Re: Re: What are the disadvantages of capillary action?

Thanks for asking about capillary action.  When we discuss advantages and
disadvantages of a particular process, it all depends upon what we are
comparing that process with.  For example, capillary action is one method
of pumping liquids from one place to another.  Compared with a mechanical
pump, capillary action produces relatively low flow rates of liquid.  On
the other hand, a mechanical pump requires some external source of energy,
so you might want to use a pump that uses capillary action when it is
difficult to obtain electricity or some other source of energy.

A good discussion of capillary action can be found here: http://www.madsci.org/posts/archives/1998-02/887637827.Ch.r.html
 The
distance that a liquid rises within a capillary can be used to determine
the surface energy or surface tension of the liquid in contact with the
material of the capillary.

Capillary action relies upon being able to wet surfaces in order to develop
a pumping action, and the better it is able to wet a surface, the higher
pumping pressure that can be developed.  On the other hand, if the liquid
you are trying to pump doesn't wet the surface within the capillary, then
there is no pumping action.  For example, say you are trying to pump water.
 If a capillary is lined with wax (like wax paper) the water won't wet the
surface and no pumping will take place.  If the liquid you are trying to
pump has certain types of contaminants that might coat the surface of the
capillary with a coating that is non-wetting, then a capillary again might
stop working.

Another consideration of capillaries is that they are quite small.    If
the liquid your are trying to pump using a capillary contains solid
particles, or even if it contains materials that might for a solid particle
if some of the liquid evaporates or cools too much, the capillary might
then get plugged.

A related problem is with the size of the capillaries.  Very small
capillaries develop higher pressures than larger capillaries.  The smaller
the diameter of the capillary, the higher the liquid will rise within the
capillary.  It is, however, more difficult for liquid to move through a
small capillary, compared with a larger capillary (friction).  So, if you
want a higher flow rate through a capillary, you will have to settle for a
lower pumping pressure.

I hope that helps explain some of the trade-offs in using capillary action,
as opposed to other types of pumping techniques.