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Tag: aerodynamic Leidenfrost effect

  • Bouncing Off a Moving Wall

    Bouncing Off a Moving Wall

    There are many ways to repel droplets from a surface: water droplets will bounce off superhydrophobic surfaces due to their nanoscale structures; a vibrating liquid pool can keep droplets bouncing thanks to its deformation and a thin air layer trapped under the drop; and heated surfaces can repel droplets with the Leidenfrost effect by vaporizing a layer of liquid beneath the droplet. But all of these methods will only work for certain liquids under specific circumstances. 

    More recently, researchers have begun looking at a different way to repel droplets: moving the surface. The motion of the plate drags a layer of air with it; how thick that layer of air is depends on the plate’s speed. (Faster plates make thinner air layers.) Above a critical plate speed, a falling droplet will impact without touching the plate directly and will rebound completely. This works for many kinds of liquids – the researchers used silicone oil, water, and ethanol – across many droplet sizes and speeds. The key is that the air dragged by the plate deforms the droplet and creates a lift force. If that lift force is greater than the inertia of the droplet, it bounces. (Image and research credit: A. Gauthier et al., source)

  • Aerodynamic Leidenfrost Effect

    Aerodynamic Leidenfrost Effect

    If you place a droplet on a surface much hotter than its boiling point, that droplet will skitter and float almost frictionlessly across the surface on a thin layer of its own vapor. This is what is known as the Leidenfrost effect. But you don’t have to heat a surface to get this behavior. There’s also an aerodynamic Leidenfrost effect, shown above, when the surface is moving. As the surface moves, it drags a layer of air along with it, and that layer of air is capable of keeping droplets aloft indefinitely. The thickness of the air layer depends on speed; the faster the plate moves, the thicker the air layer underneath droplets. The aerodynamic forces generated are large enough to drive a droplet up an incline against the force of gravity (bottom image). (Image credit: animation – M. Saito et al., source; chronophotograph – A. Gautheir et al., pdf)