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.) #
Tag: droplets
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.
Impinging Without Coalescing
Three impinging jets of silicone oil rebound without coalescence due to thin-film lubrication between the jets. The motion of the oil replenishes the thin layer of air separating the streams. The same phenomenon keeps droplets from coalescing as well. (Photo credit: BIF Lab, Department of Engineering Science and Mechanics, Virginia Tech) #
Water Spray from a Tire
The spray thrown up by a rolling tire is simulated in the lab by running a single-grooved tire (top) against a smooth tire (bottom) that simulates the road. A supply of water flows from the left at the speed of the rolling tires (6 m/s). The resultant sheet of water is a familiar site to motorists everywhere. Holes in the the sheet of water collide to form the smallest droplets, whose diameters are comparable to the thickness of the sheet, of the order of 100 microns. Thicker parts of the sheet form ligaments and break down into large droplets through the Plateau-Rayleigh instability. (Photo credit: Dennis Plocher, Fred Browand and Charles Radovich) #
Fishbone Jet Collision
The collision of two jets of radius 420 μm results in a fishbone-like structure. The fluid contains a dilute polymer mixture whose viscoelastic effects resist the tendency of the droplets to detach from the ligaments. The breakup of the jets into droplets is important for applications in inkjet printing. The photo has been rotated 90-degrees for effect. (Photo credit: Sungjune Jung)
