Surface tension – just relax

At work last week, my colleague came to the bar and said ‘I have a science question I’ve been saving for you,’ which immediately made me feel very smug and smart (and slightly worried that I wouldn’t be able to answer it). She asked me why her glass of water was able to rise over the top of the glass without spilling; an everyday effect which I’m sure many of us have noticed. I knew the effect was called surface tension, but decided to look further into it anyway because I take intermolecular forces very seriously. And so should you. Hence this blog post.

The answer I gave her at the time was a rambling interpretation of how hydrogen bonds work. As a chemist I am a huge fan of hydrogen bonds but I had to double check if I was right – and did you know there is SO MUCH MORE to surface tension than you’d think? Of course there is. Silly question.

Definition: Surface tension is the elastic tendency of a fluid surface which makes it acquire the least surface area possible. (Shout out to Wikipedia, you are responsible for a large part of my degree).

Surface tension only occurs where there is an interface between different media, so in the case of a glass of water it is the interface between water and air. The water molecules are attracted more strongly to each other than they are to the molecules of air – cohesion vs adhesion. The water molecules at the surface therefore experience a force pulling them towards the rest of the water molecules, and away from the air. This creates the effect we see of a ‘membrane’ at the top of the glass of water.

This is also the reason that liquids form droplets as opposed to any other shape – the spherical shape minimises surface area, reducing surface tension (and therefore energy) as much as it can.

The surface tension of water decreases significantly as temperature increases, so a full glass of hot water would become more likely to overflow than to hold its curved shape under the ‘membrane’ like the glass of cold water does. On an interesting side note, this is why hot water is more effective for cleaning: reduced surface tension makes it more able to enter pores and fissures to clean them, rather than bridging over them because it is unable to break the ‘membrane’ as in cold water.

The reason that the surface tension effect is so visible with water is because of the tendency towards hydrogen bonding and also the polar nature of the molecules, both of which causes them to bind together much more strongly than most other liquids. Water therefore has an unusually high surface tension.

This is not the only unusual thing about water. Welcome to one of my all-time favourite facts: despite it covering 71% of the Earth, and being the most well-recognised and most common liquid any of us can imagine, water is actually a total anomaly. Water has properties that are different to pretty much every single other liquid. An example of this is that solid water (ice) has a larger volume than liquid water. Putting too many ice cubes into your glass of water will make the glass overflow, right? But if you were to fill up a glass to the brim with ice and water and wait for the ice to melt, the glass would not overflow. In fact, you would see the water level go down. Water is one of the only liquids that this will happen to. There are many effects that you can observe between the solid and liquid phases of a substance, most of them easily illustrated with a neat graph showing a neat trend that most substances adhere to. Water almost never follows the trend!

Anyway, let’s get back to surface tension. I am now going to talk about wine, because not only is it great but it also has an interesting phenomenon associated with it, named Tears of Wine. (I’m not talking about the phenomenon that wine sometimes turns you into a weepy drunk, that is just a magical coincidence of the name).

Tears of wine look like this:

 Why does wine cry? | The Horse's Mouth

The effect is easier to observe the more alcoholic your wine is. It actually has a proper scientific explanation: the Marangoni effect.

The effect was first identified by James Thomson, who seems like the kind of wine-based scientist I could get along with. He was a great physicist but his younger brother was Lord Kelvin, who was made into a Baron because of his achievements in thermodynamics and who literally has units of temperature named after him, so can you blame James for being a bit of a wino?

The effect is caused by the two liquids in wine – water and alcohol (ethanol). They have different values of surface tension, creating a gradient, which in turn creates mass transfer. The water has a higher surface tension, meaning it pulls together more strongly than the molecules of ethanol. The water will flow away from areas of high alcohol concentration. This causes the wine to travel up the sides of the glass, away from the alcohol in the bulk of the glass. As the wine climbs the glass, the alcohol evaporates faster, leaving the water. Eventually, the collected water gains too much weight and falls back into the wine as the ‘tears’ we observe. The effect is accelerated by the evaporation of the ethanol, because this strengthens the gradient, so can be prevented simply by covering the glass!

So in summary, surface tension is much more complex and amazing than I realised. And putting a lid over your wine will stop it from crying. And potentially stop you from crying because you won’t spill it all over your nice white top. Be safe out there.

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