There are three main methods used to measure surface tension: Du Noüy ring, Wilhelmy plate and pendant drop. Du Noüy ring and Wilhelmy plate are force‑based methods run on a force tensiometer. The pendant drop method is an optical technique based on droplet shape.
Surface tension plays a key role in several industrial processes, including for example detergents, coatings and oil recovery. When you develop new formulations or need to check product quality, measuring surface or interfacial tension is often an essential step. Here is a closer look at the three main methods.
The Du Noüy ring method utilizes a thin platinum ring as a probe in a force tensiometer. The ring is attached to a highly sensitive balance and brought into contact with the liquid surface (or liquid–liquid interface). In a typical measurement:
The ring is positioned above the interface and the force is zeroed.
The sample stage is raised so that the ring is immersed below the surface.
Figure 1. From force vs. time curves different stages of the experiment can be identified: 1) At the beginning, the ring is above the interface and the force is zeroed. 2) The ring touches the interface and there is a slight positive force due to the adhesive force between the ring and the surface. 3) The ring must be pushed through the interface (due to the surface tension which will cause a small negative force). 4) The ring breaks through the interface and a small positive force is measured due to the supporting wires of the ring. 5) When lifted through the interface the measured force starts to increase. 6) The force keeps increasing. 7) The maximum force is reached. 8) After the maximum there is a small decrease of the force until the lamella breaks, or the ring is pushed back below the surface.
If you look at the force vs. time curve (Figure 1), you can clearly see the different stages: first contact with the surface, immersion of the ring, breakthrough of the interface, and the increasing force as the meniscus stretches until it breaks.
Original calculations for the Du Noüy ring treated the ring as if it had an infinitely thin wire and did not fully account for the extra liquid that is pulled up between the two sides of the ring. Today, modern tensiometers automatically apply correction factors that take this into account. To use these corrections correctly, the density of the liquid – or the density difference between two liquids in interfacial tension measurements – needs to be known.
The Wilhelmy plate method also relies on force measurement but uses a roughened platinum plate as the probe. The method assumes that the plate is completely wetted by the liquid, so the contact angle is close to zero.
For the measurement, the plate is attached to the balance and positioned above the liquid surface. The sample stage is then raised until the instrument detects the first contact with the liquid, seen as a small change in force. This position is set as the zero depth of immersion.
Next, the plate is immersed to a set depth below the surface and then moved back to the zero depth. At this position, the measured force is directly related to the surface tension and the wetted perimeter of the plate. Because the geometry is simple and well defined, the calculation is straightforward once the plate dimensions are known.
If sample volume is limited, a platinum rod can be used instead of a plate. The rod works in narrower vessels, but the smaller wetted length and less precisely known geometry usually give slightly lower accuracy than a plate. Some tensiometers also support disposable paper Wilhelmy plates, which can be a practical choice when running many samples or in teaching lab settings where cleaning between measurements is impractical.
Wilhelmy plate measurements can be done in a static mode, or in a continuous mode, where the plate remains at the surface and the force is followed over time — useful when you need to follow how surface tension evolves over time.
With modern force tensiometers, both Du Noüy ring and Wilhelmy plate measurements can be fully automated.
The pendant drop method measures surface and interfacial tension optically by analyzing the shape of a hanging droplet.
A small droplet is formed at the tip of a needle or capillary. Gravity pulls the droplet downward, while surface tension acts to keep it attached and minimize its surface area. The resulting shape – typically a pendant or tear shape – depends on the balance between gravitational forces and surface tension.
An optical tensiometer captures an image of the droplet and fits its profile with the Young–Laplace equation. When the density difference between the two phases is known, the surface tension can be calculated from this fit.
Pendant drop measurements are particularly attractive when sample volume is limited or when you already use an optical tensiometer for other measurements. The method provides a quick way to check surface and interfacial tension with minimal sample preparation — and with disposable tips, no probe cleaning is needed.
Du Noüy ring, Wilhelmy plate / rod and pendant drop are the three most commonly used methods for measuring surface tension and interfacial tension. They share the same physical principles but suit different situations:
Du Noüy ring – classic force method, widely referenced in standards. A good choice for routine measurements on low-viscosity liquids.
Wilhelmy plate / rod – robust force method, well suited for surfactant solutions, viscous samples and time-dependent measurements where you need to follow how surface tension evolves.
Pendant drop – optical method based on droplet shape, ideal when sample volume is limited or when you already have an optical tensiometer available.
For a deeper comparison of all three methods, including advantages, limitations and typical application areas, you can download our white paper “Surface and interfacial tension – How to select the best measurement method for your application?” at the end of this post.
Surface and interfacial tension are measured with tensiometers. Force tensiometers use probes such as Du Noüy rings, Wilhelmy plates or platinum rods attached to a sensitive balance. Optical tensiometers use cameras and image analysis to determine surface or interfacial tension from the shape of a droplet, as in the pendant drop method.
All three methods can give accurate results when used correctly. In practice, accuracy depends more on sample preparation, cleanliness of the probe, and correct density values than on the method itself. Wilhelmy plate is generally considered the most reproducible for equilibrium measurements due to its simple, well-defined geometry. Du Noüy ring results depend on applying the right correction factors. Pendant drop accuracy relies on good image contrast and knowing the density difference between phases.
Yes. Du Noüy ring, Wilhelmy plate / rod and pendant drop can all be used to measure interfacial tension between two immiscible liquids. The underlying principles are the same, but there are some practical differences in setup. For more information on interfacial tension measurements, visit our interfacial tension measurement page.
Du Noüy ring and Wilhelmy plate measurements typically require a few tens of millilitres so the probe is fully immersed and does not interact with the vessel walls. A Wilhelmy rod works with smaller volumes in narrower vessels. Pendant drop can often be done with only a few microlitres per drop.
Yes, but some methods suit this better than others. Wilhelmy plate is often preferred for viscous liquids because the plate can remain at the interface while the sample slowly wets it. Pendant drop can also work if a stable droplet shape can be formed and imaged. For very viscous or structured systems, you may need to extend measurement times to reach equilibrium.
Susanna is an Application Scientist at Biolin Scientific. In her PhD thesis, she developed fabrication methods for a new type of inorganic-organic polymers. Microfabricated polymer chips were utilized as tool for biomolecule separation in analytical chemistry.
