The Cheerios Effect Makes Floating Objects Stick Together
Sometimes the most interesting science is right in front of us. Namely, why do the last bits of breakfast cereal cling together on the milk's surface? In 2005, a pair of Harvard scientists published a study in the American Journal of Physics to explain this phenomenon, which is called "the Cheerios effect." On the surface of a liquid like milk, the molecules all exert a weak force on each other known as surface tension. There are other forces, too: the air molecules above the milk exert a weak upward pull while the milk molecules below the surface exert a strong downward pull, thereby making the milk's surface cave in slightly. The sides of a container also pull on the molecules, creating an upward curve in the milk's surface wherever it touches the side. Cheerios float because they're buoyant, meaning they're less dense than milk and exert a constant upward force whenever they're on the milk's surface. This means that when they encounter a curve in the surface like the one on the side of the bowl, they'll want to travel up that curve as high as they can go. The weight of a single Cheerio also creates a small curve in the surface beneath it, so when other Cheerios come along, they'll all try to travel up each other's curves and appear to cling together.
from Arvind Gupta
Key Facts In This Video
If you drop a styrofoam ball in the center of a glass of water, it immediately tries to travel toward the edge. (0:22)
Water molecules attract each other more than they attract air molecules. This causes surface tension, an effect that makes water behave as if the surface is covered by a membrane. (0:42)
In a completely full glass, the surface is convex, so the balls move toward the highest point -- the center. In a partially filled glass, the concave surface's highest points are at the edges, which is why the balls move there. (1:45)