the same and, essentially, just creating more of the interface surface.
Therefore, the tension and the force are kept constant as well. Here
we should also mention that, to be precise, we should consider the
length £ to be double the length of the slider since there are actually
two sides to the film. We will think of this technicality as built into
the definition of surface tension, rather than write 2£.
The surface tension will be uniform, tangent to the surface and
perpendicular to any line drawn in the surface interface. Furthermore,
the inward force which creates surface tension also causes the interface
surface to shrink in area as much as possible. Thus, surface tension
creates a tendency to minimize area. (This is, in fact, equivalent to
energy minimization here.) The units of surface tension are in dynes
per centimeter (force per unit length). The following table [Gar95]
lists some surface tension values for various fluids (where air is the
other interface fluid). (Also see [SZ47].)
Liquid
grain alcohol
glycerine
mercury
olive oil
soap solution
water
water
water
helium
Temp
20
20
20
20
20
0
20
60
-269
Surface Tension I
26.8
63.1
465.0
32.0
25.0
75.6
72.8
66.2
0.12
Soap solution has much lower surface tension than water because
the non-polar hydrocarbon chains inhabit the interface between the
solution and the air. If the soap film is perturbed (by shaking, say),
then the surface area of the interface increases, decreasing the density
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