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The Death of the Teapot Effect

Fluid dynamicists have worked out how to stop teapots from dribbling, once and for all.

Teapot technology is largely ignored by mainstream media (some say unfairly). But today, scientists in France unveil a technique that should breath hi-tech life into a new generation of bespouted objects.

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”.

Previous studies have shown that dribbling is the result of flow separation where the layer of fluid closest to the boundary becomes detached from it. When that happens, the fluid flows smoothly over the lip. But as the flow rate decreases, the boundary layer re-attaches to the surface causing dribbling.

Previous studies have shown that a number of factors effect this process such as the radius of curvature of the teapot lip, the speed of the flow and the “wettability” of the teapot material. But a full understanding of what’s going on has so far eluded scientists.

Now Cyril Duez at the University of Lyon in France and a few amis, have identified the single factor at the heart of the problem and shown how to tackle it. They say that 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-cappillary effect.

So how to overcome it? There are two ways say Duez and co. The first is to make the lip as thin as possible. That’s why teapots with spouts made from thin metal are less likely to dribble.

The second is to coat the lip with the latest generation of superhydrophobic materials which strongly repel water. Duez and co show how this stops dribbling at a stroke. “Superhydrophobic surfaces fully avoid dripping, and thus beat the “teapot effect”,” they say.

(Of course, there are one or two other potential applications in shaping the fluid flow in microfluidic machines but these pale into insignificance compared with the teapot revolution in hand.)

The really exciting news, however, is that in certain materials the hydro-capillary effect can be controlled electronically. That raises the possibility of a teapot design in which dribbling can be turned on and off with the flick of a switch–an object of desire on a par with the iPhone, USB catapaults and personal hovercrafts. (The iPot, perhaps?)

If this doesn’t win these guys an IgNobel, I don’t know what will.

Ref: arxiv.org/abs/0910.3306: Beating the Teapot Effect

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