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Spin a hard boiled egg in a puddle of milk and you get a sprinkler. But how? The science starts at the surface. When the egg spins, the fluid touching its surface is dragged along due to friction, and, because of the fluid’s viscosity, other parts of the fluid will also be spun. Dynamics tells us that the velocity at the surface of the object varies with radius; the velocity at the bottom of a spinning sphere is much smaller than that at its equator because a particle at the equator traverses a larger distance in a single rotation. Likewise, the fluid touching the bottom of the egg is spun slower than the fluid just above it. Bernoulli’s principle tells us that, for an incompressible fluid, the pressure decreases as velocity increases, meaning that a favorable pressure gradient exists along the spinning convex surface. It is this pressure gradient that draws the fluid up the sides of the object. Near the equator, the pressure gradient is weakest and centrifugal force flings the the fluid outward. Surface tension, angular velocity, and viscosity all play a role in the jets and sheets created by the sprinklers. (Video credit: NPR Science Friday with Tadd Truscott et al)

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