[eazye1334]

Our electroforming department has to measure surface tension on our plating tanks as part of tank maintenance. We typically use a stalagmometer to measure it, but there are some inherent difficulties in using one and the workers have asked me if I can develop a new method.

I don't need the measurement to be incredibly precise, so I'm trying to go with ease of use more than anything. As such, I'm trying to develop a measurement using capillary rise. However I'm not having a ton of luck. I've set up a vessel and have the technicians pull a capillary tube (from a container filled with DI water), shake out the water (most difficult part of the process), drop it into the vessel and then measure the height. Through a calculation, they can get the surface tension. However, my results just aren't matching up well with the stalagmometer.

Has anyone done surface tension in a similar manner with any success? Or is there another reliable method that I'm overlooking that might be easier to set up?

[curiouscat]

I've set up a vessel and have the technicians pull a capillary tube (from a container filled with DI water), shake out the water (most difficult part of the process), drop it into the vessel and then measure the height.

Use oven-dried tubes?

PS. Out of an abundance of caution: Do you get the Surface tension for DI Water right at least?

[curiouscat]

Also, do you know the contact angles accurately a priori?

However, my results just aren't matching up well with the stalagmometer.

Safe to exclude the possibility that hard-to-spell-meter is wrong?

[fledarmus]

Is the capillary systematically wrong or randomly wrong? Since you are developing your own method, if it is systematically wrong, just invoke the appropriate correction factor to make them agree.

[eazye1334]

curiouscat: I minimized any effects from the contact angle by using a fairly small tube as a measurement vessel. It forces the capillary to stand up for the most part. It's hard for me to tell whether the DI water is giving the right result as I'm never totally sure what the bore of the capillary tube is (too small to measure, and naturally VWR is completely unresponsive when I try to ask).

I have to assume the stalagmometer is close to correct as it's the way surface tension has been measured here for decades. I don't necessarily need it to be correct though. Because this is an established operation, we know how much wetter to add when we get a result from the stalaglmometer; we don't need an absolute answer, just a relative answer. The only reason I'm doing this is to try and make it easier for the technicians to operate. With the stalagmometer, they have problems with bubbles getting in the tube, which forces a complete restart. To do the measurement they also have to count drops of plating solution, anywhere between 50-80 drops each test which, considering this is a production facility with a lot going on, can be pretty easy to lose count of. This system I've come up with just requires them to drop in a capillary tube and read a measurement, so it's much simpler. I just can't get the two results to be consistently close together.

fledarmus: Unfortunately, it appears it is more random than systematic. About 1 out of every 3 measurements is significatly off; the other 2 are at least reasonably close or often dead-on. I don't think I can really come up with a correction factor that would rectify the number problems. Granted, testing so far is limited because we only test surface tension once a week for each tank so a systematic error could present itself I suppose, but I'm not exactly confident that will happen.

[curiouscat]

Surface tension is notoriously sensitive to tube cleanliness. That's why my suggestion you dry tubes in an oven. The wash-in-DI and use might be contaminating tube surfaces.

[curiouscat]

Through a calculation, they can get the surface tension.

Out of curiosity, can you show the calculation?

[eazye1334]

I'm using γ = (ρgrh)/2

ρ = density (g/cm³)
g = accel. due to gravity (981 cm/s²)
r = capillary bore (cm)
h = height of solution in capillary (cm)

I got it from here.

As for drying the tubes, I could wash them all myself first and then dry them I suppose. However, if I have to store them in DI and then dry them each time, it's an extra step that would make the process essentially unusable for us.

[curiouscat]

What are you using for r? I think it is extremely hard to get accurate and consistent diameter cappilaries.

A better way may be to always take a ratio against a standard fluid to eliminate r from the equation entirely.