Why do tears run down your face slower than a drop of normal water?

03 October 2010

Question

Why do tears run down your face slower than a drop of normal water?

Answer

Dave - Yeah, it's an interesting one. It's something I've noticed. I think probably the biggest thing is that if you put droplets of water on your face, they tend to be bigger before they run down. And the thing with water flowing anywhere is that a big river will flow a lot faster than a small river. So a big droplet of water has much more gravity pulling on it, so much larger force pulling it down your face, but the forces which slow it down increase much more slowly. So, a big drop will run down your face a lot more and it's possible that the droplets are a lot bigger with normal water than tears because there's probably some surfactants and some proteins, meaning that the droplet will break up and start running down your face when it's much smaller than just normal water. Chris - So to summarise Dave, is it fair to say then we think that water droplets are going to be larger than teardrops which are going to be a bit smaller because there are other chemicals in the tears that mean that the water forms smaller droplets with tears because the smaller droplets are less heavy, so they get dragged down your face slightly more slowly, so they run more slowly. Dave - That is I think which should be going on.

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