Pouring wine… with a twist

It’s coming into the summer imbibing season, so expect a lot of wine to be poured. And as the wine was poured at a recent pre-summer BBQ, I was asked the question: what’s up with that little twist thing that waiters do when they finish pouring your glass? Is it just for show, or does it really stop the drips?

Now don’t get me wrong, it is showy; but the showiest I’ve seen also involved a snazzy napkin around the bottle, which does raise doubts about its anti-drip efficacy… Maybe science can answer the question!

Well, as far as I’ve found there isn’t any actual research on this topic—but there is some related work on teapots, which I shall attempt to adapt.

In 2009, some French physicists published a paper on what they called the “teapot effect”; that’s when your teapot dribbles when you finish pouring, leaving a puddle on the saucer or on the tabletop (Duez C, Ybert C, Clanet C & Bocquet L 2009, “Beating the teapot effect”, Physical Review Letters, vol. 104, 084503, arXiv:0910.3306).

They found it was due to the relationship between inertial flow from the spout and surface wetting, i.e., liquid sticking to the surface. As the flow decreases, the water forms drops that cling to the lip of the spout.

Beating the teapot effect with a superhydrophobic coating. Top: water flow under the spout of an (hydrophilic)
(a) A normal teapot, pouring tea—note the stream bending back towards the spout; (a’) as the flow decreases, the drips stick to the spout. (b) A spout with a superhydrophobic coating (they used black soot) has a smooth stream, (b’) even as the flow slows. (Image: Lydéric Bocquet lab, Ecole Normale Supérieure, Paris)

Their solution was to use a superhydrophobic surface. OK, so in the science of wetting, you classify surfaces in different ways. There are surfaces that like to get wet; they’re called hydrophilic. These are usually smooth surfaces of a material that has high energy, i.e., strong chemical bonds between the molecules, which override the surface tension of the liquid. So things like metal, glass and glazed ceramic. If you think how drops of water spread out on the window—glass wets easily.

Then there are surfaces that are hydrophobic, and they usually have low energy. Things like hydrocarbons, fats and oils. Think about when you wax your car and the water forms beads: that’s hydrophobic.

Then there are superhydrophobic surfaces, which are extremely hard to wet. You find these in nature, like on plant leaves, insect wings or yes, even duck feathers. These often have surfaces that are rough on the nanoscale. The roughness stops the water drops from spreading out, forcing them to form little globules.

As the figure above shows, if you use a superhydrophobic surface on the teapot spot, the liquid doesn’t cling to it and instead it pours neatly, no matter how slow the flow.

Now, there have been other attempts to make dripless teapots by controlling the flow, like using a “speed bump” to stop it getting slow enough to dribble, or by having a really sharp edge that’s harder for drops to cling to. But I like the French research because it identifies the mechanism, which we should be able to generalise to wine. 

Red wine being poured into a glass
Note how the wine clings to the mouth of the bottle, just like the tea in the previous image (Photo: theloushe via Compfight cc)

Essentially the same thing happens: the liquid bends towards the glass when pouring, and as you stop the last few drops hang on, running down the bottle and leaving a trail over the tablecloth.

So that’s the cause, but what’s the solution? Can we insist on wine makers switching to superhydrophobic bottles?

Maybe, or maybe the twist is easier. This appears to simply embrace the fact that a drop is going to cling to the glass, but instead of letting it fall down, the twist turns the drip so it runs around the mouth of the bottle instead of down.

Wine dripping as it's poured into a glass
Or maybe it’s up to the pouree to catch the drips (Photo: sintixerr via Compfight cc)

One piece of advice I did find is to try it with a slow-flowing liquid, like honey. It’s pretty easy to do, and a great demonstration of how the twist diverts the drip; just be prepared to get sticky if you get it wrong on the first go.

But it does work, and is maybe a good way to practise before moving on to a grown-up liquid. Try it yourself!

And if you don’t like that, well one other old trick for stopping teapot dribbles is apparently to rub butter on the spout. Butter, being hydrophobic, should also reduce wetting and prevent drops.

So the same trick should also work for wine—as long as you’re not afraid of having greasy vino. Something else to try at your next BBQ.

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