Fluids: Liquids and Gases

Pressure in Liquids

Surface Tension


Water molecules have strong electrostatic forces – called van der Waals forces – that create a net attraction between adjacent molecules causing them to stick to each other. Molecules on the surface have stronger cohesiveness, or surface tension, because they are sharing the charge between fewer molecules – resulting in a surface that behaves like a thin elastic film.


Surface tension is the cohesiveness of like molecules at the surface of a liquid. Because of surface tension the water strider insect is able to walk on water.


Left: Image from Hodnett-AP





Cooler water has more cohesiveness than warmer water, so hot water is better for cleaning (the warmer molecules have less of a tendency to stick together).


Surface tension accounts for capillary action (water filling up a thin “capillary” tube), which produces the meniscus, the curve of the surface of a liquid where it touches the container. If the meniscus curves downward the cohesion or attraction of the liquid molecules to each other is greater than their attraction with the container wall. If the meniscus curves up, as for water, the cohesion to the wall is greater than to the other fluid molecules. Mercury has a significantly greater surface tension than water, and its meniscus curves down.

Left: Illustration modified from Wikipedia. 



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