Roll-Off and Sliding Angle: Hydrophobic Surface Science

Roll-Off and Sliding Angle: Hydrophobic Surface Science

You might have heard the terms “roll-off angle” or “sliding angle” before as you’ve read labels, tech data sheets or articles about ceramic coatings for cars or other applications. But what exactly do these terms mean and how are they different? What do they have to do with hydrophobic surfaces?

Scientists have been studying the interactions between liquids and surfaces for a long time to understand how a variety of factors (including chemical structure, surface tension, coefficient of friction, roughness, temperature and more) affect the behavior of different liquids that make contact with a surface. Will the liquid form droplets, or sheet out? If the liquid forms droplets, what size and shape? Will the droplets stick to the surface or will they roll or slide? If so, how easily? It is the answers to these questions and more that help scientists to develop new surface materials with practical benefits, such as hydrophobic surface finishes.

To measure how easily a drop will move across a surface, scientists often measure “roll-off angle” or “sliding angle.” While these terms sound similar, they technically refer to two different measurements.

The roll-off angle is the angle of inclination of a surface at which a drop rolls off it. It is used to characterize super-hydrophobic surfaces, that is, with very high contact angles (typically greater than 120°.) In this case, a drop of water that falls on the surface will be approximately spherical. How is the measurement made? The sample of the material or coating is placed on the platform of the instrument and positioned exactly level, at 0o. Then, a drop of water is placed on the surface of the sample. The platform, and therefore the surface of the sample, is then slowly

tilted and the angle at which the drop begins to “roll” is recorded as the “roll-off” angle. Lower “roll-off” angle means the drop moves more easily.

When a drop of water lands on a surface that is not super-hydrophobic (typically less than 120°) it does not form such a spherical drop. The drop is flat on the contact surface. So, it doesn’t really roll, it slides across the surface. Therefore, one measures the “sliding” angle. How is the measurement made? As with “roll-off,” the sample to be measured is placed on the platform of the instrument and positioned exactly level, in other words 0o. Then, a drop of water is placed on the surface of the sample. The platform and therefore the surface of the sample is then slowly tilted and the angle at which the drop begins to “slide” is recorded as the “sliding” angle.

When a ceramic coating has a low “sliding” angle it is a good thing. It means water will evacuate the surface faster, taking along contaminants and minerals in the water or on the surface. If the ceramic coating is on the windshield, a lower “sliding” angle means water droplets will blow off the surface easier and faster at lower speeds.

Again, some manufacturers will use the terms “roll-off” and “sliding” angle interchangeably. But technically speaking, when it comes to ceramic coatings for cars, true “roll-off” angle does not apply. No one has a ceramic coating with a high enough contact angle for water to roll as spherical droplets. Our DS1500 Extreme is the most hydrophobic surface coating on the market but water drops still slide (rather than roll) off it. So, when investigating claims of “roll-off” angle, dig a little deeper and make sure the data is sound.

When it comes to “sliding” angle, one needs to be careful about the claims that are being made, as some factors can skew data significantly. It is very important to note that the method is based on gravity, so the volume of the droplet used in the test is critical. The bigger the drop, the

easier the drop will slide. In testing, it is important to use the same drop size if results are to be compared. Real scientists will use a specific and consistent drop size. At DuraSlic, we use 12 µl (microliters) as a drop size. This size can be found in much of our scientific literature on hydrophobic surface coatings, so our measurements have meaning. Without knowing the specific water drop size you cannot compare one measurement with another. A claim of a “sliding” angle without the drop size is essentially meaningless.

So, the takeaway is this; if you see a claim of “roll-off” angle, they probably mean “sliding” angle. If they mean “sliding” angle you cannot take it at face value without knowing the drop size.

With all that in mind, there is also something very, very important to remember.

Sliding angle should not be the sole measure of how good a ceramic coating is. As stated above, a low “sliding” angle is a good thing, as it indicates that water and contaminants will evacuate the surface easily. But it tells you nothing about 1) The durability of the coating; how long and how well the coating will stand up to repeated washing. Remember, a wax can have a low “sliding” angle, but will it be on the car next month? And 2) It does not necessarily tell you anything about the hydrophobicity or oleophobicity of the surface. A surface can have high hydrophobicity (i.e. water repellency) without a low “sliding” angle. In other words, a surface can be highly hydrophobic but sticky.

At DuraSlic we consider all these properties when designing our ceramic coatings. We realize that hydrophobicity, oleophobicity, sliding angle, hardness, flexibility, film thickness and more all play an important role. As with any other coating, properties must be balanced to offer the best overall value. At DuraSlic durability is our goal.

So, is it important to understand the terms, “roll-off” angle and “sliding” angle? The answer is yes. It gives you an important data point in understanding overall performance. But when you look at the number stated on the label or tech data sheet, you may have to dig a little deeper to know if it is meaningful. Ask the question, how was it determined? Can I really compare the number with that of another brand? Have all the necessary variables been disclosed? We hope the information above will give you the background you need to make the best decision when choosing a ceramic coating.