Du Noüy Ring or Wilhelmy Plate — Which Method Should You Use?

The Du Noüy ring and Wilhelmy plate are the two main force tensiometer methods for measuring surface and interfacial tension. Both are widely used, both are referenced in international standards, and both will give you a reliable measurement. So why does the choice between them matter?

The answer lies in your sample. Depending on what you are measuring, one method will give you more reliable results — or simply be easier to work with. This post walks through the key differences and helps you decide which method fits your application. For background on how each method works, see our post on the three main ways to measure surface tension. 

How the methods differ in practice

The most important practical difference between the two methods is what happens at the interface during measurement.

With the Du Noüy ring, the ring is pulled through the liquid surface and a meniscus forms beneath it. As the ring is lifted, the meniscus stretches until it breaks. The maximum force just before detachment is used to calculate surface tension. The interface is disrupted during the measurement.

With the Wilhelmy plate, the plate is immersed to a set depth and then held stationary at the interface. The force measured at this position is directly related to the surface tension. The interface is not disrupted — the plate simply sits there while the measurement is taken.

This difference — detachment versus stationary contact — is what drives most of the practical recommendations below.

When to use Wilhelmy plate

The Wilhelmy plate is the more versatile of the two methods and is well suited for a wide range of samples:

Surfactant solutions

When measuring surfactant solutions, the Wilhelmy plate generally gives more accurate equilibrium results. As the ring is pulled through the interface, it constantly creates new surface area. Surfactant molecules cannot adsorb fast enough to keep up, so the measured value may be higher than the true equilibrium surface tension — particularly at lower concentrations or with slowly adsorbing surfactants. The plate, kept stationary at the interface, avoids this problem and allows surfactants time to reach equilibrium.

However, if your application requires following an international standard, the method may already be decided for you — and most existing standards for surfactant solutions specify the ring. In that case, the ring is your starting point regardless of the above. Note that adsorption of surfactants onto the plate surface can also occur, which may affect wettability, so careful cleaning between measurements is important whichever method you use.

Viscous samples

For viscous liquids, the Wilhelmy plate is generally the better choice. The sharp, well-defined edge of the plate penetrates the surface of viscous liquids better than the ring geometry. Continuous mode, where the plate remains at the surface throughout the measurement, can also be used to give the sample additional time to reach equilibrium.

Time dependent measurements

When you need to follow how surface tension changes over time — for example, as a surfactant slowly adsorbs — the Wilhelmy plate in continuous mode is the natural choice. The plate stays at the interface and force is recorded as a function of time, giving you a complete picture of how the surface tension evolves. 

When to use the Du Noüy ring

The Du Noüy ring is well suited for routine measurements on low-viscosity liquids such as water, simple solvents, and oils. For these samples it performs reliably and the measurement is straightforward to run. If your lab has historically used the ring and you need results to be comparable over time, there is no reason to switch.

The ring is also the right choice when your application requires following an international standard that uses the ring as a measurement probe.

Standards

If your application requires following an international standard, the method may already be decided for you. Many existing standards for surface and interfacial tension measurements specify the ring method, as shown in the table below. 

Practical considerations

Cleaning

Both methods require careful cleaning of the probe and sample vessel before each measurement. Neither method has a clear advantage here — cleaning is equally important for both. However, some tensiometers also support disposable paper Wilhelmy plates, which can be convenient when running many samples or in teaching lab settings where cleaning between measurements is impractical.

Correction factors and density input

The Du Noüy ring requires a correction factor to account for the geometry of the liquid meniscus pulled by the ring. Applying this correction requires knowing the density of the liquid — or the density difference between two liquids in interfacial tension measurements. This adds a measurement step, though density can typically be measured on the same force tensiometer without needing separate equipment. The Wilhelmy plate calculation is more straightforward: once the plate dimensions are known, no correction factor is needed. 

Sample volume

Both methods require sample volumes in the milliliter range — enough to immerse the probe without it being affected by the vessel walls. If sample volume is limited, a platinum rod is an alternative to the plate, allowing measurements in narrower vessels with less liquid. 

For very low sample volumes, pendant drop method with an optical tensiometer can be considered instead.

Summary

The Wilhelmy plate is the more versatile method and is the better choice for surfactant solutions, viscous samples, and time-dependent measurements. The Du Noüy ring is well suited for routine measurements on pure, low-viscosity liquids and is required by many existing international standards.

In short:

• If you are measuring surfactant solutions → Wilhelmy plate, unless a standard specifies otherwise
• If your sample is viscous → Wilhelmy plate
• If you need to follow surface tension over time → Wilhelmy plate
• If you are measuring low-viscosity liquids → either method works; ring is a solid default
• If you need to follow an international standard → check the table above; most specify the ring

FAQs about Du Noüy ring and Wilhelmy plate measurements

What's the difference between Du Noüy ring and Wilhelmy plate?

The Du Noüy ring and Wilhelmy plate are both probes used with force tensiometers to measure surface and interfacial tension. They differ in geometry and in how they interact with the liquid interface, which determines which samples and applications each is best suited for. 

Can I switch between the ring and plate on the same instrument?

Yes. Both the Du Noüy ring and the Wilhelmy plate are standard probes for force tensiometers, and most modern instruments support both. Switching between them is straightforward — remove one probe and attach the other to the measurement hook. This makes it easy to choose the most appropriate method for each sample without needing a separate instrument.

Which method is more accurate?

Both methods give accurate results when used correctly. In practice, accuracy depends more on sample preparation and cleanliness of the probe than on the method itself.

Which method is better for surfactant solutions?

For surfactant solutions, the Wilhelmy plate is generally the better choice, as it does not stretch the interface during measurement. If your application requires following a standard, check which method the standard specifies.

Learn more

For a full comparison of the surface tension measurement methods — including optical tensiometry and pendant drop — download our white paper "Surface and interfacial tension – How to select the best measurement method for your application?" below.