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Why is it so difficult to pour wine, water or tea? It just doesn’t want to go where it’s supposed to go

Posted May 21, 2019

Tell us if this sounds familiar – you try to pour water from one cup to another and instead of going where you want it to go it clings to the side of the cup and drips down on the floor. It happens every time and not just with water. We have wine stains on our carpet, kitchen floor is covered in tea and filling a travel mug is a hassle. But why is it so difficult to pour liquids? Scientists from the University of Amsterdam, University of Twente and Saxion University of Applied Sciences have a new explanation.

Strangely, pouring wine, tea or water requires some skills – liquid just wants to cling to the outside of the vessel and, of course, drip right on your white carpet. Image credit: Fabio Ingrosso via Wikimedia (CC BY 2.0)

Yes, you counted correctly – three universities were involved in researching this phenomenon. However, scientists are not really interested in your teapot or a wine bottle. Instead they put a spin on the common ‘teapot effect’ and used it to form liquid helices. This is not just interesting – it could help us solve the puzzle of this phenomenon. The first study on the teapot effect came out almost three centuries ago and we are still not sure why or how it occurs.

Nowadays scientists agree that the teapot effect is a combination of surface wetting and a small depression in the fast turning liquid. You know how air behaves around airplane wings? Well, air does behave a lot like liquids and a fast flow over a particular shape objects does tend to change its shape. But even so we don’t have a theory, which could reliably explain when the clinging occurs. In fact, we cannot even predict it that well – it sort of happens in front of our eyes and we just accept it as we see it. But now scientists made some steps towards better understanding of the teapot effect.

Researchers shot jets of coloured water onto the sides of vertical cylinders. Then they observed how water clings to the side of the cylinder and proceeds to flow down in a swirl. It forms a helix and scientists managed to learn properties of that helix. Then they were able to alter those properties by changing the flow of water. This is how the research team learned to predict the exact clinging behaviour for the first time. In other words, now we know in what pattern water clings to the sides of solid objects, even if we are still not sure why it does that.

Water clings to the side of the cylinder and moves down in a helix pattern. Image credit: University of Amsterdam

This, of course, has tremendous implications on everything. Teapots can be designed better, we can create mugs that will pour liquids like nobody’s business and wine will have no reason to meet your fluffy white carpet. More importantly, this could change some industrial equipment, using in pouring concrete or 3D printing. And children will be performing water helix experiments in their school’s science fairs.


Source: University of Amsterdam

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