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|How to Improve Surface Tension Measurements
|If you use a ramé-hart instrument
to measure surface tension, we'd like to provide ten tips for improving results and ensuring accuracy.
1. Make sure your instrument is properly calibrated. This is more important than many realize. All distances reference back to the calibration device used during calibration. If the calibration was performed incorrectly, an invalid calibration will translate to inaccurate surface tension results. Check out the video below to review the correct steps for calibrating. Note that the Aspect Ratio should be very close to 1.0000.
2. Lighting is important. If you are measuring surface tension on a pendant drop, the drop should be solid black and the background should be solid white.
3. Focus is also important. Before dispensing, focus on the needle. Since a pendant drop is inline with the needle, no additional focus should be necessary after dispensing sufficient volume to form a pendant drop.
4. Use the correct needle size. We recommend 22 gauge for water and aqueous liquids. Liquids with a lower viscosity will require a higher gauge (smaller diameter) needle. Likewise, thick liquids may require lower gauge (larger diameter) needles and slower dispensing. Note that there is no correlation between surface tension and viscosity.1
5. Drop volume is important. If the drop is too small, it's not possible to accurately capture the effects of gravity on the drop - which is how surface tension is measured using the Young-Laplace equation. If the drop is too big, it may fall off during your experiment - especially if you are taking multiple measurements over time. Here's a method we use successfully: produce a pendant drop adding volume slowly until the drop releases. Note the volume at the time of release and then repeat the drop using 80-90% of that maximum volume.
6. Make sure you have the red line and right line options turned off (in the Settings Menu of the Contact Angle Tool).
7. Ensure that your syringe, needle, or tip are all clean. Even the slightest amount of contamination can adversely affect surface tension measurements.
8. Ensure that your liquid is pure and unadulterated. Even a small amount of impurity can lower the surface tension of water.
9. Surface tension is temperature sensitive. Ensure that you report the test temperature when you report your surface tension value. A surface tension value without a temperature is not so meaningful.
10. The donut hole. In some cases,
a pendant drop may act as a lens producing a white hole in the
middle of your pendant drop. If this happens, simply pass your
vertical line to the left or right of the donut hole prior to
clicking on "measure".
|Product of the Month: Dispensing Needles
Tip number four in the section above makes reference to using the correct needle size. We keep every size needle in stock from 5g all the way up to 33g. For our Straight 304 Stainless Steel Reusable Dispensing Needle see this link. For our Inverted Needle, available in gauges 10 to 28, see this link. For messy liquids that hard to clean up after, we offer disposable needles as well. If you prefer a hydrophobic needle, we offer solid PTFE needles in sizes from 14 to 22 gauge. If you're not sure what size to use, consider our Needle Assortment which includes all even gauges from 14 to 30 gauge. If you wish to use needles with our Automated Dispensing System, you'll need a fixture adapter p/n 100-22-37. Lastly, we offer a large array of coaxial and triaxial needles, both prebuilt and custom-made configurations. See our website for spinnerets here: https://www.customspinnerets.com/