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June 2011 

Static Contact Angle Is Not Always Enough  
A customer sends us two samples with a request for us to measure static contact angle on them. We take the measurements and send them the results. The average contact angle for Sample A was 82.3°. For Sample B, the average contact angle came in at 82.8°. The customer calls us back to discuss the results. They had concluded, based on our static contact angle measurements, that the wetting properties were virtually the same for A and B and thus the surfaces are comparable. I urged them to hold off on their conclusion and offered to take some additional measurements for them. They agreed. So, using one of our Model 290 instruments with an included Automated Tilting Base, we conducted a tilt study and determined that for Sample A, the drop would not ever roll off, even at a full 90° tilt. At the 90° tilt, the average advancing contact angle measured 84.4° and the average receding contact angle measured 72.7°. The contact angle hysteresis measures 11.7°. With Sample B, however, even though the static contact angle was nearly the same, the average rolloff angle was 27.3° and with a contact angle hysteresis of only 4.2°. We emailed the additional results to the customer and, as expected, they called to discuss them. How could the static contact angle be virtually the same for both samples, but the rolloff angles be so different? They asked for us to explain this phenomenon. So, this is what we told them: The expectation is that as the contact angle increases, the rolloff angle decreases. It seems logical that a drop on a hydrophobic surface with a high mass to interfacial area ratio is more likely to roll off than a low contact angle drop with a very low mass to interfacial area ratio. And, in fact, this is often the case. But not always. We've seen drops on hydrophobic ﬂuoropolymer surfaces which produce water contact angles in excess of 115° stay pinned at a full 90° tilt and we've seen other drops on polydimethylsiloxane surfaces with contact angles in the 90's roll off at a tilt of less than 50°. We explain that contact angle generally describes wetting behavior and surface energy while rolloff angle generally describes the adhesive properties between the liquid drop and the solid substrate  two different but interdependent sets of forces. We further explain that adhesion between the drop and the solid relates primarily to the interfacial area  where the drop is in contact with the solid. We explain that wetting relates primarily to what's happening at the threephase line  that is, the perimeter of the drop at the solid. We then explain how the rolloff angle is more sensitive to drop volume than contact angle is. Suppose that we have a 5µL drop on a surface which produces a 90° contact angle. Geometrically this is easy to calculate as the drop will be very close to a half a sphere. If we solve for the area at the liquidsolid interface (the base of the half sphere), it's 2.72mm^{2}. Now, let's suppose we double the drop volume to 10µL and let's suppose as expected, the contact angle persists at 90°. Now we solve for the area again and we note that it increases by 57.7% to 4.29mm^{2}. So while the volume (and mass) increases by 100%, the area that's holding it to the surface is increasing by a bit more than half that amount. This explains one of the reasons why larger drops roll off sooner smaller ones. However, in fairness, we have to disclose that our theoretical calculation is based on a perfect circular area. In reality, once tilt is applied, the threephase line that defines the interfacial area changes from a circle to an ellipse and increases in area thus minimizing the perceived increase in mass to interfacial area ratio. (It would be worthwhile to study the impact drop volume has on rolloff angle  and perhaps we'll do that for a future newsletter. For now, we'll just say as a general rule that bigger drops roll off easier than small ones.) Finally, we note that while contact angle explains wetting and surface energy, other surface characteristics (such as roughness, porosity, and nanoscale topology) can be characterized by looking at a variety of surface energetics for a given liquidsolid system which can be captured by the rolloff angle, advancing and receding contact angles, and other dynamic studies. One final observation, even with superhydrophobic surfaces, researchers^{1} are developing methods using plasma processing for creating surfaces which, unlike conventional superhydrophobic surfaces with low rolloff angles, exhibit a "sticky" property with high rolloff angles. ^{1} Langmuir, 2008,
24 (9), pp 4785–4790 

How to Level the Specimen Stage  
One of the most important parts of measuring contact angle is ensuring that the specimen stage is level prior to taking measurements. The task of leveling the stage is the same for all of our instruments. The best way to level the stage, however, requires the use of the baseline setup tool which is part of every edition of our DROPimage software. This month's new video walks through the steps involved in leveling the specimen stage  not only left to right but also front to back. Click the video above to watch or point your browser to: http://youtu.be/scIFT2c8pg 


