We’ve seen that if an object is less dense than water, it will float partially submerged.
But a paper clip can rest atop a water surface even though its density is
several times that of water. This is an example of surface tension: The surface
of the liquid behaves like a membrane under tension . Surface tension
arises because the molecules of the liquid exert attractive forces on each
other. There is zero net force on a molecule within the interior of the liquid, but a
surface molecule is drawn into the interior (Fig. 12.15). Thus the liquid tends to
minimize its surface area, just as a stretched membrane does.
Surface tension explains why raindrops are spherical (not teardrop-shaped):
A sphere has a smaller surface area for its volume than any other shape. It also
explains why hot, soapy water is used for washing. To wash clothing thoroughly,
water must be forced through the tiny spaces between the fibers (Fig. 12.16). This
requires increasing the surface area of the water, which is difficult to achieve
because of surface tension. The job is made easier by increasing the temperature
of the water and adding soap, both of which decrease the surface tension.
Surface tension is important for a millimeter-sized water drop, which has a
relatively large surface area for its volume. (A sphere of radius r has surface area 4pr2 and volume (4p/3)r3
. The ratio of surface area to volume is 3/r, which
increases with decreasing radius.) But for large quantities of liquid, the ratio of
surface area to volume is relatively small, and surface tension is negligible compared
to pressure forces. For the remainder of this chapter, we’ll consider only
fluids in bulk and ignore the effects of surface tension.