A drop of red dye falls into a thin layer of milk, forming a crown splash. Notice the pale edges of the droplets at the rim of the crown; this is milk that has been entrained by the original drop. The rim and satellite droplets surrounding the splash are formed due to surface tension effects, chiefly the Plateau-Rayleigh instability—the same effect responsible for breaking a falling column of liquid into droplets like in a leaking faucet. The instability will have a most unstable wavelength that determines the number of satellite droplets formed. (Photo credit: W. van Hoeve et al., University of Twente)
Today many a glass of champagne will be raised in honor of the end of one year and the beginning of a new. This French wine, known for its bubbly effervescence, is full of fascinating physics. During secondary fermentation of champagne, yeast in the wine consume sugars and excrete carbon dioxide gas, which dissolves in the liquid. Since the bottle containing the wine is corked, this increases the pressure inside the bottle, and this pressure is released when the cork is popped. Once champagne is in the glass, the dissolved carbon dioxide will form bubbles on flaws in the glass, which may be due to dust, scratches, or even intentional marks from manufacturing. These bubbles rise to the surface, expanding as they do so because the hydrodynamic pressure of the surrounding wine decreases with decreasing depth. At the surface, the bubbles burst, creating tiny crowns that collapse into Worthington jets, which can propel droplets upward to be felt by the drinker. For more on the physics of champagne, check out Gerard Liger-Belair’s book Uncorked: The Science of Champagne and/or Patrick Hunt’s analysis. Happy New Year! (Video credit: AFP/Gerard Liger-Belair)
Here researchers simulate rain-like droplet impacts with large drops of water falling into a tank from several meters. The momentum of such an impact is significantly higher than many other droplet impact examples we’ve featured. In this case, the coronet, or crown-like splash, caused by the collision collapses quickly, closing the fluid canopy around a trapped bubble of air. The remains of the coronet fall inward, preventing the development of the usual Worthington jet associated with droplet impacts. Instead, the air bubble takes on an unstable donut-like shape. (Video credit: M. Buckley et al.)
Everyone knows the familiar plonk of a stone falling into a pond but few realize the complexity of the physics. When a solid object falls into a pool, a sheet of liquid, the crown splash, is sent upward. Simultaneously, the object pulls a cavity of air down with it. As the water moves inward, this cavity is pinched, creating an hourglass-like shape reminiscent of the shape of a rocket’s nozzle. As the diameter of that pinched cavity shrinks, the velocity of the upward escaping air increases, resulting in the formation of an air jet moving faster than the speed of sound. This air jet is followed by a slower liquid jet that may rebound to a height higher than then original height of the dropped object. So next time you throw a stone into a pond, enjoy the knowledge that you’ve broken the sound barrier. (Photo credit: D. van der Meer; see also Physics World)
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)
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)
This stereo 3D photo shows the Worthington jet ejected when a droplet impacts a pool. The flat crowning drop is formed from an ejected droplet colliding with a falling droplet.
I suspect demonstrating fluid mechanics was not what this cookbook had in mind when they filmed creamer poured into coffee at 2000 fps, but there’s some awesome droplet breakup, crowning, roiling turbulent mixing, and even some deformed Worthington jets here. It’s a reminder that, even though we may not notice it, fluid dynamics are all around.
This stereo photo of a droplet by John Hart shows the formation of a crown and droplet breakup. It’s possible to see the picture in 3D by crossing one’s eyes. #
