Fuck Yeah Fluid Dynamics

Celebrating the physics of all that flows. Ask a question, submit a post idea or send an email. You can also follow FYFD on Twitter and Google+. FYFD is written by Nicole Sharp, PhD.

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Posts tagged "leidenfrost effect"

Science Friday takes an inside look at self-propelled Leidenfrost droplets like those we’ve featured previously. The Leidenfrost effect takes place when a liquid comes in contact with a surface much, much hotter than its boiling point. Part of the liquid is vaporized, creating a thin gas layer that both insulates the remaining liquid and causes it to move with very little friction. Over a flat surface, this underlying vapor will spread in any direction. But by covering the surface with ratchets, it’s possible to direct the vapor in a particular direction, which propels the droplet in the opposite direction. Check out the video and our previous posts for more! (Video credit: Science Friday; via io9 and submitted by Urs)

Leidenfrost drops hover and move above hot surfaces on a thin layer of their own vapor. Over a flat surface, this vapor flows radially out from under the droplet, but creating rachets in the surface forces the vapor to flow in a single direction. The vapor then acts like exhaust, generating propulsion in the droplet and making it roll. How quickly the drop moves depends both on the droplet’s size and the rachets’ aspect ratio. For a given length, deeper rachets propel a drop faster than their shallower counterparts. The droplet’s size also affects the thrust with different scalings depending on the drop’s initial size. Like all of this week’s videos, this video is an entry in the 2013 Gallery of Fluid Motion. (Video credit: A. G. Marin et al.)

The Leidenfrost effect occurs when liquids come in contact with a substrate much, much hotter than their boiling temperature. Rather than immediately boiling away, a thin layer of the liquid vaporizes and insulates the bulk of the liquid from the heat. This essentially turns droplets into tiny hovercrafts that skate over the surface. If you use a rough surface with rachets, the Leidenfrost drops will self-propel toward the steepest part of the rachet. The vapor underneath the drop is constantly trying to flow away, and the rachets in the surface prevent the vapor from escaping in the steeper direction. The vapor instead flows out the shallower side and—thanks to Newton’s third law—creates thrust that pushes the droplet the opposite direction. Here students from the University of Bath have used these effects to build a maze through which the droplets fly. (Video credit: C. Cheng et al.; via Flow Visualization FB page and several submissions)

For readers at Texas A&M University, I will be giving a talk Wednesday, October 2nd entitled “The Beauty of the Flow” as part of the Applied Mathematics Undergraduate Seminar series at 17:45 in BLOC 164. 

Water splattered onto a a hot skillet will skitter and skip across the surface on a thin layer of vapor due to the Leidenfrost effect. The partial vaporization of the droplet provides a low-friction cushion for the droplet to glide on and acts as an insulating layer that delays the vaporization of the rest of the droplet. Modernist Cuisine shows us how serene this common and sometimes explosive effect looks at 3,000 frames per second. (On the topic of cooking, you can use the Leidenfrost effect to see if your skillet is hot enough when making pancakes. If a few droplets of water skitter across the pan before sizzling away, then your pan is ready for batter!) (Video credit: Modernist Cuisine; submitted by Eban B.)

When liquids hit a surface much hotter than their boiling point, a thin layer of gas can form between the drop and surface, allowing the drop to glide along. This Leidenfrost effect is what makes drops of water skitter across a hot pan. But what happens when the pan isn’t flat? The video above shows a Leidenfrost drop on a ratchet-like surface. Instead of gliding or skittering randomly, the drop self-propels toward the steepest section of the ratchet  This behavior allows researchers to design surfaces that guide the drops on an intended path. (Video credit: G. Lagubeau and D. Quéré)

This combined video shows the fall of a heated centimeter-sized steel sphere through water. From left to right, the sphere is at 25 degrees C (left), 110 degrees C (middle), and 180 degrees C, demonstrating how the Leidenfrost effect—which vaporizes the water in immediate contact with the sphere—can substantially reduce the drag on a submerged object. In the middle video, the vaporization of the water around the sphere is sporadic and incomplete, only slightly reducing the sphere’s drag relative to the room temperature case. The much hotter sphere on the right, however, has a complete layer of vapor surrounding it, allowing it to travel through a gas rather than the denser liquid. (Video credit: I. Vakarelski and S. Thoroddsen; from a review by D. Quere)

When a drop of water touches a very hot pan, it will skitter across the surface on a thin layer of water vapor due to the Leidenfrost effect. But what happens when another chemical is added to the droplet? Researchers find that adding a surfactant to the water droplets creates some spectacular results. As the water evaporates, the concentration of the surfactant in the droplet increases causing the surfactant to form a shell around the droplet. The pressure inside the droplet increases until the shell breaks in a miniature explosion much like the popping of popcorn. (Video credit: F. Moreau et al.)

Ethanol droplets on a hot copper plate bounce under the influence of electrostatic forces from a charged rod. The temperature of the plate is high enough that the droplet is supported by a thin vapor film, which is what keeps it from wetting the plate.  Ethanol does not have the strong polarity that water does, but the hydroxyl group on one end does make it susceptible to the electrostatic charge built up on the teflon rod.  As a result, the droplets oscillate under electrostatic and gravitational forces, resulting in a dribbling effect. (Video credit: S. Wildeman et al.)

The Leidenfrost effect occurs when a liquid encounters a solid object much hotter than the liquid’s boiling point, like when water skitters on a hot griddle or someone plunges a hand in liquid nitrogen.  A thin layer of vapor forms between the liquid and the solid, thereby (briefly) insulating the remaining liquid. The Leidenfrost effect can be static—like a droplet sitting on a pan—or dynamic, like the video above in which a droplet impacts the hot object.  The video shows both a top and a side view of a droplet striking a plate that is over five times hotter than the liquid’s boiling point.  On impact, the droplet spreads and flattens, and a spray of even tinier droplets is ejected before rebound. (Video credit: T. Tran and D. Lohse, from a review by D. Quere)

When a liquid impacts a solid heated well above the liquid’s boiling point, droplets can form, levitating on a thin film of vapor that helps insulate them from the heat of the solid. This is known as the Leidenfrost effect. Here a very large Leidenfrost droplet is shown from the side in high-speed. A vapor chimney forms beneath the drop, causing the dome in the liquid. When the dome bursts, the droplet momentarily forms a torus before closing. The resulting oscillatory waves in the droplet are spectacular. The same behavior can be viewed from above in this video. (Video credit: D. Soto and R. Thevenin; from an upcoming review by D. Quere)