Water is an amazing substance. It has many properties that have a big impact on our lives and, I think, are quite useful for us to know about. One of these is the property of surface tension, which water shares with other less prevalent liquids. So what is surface tension? Wikipedia describes it as being caused by "the attraction between the liquid's molecules" which acts at the surface of the liquid to "diminish the surface area". Do check out this site - it has nice examples of the effect of surface tension. e.g. a photo of water beading on a leaf and a movie clip of drops falling from a tap. It is surface tension that shapes the water into rounded drops.

Dew drops

In meteorology, surface tension holds rain and dew drops together, a bit like how a slightly inflated balloon holds the air inside it. Let's take a closer look at dew first, since it relates to material in an earlier post. The necessary conditions for dew to form are similar to those for frost - see the blog post on Late Frosts. But if the ground temperature doesn't fall to 0°C, then you get dew rather than frost. So dew tends to be more of a spring and autumn thing. When I passed through Hong Kong airport a couple of years ago I spotted a beautiful sculpture of a dew drop. I love the way the sculptor, Danny Lee Chin Fai, has given a sense of weight to the "water" - I can almost feel the surface tension holding the drop in its shape and preventing it from flowing out onto the floor.


Sculpture at Hong Kong international airport

The sculpture has a caption that reads: "Nature is all around us. Yet often we look but do not see. Next time you see morning dew, take a look at just one small dew drop. See your surroundings reflected there. Look closely and you will see a reflection of yourself. So why not pause and try to look at ourselves, objects and people around us from a fresh perspective?" Perhaps you have seen beautiful dew drops lying on a plant in your garden, or maybe draped on a spider web?

Rain drops

I mentioned near the start about drops falling from a tap. Not surprisingly, surface tension is also responsible for holding falling raindrops together. But in contrast to dew, raindrops are moving through the air, and quite quickly too if they're big drops. There is a limit to how large a raindrop can grow. Because raindrops fall through the air, they are affected by air resistance - if you're out running or cycling, you feel this as a turbulent draught of air rushing past your face. Once the diameter is bigger than about 5 mm surface tension isn't able to hold the drop together and the turbulence tends to break it up into smaller droplets.

Other examples

When I was a boy I made a cheap compass by magnetising a pin, then placing it onto a tray of water. Surface tension allowed the pin to float on the water (provided I didn't agitate it too much). Another instance of surface tension occurs in nature as part of the process of capillary action that draws moisture and nutrients upwards into trees. You may have learnt about this at school. A friend once told me not to use too much soap when washing my car. The reason is that soap reduces surface tension, so the water is less inclined to "bead off" after waxing. The same argument applies if you're washing your house - you really want rain to "bead off" your house to keep it dry. Surface tension is just one fascinating characteristic of water. I feel inspired to write more, perhaps in a later blog post :-)