In previous posts we’ve discussed What is a du Noüy ring and What is a Wilhelmy plate. However, when measuring surface tension or interfacial tension with a force tensiometer, how do you choose which probe is best?
When measuring interfacial tension (i.e. between two immiscible liquids such as oil and water) the du Noüy ring is most common because it is easier to manipulate at a liquid-liquid interface.
For surface tension measurement (liquid/air interface) both methods are commonly used – so how do you chose? User preference can account for many of these decisions – but if you are not sure, here’s a breakdown to help guide you based on application, accuracy, sample type and practical considerations.
- Depending on the liquids being measured, surface tension results measured with the ring and the plate may give different results – so for repeatability, always choose one method and stick to it.
- The Wilhelmy plate performs better on viscous or complex liquids since it does not require pulling through the interface.
- The Wilhelmy plate is more suited if only a small sample volume is available – but see footnote*.
- For contaminated or impure surfaces (e.g. surfactant solutions), the Wilhelmy plate is best as it is more stable and doesn’t disrupt the interface as much.
- If using a manual force tensiometer, the du Noüy ring method is easier to use as it’s a simpler and more straightforward process
- Surface tension and interfacial tension are the subject of industry standards including ISO, ASTM and others. These standards will provide detailed and specific protocols to follow when relevant. Detailed descriptions of each relevant standard are available on the Biolin Scientific website.
There are additional pros and cons/strengths and weaknesses to each method:
| du Noüy ring | Wilhelmy plate |
|---|---|
| Strengths | |
| Well established and more utilised method | No need for correction factors or densities |
| Partially takes into account liquid evaporation | Better suited for high-viscosity liquids |
| Less prone to contamination | Probe less prone to bending |
| Ideal for static measurements | Requires a smaller volume of liquid |
| Weaknesses | |
| Needs correction factors to work | Contact angle of 0° assumed with the probe and the liquid |
| The probe is more prone to bending which will influence the result | Results depend on the height resolution of the sample stage |
| The density values of both phases must be known | Interfacial tension measurement is more complicated due to buoyancy factors |
| The probe can occasionally break meniscus ending the measurement prematurely leading to a false result | The probe gets contaminated more easily |
Summary
The choice depends on your specific application, available equipment, regulatory requirements – and just maybe what you are used to. Cleaning the probe also plays an important role in the measurement process and should not be overlooked.
If you are looking to measure surface and interfacial tension and need help in choosing the best method please get in touch.
* When sample volumes are very small, surface tension can also be measured using the sessile drop method with a benchtop optical tensiometer.
Categories: Surface Tension
