This video creates the illusion of a jet of water frozen in mid-air. The effect is achieved by vibrating the water at the frequency of the speaker, then filming at a frame rate identical to the vibrational frequency. Thus the water pulses at the exact rate that the camera captures images, making the water appear stationary even though it is moving. (submitted by Simon H)
Tag: vibration
Science off the Sphere: Thin Films
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.
Making Mixed Emulsions
Ever tried to mix oil and vinegar? Anyone who has ever dealt with salad dressings knows the difficulty of evenly distributing immiscible fluids; the key is to shake them and create an emulsion, where droplets of one fluid are distributed throughout another. In this video, researchers create a double emulsion–oil in water in oil–without touching the two fluids. First they suspend a drop of water on a wire and then coat it with oil. Below, they place a bath of silicone oil, which they vibrate. When the oil-coated droplet falls onto the bath, it bounces on the surface rather than coalescing because a thin layer of air–constantly refreshed due to the vibration of the surface–separates the droplet from the bath. When the amplitude of the vibration is large enough, the oil coating penetrates the water during the bounce, leaving behind a tiny droplet and creating the emulsion. (Video credit: D. Terwange et al; Research paper)
Ultrasonic Levitation of Drops
This video shows an ultrasonically levitated 3 mm drop of propylene glycol changing shape. A couple of things are happening here. Firstly, the drop is suspended due to the acoustic radiation pressure from intense ultrasonic sound waves being produced by a transducer vibrating at 30kHz. Then the power input to the ultrasonic transducer is increased, which strengthens the acoustic field, and this is what causes the drop to flatten. Currently, acoustic levitation is used for containerless processing of very pure materials or chemicals. As with many methods for levitation, it is currently restricted to objects of relatively light weight. (Video credit: J. R. Saylor et al, Clemson University)
Sound Sculptures
This is another fun and artistic use of non-Newtonian fluids (paint) vibrating on a speaker cone for advertising purposes. The shear-thinning viscous properties of the paint vie with surface tension to create lovely instantaneous sculptures of color. Check out Canon’s Pixma ads for similar artwork.
Wave-Particle Duality in Bouncing Droplets
A droplet atop a vibrating pool is prevented from coalescing by the constant influx of air into a thin lubrication layer between it and the pool. But that is not the strangest aspect of its behavior. Researchers have found that this system demonstrates some aspects of the mind-bending wave-particle duality at the heart of quantum physics. (Submitted by Dan H.) #
Water Balloon Physics
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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.
Sloshing Dynamics
[original media no longer available]
Sloshing refers to the motion of a liquid inside a moving container, for example, in tanker trucks or inside a spacecraft’s fuel tank. The motion of the liquid payload can drastically affect the dynamics of the vehicle carrying it due to the ever shifting center of mass. In the video above, dyed water is being oscillated horizontally to and from the camera. As the frequency of this oscillation changes, the modes of sloshing–the shapes the liquid surface assumes–change dramatically.
Aeroelastic Flutter
Flutter is a rather innocuous term for a potentially dangerous phenomenon that can occur for any flexible structure in a moving flow. Aeroelastic flutter occurs when aerodynamic forces and a structure’s natural modes of vibration get coupled: the surrounding flow causes the object to vibrate, which alters the nature of the aerodynamic forces on the object, which, in turn, feeds into the object’s vibration. In some cases, damping will contain the motion to a limit cycle, but under other conditions, flutter results in an uncontrollable self-exciting oscillation that persists until destruction, as in the famous Tacoma Narrows Bridge collapse.
Playing Pac-Man with Water Droplets
The vibrations of a plate in the horizontal and vertical directions can be used to control the motion of a drop placed on the surface. Here a droplet of water on a superhydrophobic surface is controlled by joystick a la Pacman. For more, see papers here and here.