The definition of Liquid Surface Tension is simple.
It's the force that keeps a liquid from flying off into space.
However, the measurement of surface tension can take many forms, which can be confusing. In an attempt to get some clarity, we have articulated two principle measurement concepts in this article: drop-based measurement and force-based measurement.
One of the concepts is based on the analysis of a drop. This analysis can be:
These methods either require advanced mathematical analysis, or are hard to perform.
The other concept is based on the force exerted on a foreign body by the surface of a liquid. The underlying task is the measurement of the this force.
The other major force method uses a thin rectangle. The rectangle and the test process are referred to as Wilhelmy Plate. When the plate is lowered to touch the liquid, the surface tension grabs the plate and pulls down. The level of this force is used to calculate the surface tension of the liquid.
Spinning Drop is useful for very low surface tension liquids. Pendant drop analysis has the benefit of characterizing network properties. Bubble pressure instruments are particularly useful for in-line measurement and for time-phased changes.
The other techniques that include capillary rise and drop volume are used for special applications and education.
Our target is techniques of force measurement. Some of the earliest measurements were with the DuNouy Ring. This measurement starts when a ring is fully below the liquid surface.
At that juncture
Some of the early surface tension instruments used a torsion balance mechanism to determine this force. This mechanism is still used today. When calibrated, these torsion balances provide a sensitive, solid, geo-mechanical structure to ensure precision and repeatability. Newer designs use electronic balances to measure the forces.
The plate technique (Wilhelmy Plate) uses a thin rectangle, often made of platinum.
The calculation of surface tension is usually in terms of Dynes/cm as follows:
The DuNouy Ring method measures apparent surface tension. A conversion formula is needed to obtain actual surface tension. The correction factor ranges from 0.8 times the apparent surface tension at low levels to more than 1.3 times the apparent surface tension for liquids with high surface tension. In many industrial applications, standards are based on apparent readings.
Wilhelmy Plate results are actual surface tension and do not need correction.
Force-based surface tension instruments have a wide application range. Here is a sample of applications:
In summary, we defined two categories of surface tension instruments.
Drop-based instruments, which include Spinning and Pendant Drop instruments, use complex optical drop shape analysis. Bubble Tube instruments are suited to in-line and portable use. There are a number of instruments based on capillary action used for special measurement and education.
Force-measuring surface tensiometers, which include DuNouy Ring and Wilhelmy Plate instruments, have a
If you find this article useful, please share it with associates who deal with liquid surface tension.
I start on these subjects that seem to be clear-cut and easy to describe, but they end up with snags that mystify me.
Hope this treatise is useful,
Art
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