FYFD

Tag: self-propulsion

  • A Groovy Hovercraft

    A Groovy Hovercraft

    Not long ago, researchers discovered that droplets hovering over a hot grooved surface would self-propel. The extension to this was to investigate a hovercraft on a grooved, porous surface (top half of animation)–think an air hockey table with grooves. In that case, air inside the grooves flows from the point toward the edges, and it drags the hovercraft with it, thanks to viscosity. So the hovercraft travels in the direction opposite the points. This raised an obvious question: what happens if the hovercraft is grooved instead of the surface?

    That’s the situation we see in the bottom half of the animation. Air flows from the table and interacts with the grooves on the bottom of the hovercraft. And this time, the hovercraft propels in the direction of the points. That means there’s a completely different mechanism driving this levitation. When the grooves are onboard the hovercraft, pressure dominates over viscous effects. The air still gets directed down the grooves, but now, like a rocket, the exhaust pushes the hovercraft in the other direction – toward the points. For more on this work, check out the mathematical model of the problem and our interview with one of the researchers in the video below. (Research credit: H. de Maleprade et al.; image and video credit: N. Sharp and T. Crawford)

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    Bouncing, Floating, and Jetting

    Get inside some of the latest fluid dynamics research with the newest FYFD/JFM video. Here researchers discuss oil jets from citrus fruits, balls that can bounce off water, and self-propelled levitating plates. This is our third entry in an ongoing series featuring interviews from researchers at the 2017 APS DFD conference. Missed one of the previous ones? Not to worry – we’ve got you covered. (Video and image credit: N. Sharp and T. Crawford)

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    Swimming Microdroplets

    Simple systems can sometimes have surprisingly complex behaviors. In this video, the Lutetium Project outlines a scheme for swimming microdroplets. Most of the droplets shown are just water, but they’re released into a chamber filled with a mixture of oil and surfactants. All flow through the chamber is shut off, but the droplets swim around in complicated, disordered patterns anyway. To see why, we have to zoom way in. The surfactant molecules in the oil cluster around the droplets, orienting so that their hydrophobic parts are in the oil and their hydrophilic parts point toward the water. They actually draw some of the water out of the droplets. This creates a variation in surface tension that causes Marangoni flow, making the droplets swim. Over time, the droplets shrink and slow down as the surfactants pull away more and more of their water and the variations in surface tension get smaller. (Image and video credit: The Lutetium Project; research credit: Z. Izri et al.)

  • Rolling Along

    Rolling Along

    Leidenfrost drops – droplets deposited onto a surface much hotter than their boiling point – are known for their mobility. With the right surface, they can be propelled, trapped, and even guided through a maze, typically by directing the vapor layer that cushions them. But new work shows that these drops have internal dynamics that also contribute to their propulsion.

    By adding tracer particles to each droplet, researchers can visualize flows inside the droplet. Large drops tend to have a flatter shape and contain two or more rotating vortices. Such drops won’t propel themselves without another force in play. But smaller droplets are more spherical and contain only a single rotating flow. Once these drops detach, they roll away! Despite the similarity to wheels, these liquid drops aren’t moving the same way. Remember that the drop is not actually in contact with the surface. To see what sets the drop’s direction, researchers examined the shape of the bottom of the drop. They found that it sits at a slant on its vapor cushion. That pushes evaporating gases out one side, propelling the drop the other way. (Image and video credit: A. Bouillant et al., source)

  • Controlling Leidenfrost Drops

    Controlling Leidenfrost Drops

    On a surface much hotter than their boiling point, droplets can surf on a layer of their own vapor due to the Leidenfrost effect. Recent research has shown that textured surfaces like ratchets can create corrals, traps, and mazes for such droplets. Here, researchers manipulate the propulsion of Leidenfrost drops using non-parallel grooves instead. When placed between two non-parallel plates, the droplet is squeezed by side forces perpendicular to the walls, with the resultant force in the direction where the gap widens. In most states, friction forms an opposition to this squeeze, but for Leidenfrost droplets that frictional force is negligible. Instead, the squeezing from the plates launches droplets toward the wider end of the groove, allowing researchers to design repellers (top) and traps (bottom) for the fast-moving drops. (Image credits: C. Luo et al., source)

  • Self-Propelled Hovercraft

    Self-Propelled Hovercraft

    When placed on an extremely hot substrate, some drops levitate and can be propelled over specially textured surfaces. Inspired by this work, researchers are using similar principles to explore manipulation of levitating plates using surface texture. Their apparatus consists of a semi-porous, grooved surface that ejects air upward to levitate Plexiglas objects – think air hockey table with grooves. With enough airflow, the Plexiglas levitates. The grooves force air in a particular direction – in the case of the herringbone pattern, this is in the direction of opening – and, as the air moves, it drags its Plexiglas hovercraft along. As shown in the second animation, grooves can do more than move the glass linearly; with patterns offset by 90-degrees, they can make the hovercraft rotate.

    Here’s an interesting next step for anyone out there with an air hockey table and a 3D printer: does the directional manipulation work if the grooves are on the object and not the table? In other words, can you create an air hockey puck that preferentially goes to your opponent’s goal? (Image and resource credit: D. Soto et al., source)