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Author Topic: Why does milk rise inside a straw?  (Read 2260 times)

Offline thedoc

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Why does milk rise inside a straw?
« on: 18/07/2012 21:30:02 »
Govindarajan Rajagopal  asked the Naked Scientists:
Hi Chris,

First for all I thank God for luckily seeing this Naked Scientist website.

My name is Govind. I am an Indian.

I am a guy who likes these kind of things. My parents and relatives tell me that, during my childhood (as well as now) I asked lots and lots of science questions which are difficult to answer.

1. Take a cup of milk (not fully filled)
2. Take a stirrer (the plastic one, which has small hole to suck the milk)
3. Now, gently insert the stirrer into the milk
4. Observe that not only the milk will be pushed inside the milk, but also the level of the milk inside the stirrer is slightly higher than the milk contained in the cup.
Why the level is different?

Eagerly waiting for the explanation Chris.

Thanks and regards

What do you think?
« Last Edit: 18/07/2012 21:30:02 by _system »


Offline chris

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Re: Why does milk rise inside a straw?
« Reply #1 on: 20/07/2012 09:21:23 »
Hi Govind

What you are seeing with your straw in the milk is a manifestation of capillary action. The milk is water-based and has a high of surface tension (meaning it's a sticky fluid). When it rises up inside the straw, interactions between the milk and the sides of the straw mean that the liquid sticks and spreads out. Because it is itself sticky (surface tension) more liquid is pulled with it, up the side of the tube. The result is a U-shaped bulge inside the straw above the level of the liquid into which the straw has been immersed. Similar science helps water to rise inside trees.

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Re: Why does milk rise inside a straw?
« Reply #1 on: 20/07/2012 09:21:23 »


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