What are the differences between using a Wilhelmy plate or a Du Noüy ring in surface / interfacial tension measurements?

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If you compare the results measured with the ring and with the plate, depending on your liquid you may get different results. This is especially true when measuring surfactant solutions. This is because of the differences in the measurement methods. With Wilhelmy plate, the plate is stationary during the measurement which means that the surfactant molecules have time to arrange themselves at the interface lowering the ST value. If you measure with the ring, the interface is constantly changing since the ring is moving during the measurement. This is the reason why the ST values measured with the ring are often bit higher than with the plate. This can be seen even with water that has some contamination in it. With surfactant solutions Wilhelmy plate is preferred.

Ring

+ More standardized and more utilized method

+ Partially takes into account liquid evaporation

+ Less prone to contamination

– Needs correction factors to work

– The probe is more prone to bending

– The density values of both phases must be known

– The probe can occasionally break meniscus à measurement ends prematurely

Plate

+ No need for correction factors or densities

+ Better suited for high-viscosity liquids

+ Probe less prone to bending

– Contact angle of 0° assumed with the probe and the liquid

– Results depend on the height resolution of the sample stage

– Interfacial tension measurement more complicated due to buoyancy factors

– The probe gets contaminated more easily


How should I clean a Wilhelmy plate?

Clean the Wilhelmy plate by flushing it with pure ethanol and water and subsequently burning it with a Bunsen burner (ca. 1000?C flame). Low heat flames will leave residues on the plate which can lead to significant error. Hold the plate in the hottest part of the flame for a few seconds until the plate glows red-hot. Take the plate away from the flame before turning the flame off to avoid residues from low heat. Clean the plate before and after use.


How should I clean a Du Noüy ring?

Clean the Du Noüy ring by flushing it with pure ethanol and water and subsequently burning it with a Bunsen burner (ca. 1000?C flame). Low heat flames will leave residues on the ring which can lead to significant error. Hold the ring in the hottest part of the flame for a few seconds until the plate glows red-hot. Take the ring away from the flame before turning the flame off to avoid residues from low heat. Clean the ring before and after use.


What kinds of samples are suitable for powder wettability studies?

The particle size of the powder should be larger than the pore size of the sample holder. The glass holder available has a pore size of 1μm, and the steel holder (only available for Sigma 700) has a pore size of 5μm. The powder shouldn’t be soluble in the measurement liquid, nor interact with the liquid in any way. In addition, the contact angle of the powder shouldn’t exceed 90 degrees in order for the measurement liquid to rise up in the holder.

There is no exact viscosity range for Sigma because the measurability depends also on other properties, such as liquid density and elasticity, probe type and measurement parameters. As a general rule, viscosities up to 1000 mPas can usually always be measured with Sigma. Anything above 10 000 mPas will most likely not work. Viscosities in between require pretesting to determine the compatibility. 


How should I clean a density probe?

Clean the density probe by flushing it with ethanol and distilled water. The density probe does not tolerate a hot flame such as a Bunsen burner.