Froth on Killandean Burn

Image: © Ian Dodds Taken: 12 Sep 2022

Like almost all of the small but significant watercourses in West Lothian this burn regularly has immobile frothy bits dotted about its edges; there is one in the foreground in this picture and one in the distance, both on the left. Due to the amount of them, I have always presumed that they must be natural, although they often look pretty horrible to be honest. And that is where tonight's research began... I am now more confident, after a bit of online delving, that these two frothy patches are a natural occurrence (though it is hard to be completely sure). Polluting foam apparently is whiter than natural foam, and smells like detergent - because that is normally what it is - whereas natural foam is off-white, and smells 'earthy or fishy' (I was on the wrong bank to do any sniffing). Both types are formed by fats (or lipids if being scientific). I am still no chemist, but having read around the subject a bit just now it seems to all be about surface tension. Water has a high surface tension, which means that water molecules stick together strongly - this is why you can overfill a glass with water a tiny bit and it forms a slight dome without spilling out. Fats break down the surface tension of water. So it turns out that soap and washing-up liquid aren't actually the things that do the cleaning; they just free up the water more so all its molecules can wriggle about a bit more than normal to wash off the dirt. The reason the frothy bits don't move much is that the water 'wants' to keep its surface tension, and therefore is repelled by the froth, meaning that there isn't enough of it about to wash the froth downstream. That is my very basic understanding of it all at least! In periods of heavy rainfall there is more froth because more things get washed into the water, and when dead things break down they release fats, particularly plant cells, according to those in the know. It is referred to as the 'Marangoni Effect' by scientists, and can be observed quite easily in a simple kitchen experiment (thankyou YouTube!) Get a bowl and pour water in it. Then sprinkle some pepper into the bowl (ground pepper works just fine) - pepper doesn't dissolve in water and just sits on the top. Then put a bit of washing-up liquid in the centre of the bowl. Quick as a flash the water that was supporting the pepper gets as far away from the soap as it can, and all the pepper whizzes to the edge of the bowl.

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