We're post-Easter, but it's never the wrong season to watch blue-colored milk seemingly defy gravity to climb up the curve of a hard-boiled egg.

This is a trick from Professor Tadd Truscott, who conceived of it while spinning billiard balls in everything from water to treacle-thick glycerin. Depending on the speed of the spin and the thickness of the fluid, results varied; faster speeds pulled liquid higher, and even faster speeds could make sheets of water fly away from the center of the balls. Thicker liquids would fly away from the sphere in sheets that stretched multiple feet in diameter.

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The process begins with friction. The egg can't slip frictionlessly past the milk, so part of the milk clings to the egg's surface. But the milk isn't only touching the egg, it's touching the air. According to Truscott, the air pressure is what causes the milk to climb.

The Bernoulli effect indicates that when air moves quickly past a surface it exerts less pressure than it does when it moves slowly past a surface. (A common example of the effect is high winds causing a window to explode outwards during a hurricane.) This is true whether the air is moving past the surface or the surface is moving past the air. When the milk is at the bottom of the egg, it is spinning around in a little circle. Because the circle is little, the milk is spinning relatively slowly, so the pressure on it is high. If it climbs a little farther, the circumference of the egg is bigger. Now the milk is moving faster around a bigger circle, and the pressure on it drops. It's a bit like squeezing a spinning tube of toothpaste ‚ÄĒ when you put more pressure on the bottom of the fluid, it's going to climb up to where the pressure is lower. The speed, the friction, and the curve of the egg all determine how high it's going to go.

[Via Eggs In Milk, Cracking the Egg Mystery, Spinning Spheres]

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