Liquid surface tension has a straightforward definition with a dramatic job description: it’s the force that politely stops a liquid from flinging itself off into the universe.
Measuring it, however, is far less straightforward. There are multiple ways to quantify surface tension, and they can quickly turn an easy concept into a small maze of methods. To bring some order to the chaos, this article focuses on two main measurement approaches: drop-based techniques and force-based techniques.
One of the concepts focuses on what a single drop can reveal. That “simple” drop can be:
- The subject of a complex optical inspection of its shape,
- A partner in watching how much pressure it takes to make a bubble finally burst, or
- A timekeeper as a series of drops travels through a narrow tube.
These methods either call for mathematics that do not usually come free with a lab coat, or they are simply demanding to carry out in practice.
The other concept takes a more hands-on view: it watches how firmly the liquid’s surface hangs on to anything bold enough to touch it. In simple terms, the task is to quantify that tug.
One of the best-known approaches, the DuNouy Ring method, starts by quietly parking a ring just below the liquid surface. The ring is then drawn upward through the interface between
The other major force method swaps the ring for a slim rectangle, known—along with the procedure itself—as the Wilhelmy Plate. When the plate is lowered to meet the liquid, surface tension effectively “shakes hands” with the plate and pulls it downward. The strength of that handshake is what we use to calculate the liquid’s surface tension.
The test routines we group as drop-based include Spinning Drop, Pendant Drop, Bubble Pressure, and investigations of capillary rise and drop volume.
Spinning Drop shines when you are dealing with very low surface tension liquids. Pendant drop analysis does double duty by helping characterize network properties. Bubble pressure instruments are especially helpful when you want in-line measurements or need to watch how surface tension changes over time.
The remaining techniques, such as capillary rise and drop volume, are the specialists of the group—ideal for targeted applications and for teaching how these principles behave in the real world.
Our focus here is on techniques that quite literally feel the force. Some of the earliest surface tension measurements were made with the DuNouy Ring. The test begins with the ring resting quietly below the liquid surface. At that point, aside from gravity, nothing much is happening. Then a force mechanism pulls the ring upward through the surface. As the ring approaches the interface, the liquid starts to resist, building to a maximum just as the ring finally breaks free. That peak in stubbornness is what we use to calculate surface tension.
Early
The plate technique (Wilhelmy Plate) trades the ring for a thin rectangle, often platinum. As the plate is lowered into the liquid, surface tension takes hold and pulls it downward. That downward pull becomes the basis for the final calculation, with an electronic balance again acting as the impartial referee.
Surface tension is usually reported in dynes per centimeter:
- For the DuNouy Ring, the formula uses the ring’s inner a
- For the Wilhelmy Plate, it uses the downward force on the plate and the plate’s perimeter.
A recent twist on the Wilhelmy approach is the use of platinum rods instead of plates. The idea stays the same: measure the force around a known circumference and let the surface tension reveal itself.
The DuNouy Ring method measures apparent surface tension. A conversion formula is needed to obtain actual surface tension. The correction factor ranges from 0.8 times the apparent surface tension at low levels to more than 1.3 times the apparent surface tension for liquids with high surface tension. In many industrial applications, standards are based on apparent readings.
Wilhelmy Plate results are actual surface tension and do not need correction.
Force-based surface tension instruments have a wide application range. Here is a sample of applications:
In summary, we have two main families of surface tension instruments.
Drop-based instruments—such as Spinning Drop and Pendant Drop systems—focus on what a single drop can reveal through complex optical shape analysis. Bubble Tube instruments step in when you want in-line or portable measurements. A further collection of tools built on capillary action round out this group, well suited to specialized measurements and to demonstrating these behaviors in training and educational settings.
Force-measuring
If you found this exploration of liquid surface tension useful, consider sharing it with colleagues who live with these measurements every day. Topics that appear straightforward at first glance often reveal unexpected twists once you start pulling on the details.
We hope this tour through the world of surface tension has earned its keep,
Hank
P.S. You can subscribe to our soliloquies, articles, and newsletters by dropping your email address into the box just to the right of the title—it may be the easiest surface tension experiment you run all day, and the only one that answers back.
P.P.S. Take a look at our lineup of DuNouy Ring and Wilhelmy Plate tensiometers—the instruments that let your liquids reveal their true character, one measured tug at a time.