Benchmark validation
Contact angle and pendant-drop methods benchmarked against KRÜSS DSA100E measurements.
See peer-reviewed validationExpert contribution
Why You Need Dynamic Surface Tension
Static surface tension is rarely the value that controls spraying, coating or cleaning. The real question is how quickly the surface tension changes at process timescales.
Written by
Prof Steven Abbott
Adhesion and surface science consultant; author of Sticking Together and the Practical Coatings / Practical Surfactants apps.
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Reviewed by
Droplet Lab Team
Droplet Lab builds precision instruments and software for surface science measurement, specialising in contact angle analysis and surface tension characterisation. Used by researchers across materials science, pharmaceuticals, coatings, and advanced manufacturing, Droplet Lab's Dropometer has contributed to studies published in peer-reviewed journals including Advanced Functional Materials (Impact Factor 19). The team combines instrument engineering with deep domain knowledge in wettability science with a focus on practical accuracy.
Read More
Reviewed By
Droplet Lab Team
Droplet Lab builds precision instruments and software for surface science measurement, specialising in contact angle analysis and surface tension characterisation. Used by researchers across materials science, pharmaceuticals, coatings, and advanced manufacturing, Droplet Lab's Dropometer has contributed to studies published in peer-reviewed journals including Advanced Functional Materials (Impact Factor 19). The team combines instrument engineering with deep domain knowledge in wettability science with a focus on practical accuracy.
Disclosure: An unpaid, independent contribution; Prof Abbott has no commercial affiliation with Droplet Lab.
Process timescales
Why static surface tension is not enough
Measuring the surface tension of a liquid can be hard work. You often have to wait minutes for the value to stabilise, and it is very susceptible to small contaminants. It is also a waste of time for most practical processes. Almost no one has a real use for these static surface tensions.
Think about spraying, coating or cleaning. The interactions that matter take place in milliseconds. What matters is how quickly the surface tension falls to a value that helps a spray spread, a coating become even and cleaning happen.
For this, measure the dynamic surface tension. You will quickly find something counterintuitive: many “good” surfactants with low critical micelle concentrations (CMC) and low equilibrium surface tensions are useless. Instead, you often choose a “poor” surfactant. You can explore this behavior using my free dynamic-surface-tension app.
The left-hand graph is the classic curve of surface tension with concentration. With a CMC of about 100 μM, this looks like a “good” surfactant. Now look at the right-hand graph. After one second, the surface tension has dropped only to about 60 mN/m. Your process has probably finished by this time, so the surfactant has had no time to help.
Why is it so slow reaching the surface? It has nothing to do with diffusion—most surfactants diffuse at similar rates. Instead, most of the surfactant is tied up in large, lumbering micelles, with very little free surfactant able to reach the surface.
Now let's leep everything the same but change the CMC to about 470 μM. By 100 ms, the surface tension has fallen to about 30 mN/m. The reason is that there is a large amount of free, non-micellar surfactant that can quickly move to the surface.
That is why dynamic surface tension, or DST, is the measurement that matters.
For a fuller explanation, see James K. Ferri and Kathleen J. Stebe, Which surfactants reduce surface tension faster? A scaling argument for diffusion-controlled adsorption, Advances in Colloid and Interface Science 85 (2000), 61–97.
Pendant-drop method
Measuring Dynamic Surface Tension
In principle, measuring DST is easy. The shape of a droplet hanging from a tube "a pendant drop" depends on the size of the tube, the drop’s mass and the surface tension at that instant. A high surface tension makes the drop more spherical than a low surface tension, as described by the Young–Laplace equation.
Use a syringe to produce a drop growing at a controlled rate, capture an image every 33 ms, use image analysis to determine the volume and—using the liquid density—its mass, then fit the shape to the Young–Laplace equation to extract the surface tension at that moment. At every instant, you therefore know the dynamic surface tension.
Learn how to measure dynamic surface tension with the Dropometer
It is more complicated than that in practice. Measurements at multiple speeds are needed to capture drops at an optimal size for accurate calculations, so smart algorithms must cover the desired timescales.
Affordable without compromising accuracy
Traditionally, the method required expensive equipment and was out of reach for many laboratories. But 3D printing, excellent digital microscopes, fast control software and mobile computing make it possible to combine precision with affordability. Droplet Lab has transformed this important technique into an affordable package. But the team at Droplet Labs haven’t compromised on accuracy. Producing an OK prototype was one thing. The real challenge was to create a package that delivered reliable values, using standard, challenging test cases to compare results to those from well-known high-price devices. Only when components, engineering, assembly and software gave the required precision, accuracy and reliability were they happy to put the device on the market.
Evidence
Validated measurement approach
The contact-angle and pendant-drop surface-tension methods have been benchmarked against KRÜSS DSA100E reference measurements. Droplet Lab instruments are also referenced in peer-reviewed journals, theses and conference publications.
Publication evidence
Instruments referenced in peer-reviewed journals, theses and conference publications.
Browse the full citations listThe practical rule
Never measure only the surface tension. Measure the dynamic surface tension at the timescale of your process.
If DST was previously unaffordable, the device from Droplet Lab has made it practical.
Bring dynamic surface tension measurement into your lab.
Explore the Dropometer or discuss your liquid, timescale and measurement requirements with Droplet Lab.
Frequently Asked Questions
What’s the difference between static and dynamic surface tension?
Static surface tension is the value a liquid settles at after minutes of waiting, and it is rarely what matters in practice. Dynamic surface tension (DST) is the value at the actual timescale of your process—spraying, coating or cleaning all happen in milliseconds, so what matters is how fast surface tension drops, not where it eventually ends up.
Why would a surfactant with a low CMC and low equilibrium surface tension still perform poorly in a fast process?
Because most of that surfactant is tied up in micelles, leaving very little free surfactant available to reach the surface quickly. A “poorer” surfactant with more free molecules available can lower surface tension faster, even though its equilibrium value looks worse on paper.
How is dynamic surface tension actually measured?
Through the pendant-drop method—a drop is grown from a tube at a controlled rate, imaged at fixed intervals, and its shape is fitted to the Young–Laplace equation to extract surface tension at each instant. Doing this at multiple growth speeds captures the full range of timescales relevant to a given process.
What is CMC, and why does it change how fast a surfactant works?
CMC is the critical micelle concentration—the point at which surfactant molecules start forming micelles instead of remaining free in solution. A higher CMC generally means more free surfactant is available to diffuse to a fresh surface quickly, which is why a “good” low-CMC surfactant by equilibrium standards can be the wrong choice for a fast process.
Why has DST measurement historically been expensive, and what has changed?
It traditionally required specialised, high-cost equipment. Advances in 3D printing, digital microscopes, control software and mobile computing have made it possible to build accurate DST measurement into an affordable package.