Measuring Surface Tension
Date: May 2006
What are the measurements you use to find the
surface tension of a liquid?
There are many methods for measuring the surface tension of liquids.
Each has its advantages and limitations. There is a reason why there
are different methods -- surface tension is a very complicated
property of a liquid. It depends upon many variables, some of which
are: temperature, composition of the liquid (it may be a solution or
contain small amounts of substances that affect the surface
tension), measurement time (surface tension depends on time), the
material of construction of the apparatus, the viscosity of the
liquid: And this is the short list!! Realistically, one should refer
to the surface "tensionS" (plural) of a liquid because the answer
you get can be very different depending upon the method used.
One category of methods is based on the angle of contact a
liquid makes to a solid surface. The solid may be a flat horizontal
plate, a tilted plate, a vertical plate, or the walls of a thin
tube (capillary). Assuming the variables above are all under
control, all these methods have a common limitation that the angle
of contact is difficult to measure accurately. Everybody "sees" the
angle a bit differently and this results in a different value of
the surface tension. In each case the liquid solid contact may be
stationary or may be moving.
A second category of methods is based on the shape of a drop of
the liquid. The drop may be hanging stationary, or it may be
dripping, or it may be resting on a flat plate. The "problem" with
this class of methods is that the mathematical analysis of the
shape of a drop and the surface tension of the liquid is very
complicated (in fact the relation between the shape and surface
tension must be solved numerically because the formulas don't have
a direct solution).
Each of these methods also comes in "flavors". One "flavor" is a
static measurement in which the liquid is not moving. The other
"flavor" is a dynamic measurement in which the liquid is moving.
Often the "static" and "dynamic" surface tension have quite
different values that are real, and not just experimental error. In
fact the difference between the "static" and "dynamic" value can be
very useful in understanding the surface properties of the liquid.
An interesting side light for any of these surface tension
measurements is that most all the experimental setups now use video
cameras to record the basic interface. So there is a permanent
record of the experimental results. This allows the scientist the
opportunity to review the result and refine its analysis.
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Update: June 2012