[NatureNS] hummingbird feeder question

From: uhoeger@dal.ca
To: naturens@chebucto.ns.ca
Date: Wed, 28 May 2008 23:11:37 -0300
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Original-Recipient: rfc822;"| (cd /csuite/info/Environment/FNSN/MList; /csuite/lib/arch2html)"

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Not sure if this is the best way to explain it.
It's a pressure and surface tension thing.  When the feeder is inverted after 
filling it up the column of liquid will pull down and create negative pressure 
between the bottom/now top of the feeder and its surface. At this point some 
fluid will run out till the system is balanced.
As long as the hole on the feeder tube is small enough to a) not to let air 
flowing in and ruin the vacuum in the top and b) to allow the waters surface 
tension to keep it from running out, this combination will keep the liquid from 
running out.  The bird can lick the fluid from the surface forming in the front 
of the feeder tube.  Eventually when enough fluid is consumed by the birds 
or by evaporation a bit of air will sip in, the negative pressure equalizes a bit 
and the fluid column will fall down a tad further till it created again enough 
negative pressure to hold it back.  The air going in can be seen rising as an 
occasional air bubble.
If you want to try it just fill a feeder and use a tissue to suck up the fluid from 
the feeding tube and soon you will see an air bubble going up.  The trick is to 
keep the opening through which the air can get in small enough. A fluid with 
no surface tension (like water with dish soap) should drip out.

Works with a small hummer feeder and would work with a  tanker truck as 
long as the hole you drill in the bottom of the tank is a) is small enough and 
b) the only place where air can get in.

Sure better ways to explain it but this should help.....

Ulli


> 
> We put out a couple more hummingbird feeders today, and I could not come up with a good 
> explanation as to why the water just doesn't drain out. I'm sure you all know the type of feeder I'm 
> talking about. A plastic container you fill with liquid, then you screw on a red plastic part that has 
> four perches and a flower like design. You turn the whole thing upside down and hang it. So why 
> doesn't the liquid just drain out? Why does it come out for the hummingbirds, but not for gravity? 
> Pondering the truly great question of our time in Freeport, I look forward to an answer. 
> Andy 

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