Secrets of a non-drip teapot
Why is it that all but the most carefully designed teapots send a rain of drips showering onto the tablecloth while a teacup is being filled? It has been the bane of teashops and parlours throughout the land.
The usual theories blame either surface tension or adhesion of the liquid to the surface of the spout, but last year M. Reiner in Physics Today showed that neither could be wholly to blame. Now his discovery has been taken a step further. It has been shown by the mathematician J. B. Keller of New York University that the real reason teapots drip is because it is their nature to do so 鈥� that鈥檚 just the way that fluid flows from a spout.
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The desirable, non-dripping flow of tea emerges from the spout at speed, but this form of smooth flow only works when the velocity of the liquid is relatively high. If the flow of the liquid is slow enough for gravity to act on the fluid and turn the stream rapidly downwards as soon as it leaves the spout, then another kind of flow results. Liquid flows faster closest to the spout, but this faster-flowing liquid is at lower pressure than the remaining slower-flowing fluid. The resultant pressure difference pushes the liquid against the spout and helps it to turn the corner and dribble down the edge of the spout and the pot. This leads to the distressing drops that we see on our tablecloths.
Teapot technologists should now be aiming to make the velocity of transition from one type of flow to the other as low as possible. This will require better design. However, many people will still possess badly designed teapots. Fortunately for them, an old-fashioned remedy is at hand. If one greases the spout of the pot with butter to increase the angle of contact between the tea and the spout surface, this can decrease the transition velocity leading to fewer (but perhaps greasier) tea drips.
From The New 杏吧原创, 26 September 1957