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)

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