Why do teapots dribble? French scientists say it’s all about the simple subject of surface wettability
The teapot dribble effect could be made a thing of the past, say French scientists, who it has to be said come from a country of coffee drinkers. Photograph: Graham Turner
For those who hate tea stains on their pristine linen tablecloth, succour is at hand: scientists in France have solved the perennial puzzle of the dribbling teapot. Fluids experts at the University of Lyon have produced a four-page report [pdf] that claims to offer a solution, and as often can be the case with long-unresolved problems, it is a simple one.
“Surface wettability is an unexpected key factor in controlling flow separation and dripping, the latter being completely suppressed in the limit of superhydrophobic substrates,” the report explains. “This unforeseen coupling is rationalised in terms of a novel hydro-capillary adhesion framework, which couples inertial flows to surface wettability effects. This description of flow separation successfully captures the observed dependence on the various experimental parameters – wettability, flow velocity, solid surface edge curvature. As a further illustration of this coupling, a real-time control of dripping is demonstrated using electro-wetting for contact angle actuation.”
This scientific jargon boils down to the fact that tea tends to stick to the inside of the spout as it is poured. The flow of tea then begins to stop-start, causing a dribble effect. The team, led by Cyril Duez, say the use of “superhydrophobic surfaces” – essentially water-repelling linings – on the inside of the spout can avoid dripping and “thus beat the ‘teapot effect'”.
The scientists are not the first to bend their minds towards the problem. This year the retailer Debenhams claimed to have designed a dribble-free teapot with a “multi-faceted solution” that involved a larger spout, “tea bag baffle” and redesigned lid. As far back as 1998 the British inventor Damini Kumarb was hawking her solution – the D-pot – around the BBC and other media groups. Her solution was a groove under the spout.
The latest intensive research appears to be the first to tackle the dribbling problem from an explicitly scientific perspective. The Lyon team’s verdict: marry a superhydrophobic surface with the more traditional method of using a sharp edge at the end of the spout, creating a drip- and hassle-free pot.
What about other brew-time dilemmas? In 2003 the Royal Society of Chemistry released guidance on how to make the perfect cup of tea [pdf], and in 1998 researchers from the University of Bristol published a scientific formula for dunking a biscuit.
Long may science’s dalliance with snack-based problems continue.