagate from the initial point of contact and the final point of contact (where the balloon skin peels away). As these waves come inward toward one another, the water is compressed from its original potato-like shape into a pancake-like one. In most cases, surface tension will provide a damping force on this oscillatory motion, eventually making the water into a sphere. On Earth, in contrast, a water balloon seems to hold its shape after popping. This is because the effect of gravity on the water is much larger than the effect of the propagating waves. This is one reason that it is useful to have a laboratory in space! Without a microgravity environment, it is much harder to study and observe secondary and tertiary-order forces on a physical event. (Video credit: Don Pettit, Science Off The Sphere)
Search results for: “water balloons”
Water Balloons in Microgravity
Sometimes you need microgravity in order to observe the neat effects of surface tension on a fluid. Also, I hear it’s a good excuse for popping water balloons on the Vomit Comet. #
A Water Balloon on a Bed of Nails
If you dropped a water balloon on a bed of nails, you’d expect it to burst spectacularly. And you’d be right – some of the time. Under the right conditions, though, you’d see what a high-speed camera caught in the animation above: a pancake-shaped bounce with nary a leak. Physically, this is a scaled-up version of what happens to a water droplet when it hits a superhydrophobic surface.
Water repellent superhydrophobic surfaces are covered in microscale roughness, much like a bed of tiny nails. When the balloon (or droplet) hits, it deforms into the gaps between posts. In the case of the water balloon, its rubbery exterior pulls back against that deformation. (For the droplet, the same effect is provided by surface tension.) That tension pulls the deformed parts of the balloon back up, causing the whole balloon to rebound off the nails in a pancake-like shape. For more, check out this video on the student balloon project or the original water droplet research. (Image credits: T. Hecksher et al., Y. Liu et al.; via The New York Times; submitted by Justin B.)
Underwater Explosions
Underwater explosions are incredibly dangerous and destructive, and this animation shows you why. What you see here are three balloons, each half-filled with water and half with air. A small explosive has been set off next to them in a pool. In air, the immense energy of an explosion actually doesn’t propagate all that far because much of it gets expended in compressing the air. Water, on the other hand, is incompressible, so that explosive energy just keeps propagating. For squishy, partially air-filled things like us humans or these balloons, that explosion’s force transmits into us with nearly its full effect, causing expansion and contraction of anything compressible inside us as our interior and exterior pressures try to equalize. The results can be devastating. To see the equivalent experiment in air, check out Mark Rober’s full video on how to survive a grenade blast. (Image credit: M. Rober, source)
Underwater Explosions
As dangerous as explosions are in air, they are even more destructive in water. Because air is a compressible fluid, some part of an explosion’s energy is directed into air compression. Water, on the other hand, is incompressible, which makes it an excellent conductor of shock waves. In the video above we see some simple underwater explosions using water bottles filled with dry ice or liquid nitrogen. The explosions pulsate after detonation due to the interplay between the expanding gases and the surrounding water. When the gases expand too quickly, the water pressure is able to compress the gases back down. When the water pushes too far, the gases re-expand and the cycle repeats until the explosion’s energy is expended. This pulsating change in pressure is part of what makes underwater explosions so dangerous, especially to humans. Note in the video how the balloons ripple and distort due to the changing pressure. Those same changes in pressure can cause major internal damage to people. (Video credit: The Backyard Scientist; submitted by logicalamaze)
Balloons in the Car
Destin from Smarter Every Day has just made a video on one of my favorite fluids brain teasers: what happens to a helium balloon when you accelerate in a car? Take a moment to think about the answer before watching or reading further…
Okay, so what happens? Contrary to what you may expect, hitting the accelerator with a balloon in the car will make it shift forward. This is a matter of buoyancy. As Destin demonstrates with the water bottle, when two fluids are accelerated forward, the denser one will shift backwards, which pushes the lighter one forward. Because the helium is lighter than the air filling the car, accelerating pushes the air backward (just as it does the pendulum and the car’s inhabitants) and that shifting of the air pushes the helium in the balloon forward. (Video credit: Smarter Every Day)
Water Balloon Physics
[original media no longer available]
This video explores some of the physics behind the much-loved bursting water balloon. The first sections show some “canonical” cases–dropping water balloons onto a flat rigid surface. In some cases the balloon will bounce and in others it breaks. The bursting water balloons develop strong capillary waves (like ripples) across the upper surface and have some shear-induced deformation of the water surface as the rubber peals away. Then the authors placed a water balloon underwater and vibrated it before bursting it with a pin. They note that the breakdown of the interface between the balloon water and surrounding water shows evidence of Rayleigh-Taylor and Richtmyer-Meshkov instabilities. The Rayleigh-Taylor instability is the mushroom-like formation observed when stratified fluids of differing densities mix, while the Richtmyer-Meshkov instability is associated with the impulsive acceleration of fluids of differing density.
Giant Water Balloon Physics
Playing with a giant water balloon and high-speed cameras is like a giant experiment in surface tension, right up until the tensile strength of the balloon comes into play. The rippling in the balloon is reminiscent of the motion of droplet breakup or impact on superhydrophobic surfaces. (submitted by Daniel B)
Water Balloon Photography
Photographer Edward Horsford uses high-speed photography to capture water balloons as they burst. On Earth, of course, gravity wins over surface tension, but the results are very different in microgravity. See the technical description for how Horsford gets his shots and look at more of his work on Flickr. (via NPR)
The Destructive Power of a Blank
Removing the slug does not make a bullet harmless, as the Slow Mo Guys demonstrate in this video. They’re shooting blanks — casings that still contain propellant but no projectile. There’s still more than enough force to obliterate an egg, lunch meat, and water balloons. You really don’t want one of these fired near you.
It looks as though the burning propellant is generally the first thing to puncture in each of these. Then the gas from the explosion blows the rest of the object away. The most interesting segment, to me, was the final (pink) water balloon, where the blast wave and its aftermath are visible in a schlieren-like effect that passes over the balloon before its destruction. The sun must have been at just the right position relative to their set-up. (Video and image credit: The Slow Mo Guys)