Ask A Scientist Physics Archive

name         Jewel B.
age          15

Question -   I am doing a project on the surface tension of water, but
I do not know how to test it. So I was wondering if you could give me
some ideas on how to easily test surface tension. Thanks.
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There are several laboratory instruments for measuring surface tension, but
I do not know of one that you can put together easily "from scratch".

Vince Calder
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There are numerous pretty ways to study the properties of surface
tension.  One way I often tried as a young boy was to float a needle.
It is quite easily done by laying a piece of tissue paper gently on
top of the water in a glass and gently laying a needle on the paper.
Then, with another needle carefully push the paper down into the
water. You will often leave the needle floating on the water.  It is
obviously held up by surface tension since needles are made of steel
which is almost 8 times as dense as water.

You can also blow a soap bubble using a pipe or just a soda straw.
Notice that after you blow a bubble, it tries to contract if you let
the air escape.  This is because the surface tension of the water
creates a pressure inside the bubble which is 2s/r greater that the
pressure outside.  Here s is the surface tension (for water, about
0.07 N/m) and r is the radius of the bubble.  (The 2 is there because
a soap bubble has two surfaces: the inside and the outside).

For a soap bubble with a radius of 1 cm, the overpressure is 14 N/m2,
which is quite small compared with atmospheric pressure which is about
100,000 N/m2.  You might observe it by holding a small strip of tissue
paper in front of the soda straw (or bubble pipe) to observe the air
being blown out.

You might be able to measure surface tension rather directly by
putting a thin needle under water, just at the surface, and pulling it
out with a thread tied about the middle of the needle so it stays
parallel to the surface of the water.  The force needed to pull it out
will be 2sd, where d is the length of the needle.  For a 5 cm long
needle, this amounts to about 0.007 N = 0.002 lb = 0.03 oz.  You might
try to make a sensitive balance with a soda straw balanced on a needle
through its middle with the thread to the wet needle attached to one
end and weights (pins?) put in the straw at the other end.

Similar effects can be seen when a thin glass tube is put halfway into
water.  The water climbs up the walls of the tube because the water
molecules are attracted to the glass molecules more strongly that to
other water molecules.  Mercury shows the opposite effect and the
mercury level is depressed inside a thin glass tube.  You might have a
mercury thermometer where you can observe this effect.

Good luck!

Best, Dick Plano...
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