Dynamic contact angles, the advancing and receding angles, give a more complete and reproducible picture of surface wettability than a static contact angle alone. They also reveal contact angle hysteresis, which is linked to surface heterogeneity and to how easily droplets move across a surface. This matters for coatings, cleanability and self-cleaning surfaces, for example.
If you have ever measured static contact angles at a few spots on the same sample, you may have seen noticeably different results. That's not necessarily a measurement error. A real surface supports a whole range of stable contact angles, and a single static reading captures just one of them. The advancing and receding angles define the limits of that range.
What are dynamic contact angles?
Dynamic contact angles are the contact angles measured while the droplet's three-phase contact line is moving across a surface. There are two of them:
- Advancing contact angle is the largest contact angle a surface supports. It is measured as the liquid front advances over a previously unwetted part of the surface.
- Receding contact angle is the smallest contact angle a surface supports. It is measured as the liquid front retreats from a previously wetted part of the surface.
The difference between the advancing and receding contact angles is called contact angle hysteresis. A static contact angle measured on the same surface will typically fall somewhere between these two values.
Why isn't static contact angle always enough?
The static contact angle is simple to measure and works well for many routine tasks, such as checking that a surface treatment has had the expected effect. Its limitation is that it gives you only one value from a range. On a real surface, a droplet can rest stably at any angle between the receding and advancing angles, and where your static reading lands within that range can depend on how the droplet was placed.
A single static value also hides how the surface behaves when liquid moves across it. Two surfaces can show the same static contact angle, yet droplets may stay pinned on one and roll off the other with only a slight tilt. Measuring the advancing and receding angles reveals this difference, which is why dynamic contact angles are needed when you want to understand a surface more fully, or when the property you care about depends on how liquid moves.
What causes contact angle hysteresis?
Contact angle hysteresis arises mainly from surface heterogeneity. On an ideal surface, smooth and chemically uniform, the advancing and receding angles would be the same. Real surfaces are never ideal, and the main sources of hysteresis are:
- Chemical heterogeneity: areas of different surface chemistry pin the contact line as it moves.
- Surface roughness: topographical features hold the contact line back as it advances or retreats.
- Liquid–surface interactions: the liquid may adsorb onto, penetrate or swell the surface, or cause molecules at the surface to reorganize.
Because contact angle hysteresis is sensitive to chemical heterogeneity and roughness, it is often used as an indicator of how uniform a surface or coating is. It isn't a direct measure, though, since interactions between the liquid and the surface also contribute.
What do advancing and receding contact angles tell you?
Each of the two angles, and the difference between them, describes a different aspect of how water interacts with a surface1:
- The advancing contact angle correlates with how strongly the surface attracts or repels water.
- The receding contact angle correlates with how strongly water adheres to the surface.
- Contact angle hysteresis correlates with how easily a droplet moves across the surface. A moving droplet has an advancing edge at the front and a receding edge at the back, so the larger the hysteresis, the less mobile the droplet. Drop size also plays a role, so hysteresis isn't the only factor.
- Surface uniformity shows up as you measure, because the contact line sweeps across a much larger area than a single resting droplet covers. Repeating the measurement at several locations extends that coverage further.
Another practical advantage is that, unlike a static contact angle, the advancing and receding angles can be measured reproducibly when a careful protocol is followed1. That said, the receding angle is more sensitive to roughness and can be harder to measure reliably.
The two angles also respond differently when a surface isn't chemically uniform, for example because of contamination. The advancing angle is most sensitive to the low-energy, apolar components of a surface, while the receding angle is thought to be more sensitive to the high-energy, polar components.
In practice, this makes the advancing angle a useful cleanliness check. On a high-energy surface, even a small amount of low-energy contamination can raise the advancing water contact angle noticeably, and most contaminants are organic and low in surface energy. The advancing angle is also the more relevant value in applications where liquid spreads over a dry surface, such as inkjet printing and spray coating.
Example: water-repellent coatings and smart surfaces
Hydrophobic and superhydrophobic coatings are gaining interest for their anti-wetting and self-cleaning properties. On these surfaces, a high contact angle alone is not enough to produce the desired behavior. Low hysteresis and a low roll-off angle, the tilt angle at which a droplet starts to roll off, are needed so that droplets don't stick to the surface.
This matters on surfaces such as windshields, lenses and water-repellent textiles, where water shouldn't accumulate. Rolling droplets also collect dirt as they go, making the surface self-cleaning. For more on how dynamic contact angle was used to evaluate a lens coating, see our case study A proof of superior coating quality with dynamic contact angle.
Dynamic contact angles are also useful for characterizing smart surfaces, whose wettability changes in response to a stimulus such as temperature, light or an electric field. For more on this, see our article Practical guide to smart materials and wettability .
How are dynamic contact angles measured?
Dynamic contact angles can be measured with either an optical or a force tensiometer. The right method depends both on your sample and on what you want to learn about it:
- Needle method (optical tensiometer): the droplet volume is increased and then decreased while the contact angle is recorded. Gives local values, so measuring at several spots shows how uniform the surface is. The measurement can be fully automated, which reduces operator-to-operator variation and makes results more repeatable.
- Tilting method (optical tensiometer): the sample is tilted until the droplet starts to move. Also gives the roll-off angle, which is useful for hydrophobic and self-cleaning surfaces. Because the roll-off angle depends on drop size, using the same drop volume is important when comparing samples.
- Wilhelmy plate method (force tensiometer): the sample is immersed in and withdrawn from the liquid while the force is recorded. Easy to automate and free from visual judgment, it suits samples with a uniform perimeter and the same surface properties on all sides, such as plates and fibers. Results are an average over the whole immersed area rather than local values.
Learn more
To learn more about dynamic contact angle measurements, join our free upcoming webinar "What dynamic contact angle measurements can tell you" on November 12, 2026.
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