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I am conducting an experiment in my lab class and I need to examine the behavior of liquids with different properties. I want to isolate either the viscosity or the surface tension, but don't know if it is possible. Are there ways to change mostly just one of these things? Or are there liquids that have very similar viscosity and different surface tension or vice versa?

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As I recall, surface tension can be changed by an additive, such as a detergent. Viscosity probably requires dilution with another liquid.

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regarding surface tension:

Surface tension of water is easily reduced by adding chemicals called surfactants to it. The quantity of surfactant required to get a big change in surface tension is so small that it will not affect the viscosity. Surfactants are the active ingredients in dishwashing detergent liquid, which is the classic method of reducing surface tension for demonstration purposes.

In its pure state, water has a surface tension of about 78 dyne-cm and when fully "doped" with surfactant, it gets down to about 35 which is as far down as you can go. Note that a little surfactant goes a very long way, so you'll need to run some dilution experiments first to find out what range of surfactant doping is needed to hit a certain surface tension target.

You'll also need some means of measuring surface tension. The easiest method is to measure the height of fluid lift in a fine-bore glass tube. The lift height is directly proportional to the surface tension in this case, so you measure the lift in a clean tube with pure water and compare that to the lift height with a doped water sample in a fresh tube. if the lift height is half the pure water value, the surface tension is half that of pure water. This method is highly sensitive to the cleanliness of the tube, so you can't re-use the tubes once they have had doped water in them.

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  • $\begingroup$ yes Dunouy ring tensiometers and critical angle tensiometers cost an unusually high amount. Some people float shaving razors, build paper boats (counting number of pellets added), capillary tube height, how fast it wets and spreads on surface instead among other methods $\endgroup$
    – ChemEng
    Apr 10, 2021 at 17:09
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Surfactants can be added to change the surface tension (even in the ppm region). Polymers as well can affect the viscosity and there is a phenomenon known as polymer surfactant interaction that decreases the concentration required to achieve similar changes in viscosity. Typically both surfactants and polymers decrease the shear viscosity while increasing the elongational viscosity and this allows for increased pumping efficiency as seen in Fanning Friction vs Reynold number plots

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