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Concept
Tensiometer (surface tension)
Summary
In surface science, a tensiometer is a measuring instrument used to measure the surface tension (γ) of liquids or surfaces. Tensiometers are used in research and development laboratories to determine the surface tension of liquids like coatings, lacquers or adhesives. A further application field of tensiometers is the monitoring of industrial production processes like parts cleaning or electroplating.
Surface scientists commonly use an optical goniometer/tensiometer to measure the surface tension and interfacial tension of a liquid using the pendant or sessile drop methods. A drop is produced and captured using a CCD camera. The drop profile is subsequently extracted, and sophisticated software routines then fit the theoretical Young-Laplace equation to the experimental drop profile. The surface tension can then be calculated from the fitted parameters. Unlike other methods, this technique requires only a small amount of liquid making it suitable for measuring interfacial tensions of expensive liquids.
Du Noüy ring method
This type of tensiometer uses a platinum ring which is submersed in a liquid. As the ring is pulled out of the liquid, the force required is precisely measured in order to determine the surface tension of the liquid.
The method is well-established as shown by a number of international standards on it such as ASTM D971. This method is widely used for interfacial tension measurement between two liquids but care should be taken to make sure to keep the platinum ring undeformed.
Wilhelmy plate
The Wilhelmy plate tensiometer requires a plate to make contact with the liquid surface. It is widely considered the simplest and most accurate method for surface tension measurement. Due to a large wetted length of the platinum plate, the surface tension reading is typically very stable compared to alternative methods. As an additional benefit, the Wilhelmy plate can also be made from paper for disposable use. For interfacial tension measurements, buoyancy of the probe needs to be taken into account which complicates the measurement.
Official source
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