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|Six things your sessile drop would tell you - if it could only speak|
If you spend any significant time measuring the contact angle of sessile liquid drops on a solid surface, there's a good chance your sessile drops would like to tell you a few things...if you would only listen. Here's the top six things those drops would say - if they could speak:
1. "Watch me, I'm spreading." There's a certain temptation to take a single static water contact measurement, call it quits, and move on to a new drop or another surface. Amazingly, many drops - depending on the solid and liquid - will wet out over time. This behavior, dynamic wetting, is an important observation to make. Recently, we discovered a surface while doing some lab work that exhibited dynamic wetting - that is, over time the contact angle decreased while the drop width increased. It turned out there was a residue left the surface which, acting like a surfactant, slowly lowered the surface tension of the sessile drop. The cohesive forces between the water molecules in the drop weakened resulting in increased wetting behavior. After cleaning the surfaces thoroughly, we observed a reduction in dynamic wetting.
2. "Help, I'm disappearing!" Sometimes your contact angle will drop over time but the drop width remains static, another reason to take a dynamic contact angle measurement. In this case, either the drop is evaporating or it's slowly being absorbed into the surface. Sometimes both. To figure out how much of each, measure the change in volume on a similar surface using the same volume. For example, if you start with a 5µL sessile drop on your test surface and then observe that it becomes 2µL over the course of five minutes, you know that you've lost 3µL of drop volume in that amount of time. Now find a reference solid with about the same initial contact angle. If the test surface is hydrophobic, let's suppose you use PTFE for as the reference solid and with the same 5µL initial sessile drop you observe that the volume reduces to only 4µL over the same five minutes, a loss of 1µL. Thus, you can conclude that on your test surface, approximately 1µL is lost due to evaporation while twice that amount is being absorbed into the surface.
3. "Man, I'm on a rough spot." Literally. Rough spots can present extra challenges when characterizing wetting properties. Roughness amplifies wetting behavior: on hydrophilic surfaces, an increase in surface roughness will decrease the contact angle while additional roughness on a hydrophobic surface will result in an increase in contact angle.1 Advancing and receding contact angles are used to capture the contact angle hysteresis, or the difference between the highest and lowest possible metastable contact angles. As roughness increases, so too does the contact angle hysteresis.
4. "Your sloppiness has contaminated me. Now I can't do my job right." The surface tension of water is very sensitive to contaminants. Even minute traces of contaminants in the test liquid can affect the contact angle. To obtain accurate results, be sure to clean your syringe and needle or use a fresh dispensing tip if you are using the Automated Dispensing System. Ensure that our source liquid is pure and uncontaminated as well. If you're not sure, measure its surface tension (with DROPimage Advanced). It should be about 72 mN/m.
5. "Man, you made me way too small." There seems to be an obsession with picoliter and smaller-than-average drops. Except for a few rather uncommon applications (like inkjet printing), picoliter drops introduce nothing but problems. They evaporate very quickly and they are more difficult to capture optically. Unless the surface is perfectly flat, picoliter drops can easily get lost in the topology of the surface. In short, unless you have a specific application that warrants it, you should work with microliter size drops, for example in the 1 to 5µL range.
6. "You may need more than one of me." It would be nice to be able to take only one measurement per solid/liquid combination. However, since there is some variability in both the chemical composition as well as the surface topology, it pays to take a few measurements at different locations for each solid/liquid pair. If the range is greater than expected, take even more in order to obtain a more meaningful average contact angle.
The contact angle is a
function of the cohesive forces between the water molecules and the
adhesive forces between the surface and the water. When the cohesive
forces of the liquid are greater than the adhesive forces at the
surface, the result is a larger contact angle. Likewise, when
adhesive forces (or surface energy) is greater, the contact angle is
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