Re: Why is there no whirlpool when we have a bubble bath?

Dear Andrea:

Several days ago, I had a conversation with two physicist friends and a 
chemist friend about your inquiry.  I learned some interesting things 
about whirlpools (vortexes) that I didn’t know before.

The biggest whirlpools on earth are, of course, low pressure systems, 
extreme versions of which are hurricanes and tornadoes.  As for these 
systems, the circulation of the air around their center is known to be 
counterclockwise (when viewed from above) in our (northern) hemisphere and 
clockwise in the southern hemisphere.  The “cause” of the direction of 
rotation is the Coriolis Force, one of several types of “inertial forces” 
(also known as pseudo-forces) such as the “artificial gravity” created in 
rotating spaceships or space stations, or the “centrifugal force” 
which “pushes” you towards a car door when the car is making a sharp turn.

A nice description of the Coriolis Force and its consequences is presented 
in Chapter 7 of Jerry D. Wilson’s textbook College Physics, Simon and 
Schuster, 1990.  A more vivid analogy can be found at the following 
website:
 http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/crls.rxml

Coriolis forces are “exerted” on large-scale motions, but have no effect 
on small-scale phenomena, such as whirlpools in a bathtub or sink.  And 
here’s where my chat with my science friends got interesting.
It has been shown that bathtub whirlpools can rotate either clockwise of 
counterclockwise depending on how the motion is initiated.  That is, if 
you stir the water in a counterclockwise direction, the resulting 
whirlpool will be counterclockwise.  On the other hand, starting the water 
rotating clockwise, will result in that motion’s continuing as the water 
drains.

Even more fascinating was an Australian study that (according to my 
friends) showed the same phenomenon in an enormous pool of water with the 
drain in the center of the pool.  What was fascinating about this study 
was that, if the water was forced to rotate in one direction, and then the 
drain was plugged, and the pool was allowed to come to quiescence, the 
same rotation continued when the drain was unplugged, even days later.  It 
is as if the water “remembered” its original motion!

It is well known that water molecules attract each other strongly by means 
of “hydrogen-bonding”:
 http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/H/HydrogenBonds.html

If some of the molecules are caused to rotate in one or another direction, 
they “carry” their neighbors with them resulting in larger-scale 
rotation.  On the other hand, when bubble-bathwater is drained, two things 
occur:  Firstly, the soap molecules cause the water molecules to separate 
from each other, lessening their attractive influence on each other.  
Secondly, the bubbles physically block the rotation from initiating, 
because the bubbles and water together act as if they were a kind of 
powder.  You know from experience that sand (or any powdered material) 
will not go into rotation when it is being poured from one location to 
another, even if you try to start it rotating.

Most sincerely,
P. M. Fichte
Coker College
Hartsville, SC