HOW COME? Some liquids dribble when poured

When you try to pour a glass of milk over your cereal, why does it spill down the side of the glass instead? - asks a reader.While there's no need to sob over spilled milk, it's perfectly OK to be really, really annoyed. A wide-mouthed glass may be extra drip-prone, but even teapot spouts aren't immune from the dreaded dribble.

In fact, as it turns out, the problem of dribbling liquids is officially known as The Teapot Effect. And after thousands of years and billions of spills, scientists are still researching the messy problem. In 2009, French researchers added another piece to the puzzle, in a paper called "Beating the Teapot Effect."

The effect occurs when tea apparently changes its mind about pouring into a waiting cup, turning back to run down the spout. But gravity has the last laugh, as the (brown, staining) liquid is pulled off the spout, only to land on the table (or in a guest's lap). Beyond annoying, the teapot effect also causes rain water to run down and pool underneath windowsills outdoors, rotting window frames over time.

Scientists say that the teapot effect occurs mainly at low flow rates - when a liquid pours slowly and feebly, rather than quickly and forcefully. But what actually causes the detour down the spout? Scientists have suggested many possibilities.

For example, water is attracted to glass and ceramics, making tea, milk, and other watery fluids naturally cling a bit on their way out of a vessel. There may also be an effect of air pressure. A tea stream experiences higher pressure where its exposed surface meets the air, and lower pressure underneath, where out-flowing tea touches a spout's lip. If the stream is flowing slowly, the higher air pressure from above may prod the tea to run down the underside of a spout.

But when a liquid is flowing quickly, the fluid layer pressed against the spout's surface - the boundary layer - detaches easily. During a slow pour, however, fluid layers separate, with the boundary layer tending to cling to the surface. Result: a stuttering, dribbling flow.

Besides the flow speed, studies have found that the shape and diameter of a vessel's lip or spout and the "wettability" of its material can change the effect. In 2009, French scientists found that the key is a "hydro-capillary effect" that keeps the fluid in contact with the spout surface. Pouring speed, spout shape, and other factors simply strengthen or weaken this basic effect.

The effect can be reduced, the researchers say, by using a narrow, thin-lipped spout, such as those often found on metal teapots. To abolish the effect, they say, the spout's lip could be coated with a "superhydrophobic" (super water-repelling) material, ensuring that not a single drop will drip.

In their experiments, the scientists coated a spout with black soot - yes, the same soot produced by burning candles (or coal). Since soot makes for unpleasant tea parties, it might be better to spring for a metal teapot - or just pour fast, with a little twist at the end.