From: u108502@beta.lanl.gov (Andrew Poutiatine)
Subject: Re: Surface Tension Measurements
Keywords: Surface tension
Organization: Los Alamos National Laboratory
Date: Mon, 11 Jan 1993 18:24:48 GMT

In article <1993Jan8.203152.18382@hubcap.clemson.edu> panoff@hubcap.clemson.edu (r m panoff) writes:
>
>A middle school teacher has been asking about simple measurements
>of surface tension.  Is there a simple lab device that one can use to
>measure various liquids?  Does anyone know of a simple lab setup that
>can be done in the average middle school lab?  What kinds of hands-on
>explorations of surface tension ought to be suggested?  Any references
>appropriate for this level?
>
>Send any helpful comments, answers, suggestions, pointers directly to
>me at:  rpanoff@ncsa.uiuc.edu (note different from posting address)
>or directly to the middle school teacher: pedersen@cardinal.ncsc.org
>
>Thanks in advance for all your help.
>
>Bob Panoff
>-- 
>rmp, for the Bob's of the World

Surface tension is a fairly complex phenomenon, at least in normal every-day
situations, and a quantitative analysis of it may be beyond the middle school
level.  Qualitatively, one can explore whetting (hydrophilic) vs. non-whetting
(hydrophobic) contacts, i.e. drops on surfaces.  This is a function of the 
three phases involved, though, and is very sensitive to the conditions of the 
phases, i.e. how dirty is the surface, how pure is the liquid and gas?

Simple apparatuses may involve water, glass, wax paper, mercury (very poisonous)
, capillary tubes etc.

If you really want to measure the surface tension, there are a few simple
(although not too accurate) ways of doing it. 

One way is to place a capillary tube in the liquid and measure the rise of 
the liquid (or drop as the case may be) relative to the outside free 
surface.  I will try to draw a diagram:


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Measure the height above (or below) the free surface, and the densities of the
fluids, and the radius of the capillary tube, and the unknowns are then 
the surface tension (sigma), and the contact angle between the fluid and 
the tube wall (alpha).  If two different tube diameters are used (the same 
material, preferably glass), then you can solve for the two unknowns.
I am not actually sure at this point whether or not this will yield independant
equations, but I think so.  Perhaps someone else on the net could comment on
this, otherwise I could sit down and figure it out if you think this is a 
worthwhile endeavor. 
The equation for this situation is 

      h   2*(sigma)*cos(alpha)
      - = ------------------------
      a   (rhol-rhog)*g*a^2

where h is the height of the miniscus above (negative for below) the liquid
outside level, a is the tube inside radius, rhol is the liquid density and 
rhog is the as density, and g is the acceleration of gravity.

There are some other methods, but none are really that simple as far as 
calculating the surface tension, since usually they involve the contact
angle also.  It is possible to make assumptions that may make it simpler,
at the expense of accuracy, but this all sounds beyond a useful and attention
grabbing middle school level.

I hope this helps

-AIP