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ramé-hart Newsletter

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July 2018

  A Dozen Fun Facts about Surface Tension
  We love to measure surface tension. It's something we do well. If you find it interesting too, you may enjoy these fun facts about surface tension.

1. While the surface tension of water at 72.8 mN/m is higher than just about any other liquid, it's not nearly as high as the surface tension of Mercury which measures an astonishing 438.4 mN/m (on a chilly day).

2. Surface tension cannot be negative. If the surface of a drop were more energetic than the bulk liquid, the surface could not form.

3. As temperature increases, surface tension decreases.1 Surface tension reaches zero when the critical temperature (aka, the critical point) is reached.

4. Bubbles are easier to make as surface tension decreases. Think about it? Is it easier to make bubbles with pure water or soapy water? Soap is a surfactant which lowers the surface tension of water.

5. Most people think raindrops are shaped like teardrops. That's a myth. Raindrops start out spherical, due to surface tension, but as they fall they take on the shape of a hamburger bun.2 The same is true for water drops from a faucet, gutter, or anywhere else.

Raindrops are not teardrop shaped. They start as spheres and change to a flat-bottomed shape as their size increases.

6. Adding just about anything to water will lower its surface tension - except salt (and other soluble impurities). This explains why seawater has a higher surface tension than water. Since 97% of the water on the planet is salty, this is good to know - especially if you are a oceanographer. If a substance lowers surface tension, it's a surfactant; if it increases surface tension (like ionic salt) it's tensoionic.

7. Many insects rely on surface tension. For example, the water strider can walk on water due to its high surface tension. This also explains how you can get a coin or a needle to "float" on water. Add soap to the pond, and the water strider will sink.

Water Strider "walking" on water with the help of surface tension

8. It may seem intuitive that as viscosity increases, so too does surface tension. However, this is not the case. There appears to be no correlation between viscosity and surface tension.3

9. Gravity does not affect surface tension. Water has the same surface tension in space as it does on earth, for example. However, most of the methods we use to measure surface tension (e.g., pendant drop, sessile drop, Wilhelmy Plate, Du Noüy Ring, drop volume) will not work without gravity.

10. Surface tension is a force and is measured in mN/m. Note that 1000 mN/m is equal to 1 N/m. Note also that that 1 mN/m = 1 dyne/cm = 1 erg/cm2 = 1 mJ/m2.

11. Using the pendant drop method, the drop is a liquid while the external phase is a gas. If a phase is a solid, the term surface free energy, or simply surface energy is used. If the drop phase and the external phase are two immiscible liquids, then interfacial tension is being measured. Replace the external phase with a gas, and now you're back to measuring surface tension.

12. Hot soup tastes better than cold soup because the heat lowers the surface tension allowing the soup to cover a larger area of your tongue. Surface tension depends on the liquid's intermolecular forces. Temperature and impurities, as noted above, affect the surface tension.

1 More on this behavior in our July 2013 Newsletter.
2 See this article: https://pmm.nasa.gov/education/articles/shape-of-a-raindrop
3 More on this topic in our March 2014 Newsletter.

  Tools for Measuring Surface Tension

ramé-hart has been around since 1961. That's before I was born, by the way. We were the first company in the world to mass produce a contact angle goniometer and for many years we manufactured the original Model 100-00 NRL Contact Angle Goniometer.1 Thousands of these tools are still in use today.

In the 1990s we began the migration from optical microscope-based tools to camera-driven software-controlled instruments. This made it much easier to add surface tension capabilities to our products. Today six of the nine models we offer include surface tension and interfacial tension capabilities.

If you are looking for a professional lab-quality instrument to measure surface tension, please visit our website www.ramehart.com and contact us today for a quotation and more information. 

1 For more on our history, see our December 2007 Newsletter.


Carl Clegg
Director of Sales
Phone 973-448-0305
Contact us



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