Ask A Scientist Physics Archive

name         Kathy Van O.
status       educator
age          40s

Question -   We did the demonstration with the two liter bottle full
of water with three covered holes on the side.  Uncover the top hole, no
water escapes.  Uncover the second hole and air bubbles are seen at the
top hole and a stream of water comes out the second hole. Why does the
water level stop at the top of the hole?  Everyone predicted that the
water would run out to just below the hole, but every time the meniscus
of the water was at the top of the hole.  We need a scientific
explanation.
-----------------
If the hole is small enough, the surface tension of the water and the 
adhesion of the water to the bottle may be enough to support the 
additional height of water.

I suggest experimenting with different size holes.  You could also reduce 
the surface tension by adding soap to the water and see if this affects 
the results.

Greg Bradburn
=====================================================
Pretty sure the water is held back that extra 2-4 mm by the surface 
tension of the water stretched across the small exit hole.
A corollary to that is: I bet the outside of your bottle is not wet, even 
after the water streams out and dribbles to a stop.
If it was wet, the water level inside would go as low as you expect.

Paint your bottle with dish soap and include a few drops inside, too.
Then the water level will go down farther, possibly a below the bottom of 
the lowest open hole.
Gimicking the hole often changes this, too, like making the bottom edge of 
the hole rounder,
or putting a few shallow razor-blade scratches on the hole's bottom edge
and on the outside surface leading 1/8 to 1/2-inch down from the hole.

These scratches do not go through the bottle wall, they are merely concave 
parts of the plastic's surface
which attract water better than the glossy, clean manufactured surface.
Crazing the surface with acetone or nail-polish remover or sand-paper 
would have similar effects.

With your bottle un-modified, pour a gentle trickle of water down the side 
of the bottle, so that it touches the hole.
This will probably "suck" the excess water out relatively briskly, without 
appearing to work hard.

Some of my plastic sinks at work are extremely resistant to draining the 
last drop.
Can be a pesky design problem.

Try that?
Jim Swenson
=====================================================