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Name: Sushil
Status: student
Grade: 6-8
Location: MO
Country: N/A
Date: May 2006

Question:
What are the measurements you use to find the surface tension of a liquid?



Replies:
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.

Vince Calder



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