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The Teapot Effect - By Mr. Y - The True Scientist in the School

The Teapot Effect

Our pavement is eroding away. This is due to an interesting phenomena called the teapot effect. Teapot technology is largely ignored by mainstream media (some say unfairly). The problem with teapots is their annoying habit of dribbling, particularly at low rates of flow. The phenomenon has achieved such notoriety that it has been imaginatively dubbed the “teapot effect”. This I am sure we have all seen with say pouring milk from one glass into another. It will run down the side of the glass on us.

Many people, including scientists, have thought that the water spillage is caused by water tension, however, this is not the case.

In 1956, Markus Reiner performed an experiment which seemed to show this wasn't true. He placed a conical flask upside down in water and poured salty water, which sinks in normal water because it’s denser, over the top of it. Since surface tension has no effect underwater the salty water stream should have fallen straight down from the edge of the flask’s bottom, but Reiner found that the water followed the incline of the flask for a short while. He also established that flow rate was important. The slower the pour, the more likely the teapot was to drip. With a fast pour the tea has less of a chance to slow down, change direction and drip down the spout.

Like many of us, Dr. Keller had long observed the pesky problem of that little bit of tea that always seems to run down the outside of the spout and drip into our laps. Then, in 1956, he heard a lecture that inspired him. An Israeli scientist reported he asked 100 physicists why teapots drip and they all said it was due to surface tension. This scientist did some experiments that proved it couldn’t be caused by surface tension, so what is the explanation? Dr. Keller wrote a paper, ‘The Teapot Effect,’ Dr. Keller showed that it was air pressure, not surface tension, that causes drips. “It is simply that at the pouring lip the pressure in the liquid is lower than the pressure in the surrounding air,” he said, “so air pressure pushes the tea against the lip and against the outside of the spout.”

So you are probably wondering what this has to do with the pavement eroding away on our hardtop? Well, very simply, when it rains, water is forced down to this area due to gravity, and the topography of the pavement. As it hits the lip of the pavement it drips or flows along the edge and due to differential air pressure it is forced up under the asphalt lip, and it eats away at the underlying gravel and digs it out.

The teapot effect.

So as these two pictures show, the water runs over the edge where the air pressure forces it to ride along the bottom of the asphalt thus digging out the gravel underneath.

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