HOW COME? The 'Cheerios Effect' holds the world together

When you sprinkle pepper into soup, why do the grains clump together on the surface? asks a reader.Pepper grains on your soup. Goldenrod pollen on a pond. Bubbles on the surface of your root beer. Shorn beard hairs, floating in the sink. All tend to drift together, finding each other in an expanse of liquid.

Is the particle huddling just an attempt to avoid liquid loneliness? Actually, the grains, bubbles and other small objects have no choice but to form cliques. Their togetherness is forced, a side effect of surface tension. The most familiar example is milk-drifting breakfast cereals. Scientists call such clumping the "Cheerios Effect."

What causes the surface of liquids to be "tense?" Take water. Molecules at the surface of a bowl of water are strongly attracted to each other. They are also attracted to the water molecules beneath them in the bowl. However, they are much less attracted to air molecules above the surface. The result: The surface has a built-in, skinlike tension. Thanks to surface tension, some insects can stride across a pond, feet indenting the water as if it were a rubbery sheet.

When pepper grains drop into soup, pollen fall onto a pond, or corn pops bob up in milk, each creates a dent in the surface, just like a water-strider's foot. The dents transform a smooth surface into a dimpled landscape. As the particles drift near one another, their tiny troughs connect, creating even bigger dimples.

So it's not that pepper, pollen, and corn pops possess some mysterious magnetic attraction. Instead, as more particles float near, unintentionally playing connect-the-dents, the trough becomes larger and larger. Soon, there's no escape: To break free from the bunch would require each grain or Cheerio or beard hair to travel up a little hill, against gravity. So floating particles tend to stay clumped. (Unless, of course, someone comes along with a big spoon. Or swishes a razor in the sink water.)

In a large area such as a pond, researchers say, particles like pollen grains tend to congregate into floating islands, with gaps in between. But in a bowl, floating bits tend to clump together in the center. So why do pepper grains and cereal sometimes also cling to the sides of glass bowls?

Where it meets the surface of a solid, a liquid curves. So a liquid bulges into a convex shape when it's repelled by the solid, and dips in a concave shape when it's attracted. Since glass attracts water molecules, and milk is mostly water, water and milk will dip near the bowl's wall. Particles are carried up to where the top of the dip meets the wall, forming a ring.

In Ireland, scientists used the "Cheerios Effect" to create a new material. After coating copper particles with a water-repelling chemical, they dropped them into a bowl of water. Although the particles are denser than water, the coating keeps them afloat. On the surface, they clump into netlike sheets, which can support drops of water and even bits of cement.