# The Naked Scientists Forum

### Author Topic: Where does the energy behind 'capilary action' come from?  (Read 3221 times)

#### krytie75

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##### Where does the energy behind 'capilary action' come from?
« on: 19/11/2009 19:08:51 »
A recent BBC News article ( here ) reminded me how wonderful capilary action is.  It is my understanding that capilary action is the tendancy for a fluid to move through small spaces (a good example of this is that of a sponge sucking up water from a wet surface with no user intervention), with the article mentioning that this method can be used to build a pumpless blood analysis chip.

This being the case, where does the energy that moves the fluid (against gravity in some cases) come from? Surely where ever the energy is coming from, it must 'deplete' over time with some physical change to the source of the energy? How much of this mysterious energy is available and is there any way to convert it into small amounts of power (e.g. for a tiny tiny chip)?

Jon

#### Soul Surfer

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• keep banging the rocks together
##### Where does the energy behind 'capilary action' come from?
« Reply #1 on: 19/11/2009 23:48:35 »
The energy comes from the surface tension and contact angle of the fluid in the capillary tube.

In a liquid the molecules are attracted towards each other but not fixed as in a solid this attractive force means that a the surface there is an asymmetry and the forces are slightly stronger.  its a bit like a liquid having a "skin" on its surface. the liquid also contacts the glass at a particular angle  for water this is usually zero degrees which means that the water surface must bend to contact the glass  this bending of the surface creates an attractive force which becomes greater as the bending becomes sharper (as the tube gets smaller) and caused the water to rise in the tube.

So the forces to draw the liquid up the tube come from the attractive molecular forces that hold the liquid together.

#### syhprum

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##### Where does the energy behind 'capilary action' come from?
« Reply #2 on: 20/11/2009 11:36:06 »
The questioner spoke of energy the fluid moves against the force of of gravity thus gaining potential energy.
I can only think there must be a drop in temperature of the fluid and wonder what is the thermal efficiency of this process viewed as a heat engine.
From my school days I recall that 1°K rise in water tempeature is caused by a drop of 427 meters (1300 feet)
« Last Edit: 20/11/2009 15:52:48 by syhprum »

#### peppercorn

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##### Where does the energy behind 'capilary action' come from?
« Reply #3 on: 20/11/2009 12:02:46 »
The energy comes from the surface tension and contact angle of the fluid in the capillary tube.
Capillary action is one of those bits of the physical world that seems to go against our 'common-sense' perception, doesn't it? Feels like your getting something for nothing.
If the surface tension of the liquid is doing work does that mean it has less potential energy afterwards?  Do it have something to do with entropy or did I just make that up?

[Edit: posted at the same time as syhprum, so may overlap in context...]

#### syhprum

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##### Where does the energy behind 'capilary action' come from?
« Reply #4 on: 22/11/2009 16:25:44 »
I am disapointed by the lack of comments on this question, when the fluid gains potential enegy by rising against the force of gravity where does this come from ?.

#### krytie75

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##### Where does the energy behind 'capilary action' come from?
« Reply #5 on: 14/04/2010 13:49:30 »
Yeah it's a shame, no-one seems to have been able to adequately explain this one yet....

#### Soul Surfer

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##### Where does the energy behind 'capilary action' come from?
« Reply #6 on: 14/04/2010 23:23:36 »
I thought that it had been adequately explained.  it comes from the cohesive energy in the liquid.  Consider also the case of mercury with its contact angle of 137 deg the effect is that mercury is below the surface level of the liquid so the enrtgy is being used to preven the liquid rising in the tube.

#### The Naked Scientists Forum

##### Where does the energy behind 'capilary action' come from?
« Reply #6 on: 14/04/2010 23:23:36 »