Like Watching Paint Dry: Day-to-Day Surface Tension

By Finn Almandmoss

Surface Tension Research is painful sometimes. It can be slow, inaccurate, give you incorrect data for seemingly no reason at all, and did I mention slow?

Surface tension can be defined and measured in a couple of ways. It arises from the cohesive attractive forces in liquids at the molecular level. Water has a comparably high surface tension, as its polar frame and composition allows for strong hydrogen bonding interactions to occur. A substance like octane has a much lower surface tension, as its only intermolecular forces result from temporary dipoles characterized as the London Dispersion effect.

This inward force results in liquid solutions minimizing their surface areas better with higher surface tension. Liquids with higher surface tension are better able to “pull themselves together” and form more spherical droplets than those with lower surface tension, forming more elongated, traditional droplet shapes.
So, that’s all well and good, but why do we care? Well, several reasons. For one, capillary action, the process of a liquid flowing in a narrow area without external forces such as gravity, is a direct result of what we know as surface tension. You can see this effect in your day-to-day life in your Wendy’s soda, as after you take a sip of your preferred soda you can see the level of liquid in your straw begin to move.

How do we measure it then? Well, it begins by selling your soul…never mind. There are a couple of methods, but to measure liquid-liquid interfaces (the surface tension of a liquid surrounded by a liquid) we can use the pendant drop method. This method revolves around using a needle to hang a droplet of your liquid of choice suspended in front of a camera, fitting a polynomial curve to it, and using that to calculate the surface tension based on the size of the drop.

However, this can have several issues, and here’s where the “fun” begins. The camera needs to be calibrated with a sphere of known diameter to be accurate, and depending on the software you use, your computer can really suck at fitting a circle to the sphere. Light reflecting off of or coming from various sources in the surrounding area can mess with your data and cause it to report incorrect results. Your software can decide to take off for a day and not fit the curve even remotely close to the side of the droplet, resulting in some obscenely wrong data. Even if everything goes right so far, your software can also report a certain amount of uncertainty or error, so even if the fit is correct, your value can still be way off.

But even now after we’ve talked about all of that, we haven’t even mentioned the worst parts yet. The droplets coming out of the needle can form on the sides of a cuvette you’re using, the side of the needle itself, or even refuse to hang onto the needle long enough to take a measurement. Any fingerprints or oils on the cuvette itself will nullify a correctly set up measurement. Worst of all, sometimes it simply just…doesn’t work. For no explainable reason. Super fun when that happens.

Assuming everything I’ve just mentioned goes to plan, and every measurement works correctly, this process can still be incredibly painful. The drop must stop moving (even really small vibrations) for the measurement to be accurate, which results in a minimum of 5-10 minutes of waiting even after forming the drop. Any vibrations or other disruptions in the lab such as someone vortexing a sample or even closing a cabinet door can cause the drop to move around.

So, is it worth three hours of your time to obtain six useful data points? You tell me…