You may not be entertaining the Queen of England but when pouring someone a cup of tea from a proper teapot it’s annoying, nay embarrassing, when the tea drips down the spout and splashes into the saucer or onto the cucumber sandwiches (well, at least a plate of biscuits). French scientists know this feeling only too well and have employed technology to put an end to this horrendous social faux pas.
The “teapot effect” is the less than imaginative term given to items which exhibit the nasty problem of dribbling, particularly at low flow rates. At higher flow rates, flow separation occurs where the layer of fluid closest to the boundary becomes detached from it and the fluid flows smoothly over the lip. But as the flow rate decreases, the boundary layer re-attaches to the surface causing dribbling.
But a full and thorough scientific explanation of why this happens has so far eluded scientists. Previous studies have shown that a number of factors affect this process such as the radius of curvature of the teapot lip, the speed of the flow and the "wettability" of the teapot material.
Now Cyril Duez at the University of Lyon in France and his colleagues have identified the single factor at the heart of the problem and shown how to tackle it. They say the culprit is a "hydro-capillary" effect that keeps the liquid in contact with the material as it leaves the lip. The previously identified factors all determine the strength of this hydro-capillary effect.
To overcome it Duez says first you must make the lip of the spout as thin as possible. Then coat the lip with the latest generation of superhydrophobic materials that strongly repel water. And, presto - no more damp sandwiches!
Putting teapots aside, there are probably a few more applications for Duez’s discovery and other scientists will surely be interested in his results. For instance, in certain materials the hydro-capillary effect can be controlled electronically, meaning dribbling can be turned on or off. Can’t think of an application myself, but there must be plenty out there.
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