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Tag: Leidenfrost drops

  • Leidenfrost Collapse

    Leidenfrost Collapse

    When a droplet encounters a surface much hotter than its boiling point, it forms a thin layer of vapor that insulates the liquid from the surface. But this Leidenfrost effect can’t last forever. Eventually, the vapor layer destabilizes and the drop touches the surface, causing explosive boiling that destroys the drop.

    To determine how the layer destabilizes, researchers simulated the breakdown. To their surprise, they found that inertial forces in the micron-thin vapor layer were critical for destabilization. The gas inertia caused reductions in pressure that pulled the liquid toward the surface. Usually at these small scales, we’d ignore inertial effects and focus instead on viscosity, but, for Leidenfrost drops, that simplification doesn’t work. (Image credit: L. Gledhill; research credit: D. Harvey and J. Burton)

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    Leidenfrost Explosion

    When a water drop hits a surface that’s much hotter than its boiling point, part of it will vaporize immediately. Depending on the temperature, this Leidenfrost effect can be a relatively gentle process — or not. Here, the surface is so hot that the entire drop is boiling before it’s even finished spreading from impact. The vapor in contact with the surface is trying to escape, bubbling up so violently that it rips the original droplet into a spray of tiny droplets. (Video and image credit: L. Gledhill)

  • Triple Leidenfrost Effect

    Triple Leidenfrost Effect

    Droplets can skitter across a hot surface on a layer of their own vapor, thanks to the Leidenfrost effect. If two Leidenfrost droplets of the same liquid collide, they merge immediately. But that doesn’t always happen with two dissimilar liquids. A new study looks at how dissimilar Leidenfrost droplets collide. The researchers found that these drops can bounce off one another repeatedly before their eventual merger (Image 1).

    Just as a vapor layer prevents the drops from touching the hot plate, a vapor layer forms between them when they collide, preventing contact (Image 2). Because of these three distinct areas of Leidenfrost vapor (one beneath each drop and one between the drops), the researchers call this the triple Leidenfrost effect.

    Eventually, the more volatile (in other words, easily evaporated) drop shrinks to a size similar to its capillary length, at which point the drops merge. If the boiling points of the two liquids are vastly different, the merger can be explosive (Image 3). (Image and research credit: F. Pacheco-Vázquez et al.; via APS Physics)

  • Leidenfrost Without the Heat

    Leidenfrost Without the Heat

    Leidenfrost drops slide almost frictionlessly on a layer of their own vapor, generated by extremely hot surfaces nearby. But in this experiment researchers recreated many of the classic behaviors of a levitating Leidenfrost drop without the added heat. Instead, they supersaturated water droplets with carbon dioxide to create “fizzy droplets” that slide and self-propel along superhydrophobic surfaces.

    Initially, the drops don’t levitate. It takes a little while for the carbon dioxide layer to build up beneath them, as seen by the slowly appearing interference fringes in the second image. But once the layer forms, the drops behave like conventional Leidenfrost drops until their carbon dioxide is depleted. They’re even able to self-propel on a racheted surface (third image)! (Image and research credit: D. Panchanathan et al.; via Physics World; submitted by Kam-Yung Soh)

  • The Sounds of Leidenfrost Stars

    The Sounds of Leidenfrost Stars

    On a hot surface, droplets can float on a layer of their own vapor and vibrate in star-like shapes. These so-called Leidenfrost stars also make noise, with distinct beats that match the oscillations of the vapor layer beneath them. Researchers found that the frequency of the sound shifts with droplet size, increasing as the drop size decreases. Physically, the droplets act much like a wind instrument! (Image and research credit: T. Singla and M. Rivera; via APS Physics)

  • Jets Beneath Leidenfrost Drops

    Jets Beneath Leidenfrost Drops

    When a droplet impacts, it’s not unusual for converging ripples to form an upward jet, like the one seen here. But under the right circumstances, jets can form downward, too. This study looks at the ultrafast jets that can form beneath an impacting Leidenfrost drop.

