Physics

Video Prize Dominated by Superhydrophobic Surfaces

November 27, 2019• Physics 12, 136

Animal figures manufactured from salt, drops bouncing off cones, and flower-shaped movies win the highest video prize from the APS Division of Fluid Dynamics.

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A body from one of many Gallery of Fluid Movement award-winning movies reveals three examples of “crystal critters” produced by salt water evaporating on a scorching, superhydrophobic floor. (See movies beneath.)

Figure caption

A body from one of many Gallery of Fluid Movement award-winning movies reveals three examples of “crystal critters” produced by salt water evaporating on a scorching, superhydrophobic floor. (See movies beneath.)×

Like so many different nice discoveries, it occurred by chance. Massachusetts Institute of Know-how graduate scholar Samantha McBride was observing tiny droplets of salt water evaporate on a scorching, bumpy floor, when she noticed miniscule animal collectible figurines, reminiscent of elephants and crabs, kind out of the salt crystals. “First time I noticed it, I used to be very excited,” she says. The video McBride and her colleagues produced gained the 2019 Milton van Dyke Award, the prize given to the highest three movies within the APS Division of Fluid Dynamics Gallery of Fluid Movement video contest.

The researchers had been learning the issue of salt deposits damaging pipe partitions in desalination crops. They began with the fundamentals, creating salt crystals by evaporating salt-saturated water, utilizing a spread of salts, surfaces, and temperatures. One floor sort they thought-about was the so-called superhydrophobic floor, which has microscale or nanoscale bumps or different patterns that may strongly repel water. This property can probably cut back crystal formation from drying drops of salt water. McBride found that when she positioned a microliter droplet of saturated salt water on a scorching superhydrophobic floor, it fashioned a roughly spherical crystal shell because the water evaporated. However then out of the blue, when the water was principally gone, the shell started to develop legs that steadily lifted the sphere off the floor.

When a droplet of salt water evaporates on a scorching, superhydrophobic floor, shocking, animal-like buildings kind.

After a lot experimentation, McBride realized that the legs develop as a result of, as soon as the shell has fashioned, the remaining water drains to the underside of the sphere and clings to it each in and out, preferring to stay to the water-loving salt slightly than the superhydrophobic floor. However the liquid nonetheless contacts the new floor at a couple of remoted factors, the place the water quickly evaporates and leaves behind the slender legs.

Moreover making lovely video, these experiments may probably result in methods of decreasing corrosion of crucial uncovered surfaces on ships, reminiscent of navigation gear or components of the propulsion programs, says MIT engineering professor Kripa Varanasi, McBride’s supervisor. Sea spray hitting a heated superhydrophobic floor may kind crystalline buildings that merely break off, slightly than forming deposits that require periodic cleansing.

The workforce was capable of range the shapes produced by adjusting parameters such because the salt content material and the temperature. Varanasi says that it is a uncommon case the place a nanoscale phenomenon—the interface between the crystalline legs and the strong floor—is controllable by adjusting macroscopic parameters. “These are the issues I really like,” he says. “With out doing any loopy exact processes, you’re capable of management this phenomenon.”

In creating their video, the workforce selected the soundtrack, “Within the Corridor of the Mountain King,” a well-known 1875 symphonic piece by Edvard Grieg. “We needed one thing sort of dramatic for when the crystal lifts off,” says McBride. Workforce member and MIT graduate scholar Henri Girard says the music is “the best mixture of gorgeous and foolish.”

Water bounces off of a superhydrophobic cone to kind a hoop that breaks up into droplets.

One other van Dyke winner additionally used superhydrophobic surfaces, on this case to point out the shocking habits of water drops when falling onto such surfaces. Guillaume Durey, of the Lutetium Challenge and ESPCI Paris and his colleagues confirmed drop hitting a flat superhydrophobic floor bounces again as a principally intact blob. But when the floor is a cone pointing up, then the drop bounces off as a hoop that splits into smaller droplets. With the best selection of cone angle, these droplets shoot out horizontally, as a result of gravity balances the upward response drive for the surface-drop interplay. “We had been pleasantly stunned to watch liquid rings, as they’re elusive objects,” says workforce member Pierre Chantelot of ESPCI Paris. He says that the fragmentation of drops happens in lots of conditions, so learning it right here may, for instance, assist enhance understanding of the dispersion of contaminants within the ambiance by means of small drops.

Drying a colloid containing nanoparticles results in a flower-shaped movie.

The third van-Dyke-winning video, from Paul Lilin of MIT and his colleagues, additionally has one thing in frequent with McBride’s video—in each instances the researchers demonstrated the inventive outcomes of letting water evaporate. However in Lilin’s video, the fluid is a colloid—water containing silica nanoparticles—slightly than salt water. Because the movie dries, it develops inside stresses that produce symmetric flower patterns within the dry movie. The workforce may management the variety of equally-spaced cracks by adjusting the density of colloid particles. “I’m fascinated by the wealthy number of totally different patterns that happen in such a seemingly easy system,” says workforce member Irmgard Bischofberger of MIT. “I imply, it is only a bunch of tiny particles in water.”

–David Ehrenstein

David Ehrenstein is the Focus Editor for Physics.

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