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)
Tag: droplets
Fragmenting Raindrops
This numerical simulation demonstrates the fragmentation of droplets of water falling through a quiescent medium–essentially how a raindrop behaves. As the initial droplet falls, drag forces deform the droplet, contorting it until surface tension causes it to break into smaller droplets, which can themselves be broken up by the same mechanisms.
Atomizing Jets
The breakup of impinging jets into droplets (also called atomization) and the subsequent dynamics of those droplets are important in applications like jet and rocket engines where the mixing of liquid fuel with oxygen is necessary for efficient combustion. This video showcases recent efforts in high fidelity numerical simulation and modeling of such flows. The complexity of the problem requires clever ways of reducing the computational efforts required. One such method uses adaptotive meshing to concentrate grid points in areas where variables are changing quickly while leaving the grid sparse in areas of less interest. Because the flow is constantly evolving, the mesh must be able to adapt as the simulation steps forward in time. Even so, such calculations typically require supercomputers to complete. (Video credit: X. Chen et al)
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.) #
Seed-Ejection via Raindrop
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We don’t often think of plants as using fluid dynamics aside from capillary action drawing water from their roots, but many plants also use fluid dynamics to disperse reproductive materials. This high-speed video explores the efficacy of splashing raindrops at ejecting seeds from different blossoms. (Video credit: G. Amador et al)
Freezing Drops
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The physics of droplets freezing is important for understanding applications like ice formation on airplane wings. Here we see how a warm droplet deposited on a cold plate freezes. A freezing front advances through the drop, which expands vertically as it freezes. Ultimately, the expansion of the ice and the surface tension of the water create a pointed singular tip.
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.
The Disintegrating Bowl
A viscous fluid droplet impacts a thin layer of ethanol, which has a lower surface tension than the viscous fluid. A spray of tiny ethanol droplets is thrown up while a bowl-shaped crown of the viscous fluid forms. As the ethanol droplets impact the bowl, the lower surface tension of the ethanol causes fluid to flow away from points of contact due to the Marangoni effect. This outflow causes holes to form in the crown, forming a network of thin fluid ligaments. For more, see this paper (PDF) and video. (Photo credit: S.T. Thoroddson et al)
Vibration-Induced Atomization
Atomization–breaking a liquid into a fine spay of droplets–is common in engines, printers, and in the shower. Here a droplet of water is placed on a thin metal diaphragm that is vibrated at 1 kHz with increasing vibrational amplitude. Capillary waves form on the droplet, and once a critical vibrational amplitude is achieved, tiny droplets are ejected. Full atomization of the original droplet is achieved in about 0.3 seconds real-time. #
The Coalescence Cascade
When a droplet impacts a pool at low speed, a layer of air trapped beneath the droplet can often prevent it from immediately coalescing into the pool. As that air layer drains away, surface tension pulls some of the droplet’s mass into the pool while a smaller droplet is ejected. When it bounces off the surface of the water, the process is repeated and the droplet grows smaller and smaller until surface tension is able to completely absorb it into the pool. This process is called the coalescence cascade.