    These Leidenfrost drops are striking a surface much hotter than their boiling point, so a large vapor cavity forms quickly beneath them. Using x-ray imaging, the researchers were able to capture the dynamics of this cavity’s formation and collapse (Image 2). The field of view in the animation shows only a portion of the drop’s cavity, so Image 3 may help you orient relative to the drop at large.

    Initially, we see the center of the droplet hitting the surface, followed by the fast growth of a vapor cavity. Rippling capillary waves converge on top of the cavity, creating a pinch-off. From there, a bubble rises up while a fast jet shoots downward. (Image credit: water jet – A. Min, others – S. Lee et al.; research credit: S. Lee et al.)

  • The Vortex Beneath a Drop

    The Vortex Beneath a Drop

    While we’re most used to seeing levitating Leidenfrost droplets on a solid surface, such drops can also form above a liquid bath. In fact, the smoothness of the bath’s surface, combined with mechanisms discussed in a new study, means that drops will levitate at a cooler temperature over a liquid than they will over a solid surface.

    Researchers found that a donut-shaped vortex forms in the bath beneath a levitating droplet, but the direction of the vortex’s circulation is not always the same. For some liquids, the flow moves radially outward from beneath the drop. In this case, researchers found that the dominant force was shear stress caused by the vapor escaping from under the droplet.

    With other droplet liquids, the flow direction instead moved inward, forming a sinking plume beneath the center of the drop. In this situation, researchers found that evaporative cooling dominated. As the liquid beneath the droplet cooled, it became denser and sank. At the same time, the lower temperature changed the bath’s local surface tension, creating the inward surface flow through the Marangoni effect. (Image credit: F. Cavagnon; research credit: B. Sobac et al.)

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    Mixing Leidenfrost Drops

    When placed on a very hot, patterned surface, droplets will self-propel on a layer of their own vapor. Here, researchers use this to drive droplets to coalesce so that they can observe how well they mix. After their head-on collision, the merged droplets have two major forces fighting in them: surface tension, which tries to minimize the overall surface area; and gravity, which tries to flatten the large droplet. Together, these forces drive the large oscillations we see in the merged drop, and those oscillations help mix the liquid from the two original drops together. (Image, video, and research credit: Y. Chiu and C. Sun)

  • Kicking Droplets

    Kicking Droplets

    Moving the surface a droplet sits on creates some interesting dynamics, especially if the surface is hydrophobic. That’s what we see here with these droplets launched off an impulsively-moved plate.

    On the left, the drop has some limited contact with the plate and it takes time for the droplet to completely detach. When accelerated, the droplet first flattens into a pancake, the rim of which quickly leaves the plate. The center of the droplet is slower to detach, stretching the drop into a vase-like shape. When the drop does finally lose contact, it creates a fast-moving jet that shoots upward at several meters per second!

    In contrast the image on the left shows a levitating Leidenfrost droplet. Since this drop has no physical contact with the plate, the kick makes it leave the surface all at once, launching a pancake-like drop that quickly forms unstable lobes. (Image and research credit: M. Coux et al.)

  • Leidenfrost Stars

    Leidenfrost Stars

    Atop a very hot surface, liquids can instantly vaporize, leaving a drop levitating on a layer of its own vapor. These Leidenfrost droplets demonstrate all kinds of interesting behaviors, including self-propulsion, explosion, and star-shaped oscillations, like those above. The oscillation is driven by feedback between the drop and its vapor layer. 

    Interestingly, the drops are capable of sustaining more than one mode of oscillation at once, as seen above. The obvious mode (m=5) corresponds to the 5 star-like points pushing out on the drop. But notice that the drop is also stretching into an oval shape that moves up and down, back and forth. This is the second mode (m=2) present. It moves slower than the m=5 mode, completing a cycle only once for every four cycles the other has. (Image and research credit: J. Bergen et al.)