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”.
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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.
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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.
