文档介绍：2. Interface Tension and Contact Angle
Wherever liquids occur, the effects of surface tension are obvious. The theory
of surface tension, however, is still in a phenomenological state. Several mi-
croscopic arguments, in particular those based on van der Waals attraction,
are available. They yield qualitative results at best.
This situation es even worse if dynamic effects are considered, .
the creation of fresh surfaces during surface oscillations, and the motion of
a solid/liquid contact line. An advancing contact angle is steeper than the
stationary contact angle, a receding one is flatter. The main aspects of these
dynamic effects are discussed here.
Molecular Attraction and Condensation
Water drops which fall from a tap or which fall from the sky as raindrops
generally assume a spherical shape. They may oscillate owing to the preceding
formation process and may adopt a slightly oval form owing to atmospheric
resistance. The primary cause of their spherical shape is molecular attraction.
And, from the fact that a spherical shape exhibits the minimum surface area
for a given liquid volume, it may be concluded that molecular attraction
is isotropic and a surface energy exists. A further repeatedly cited effect of
surface tension is the fact that many insects are able to walk on a water
surface.
There is an attractive force not only between water molecules, but also
between any two molecules of matter. Molecules may lower their joint poten-
tial energy by contracting into a sphere. The first quantitative description of
this attraction between molecules was given by van der Waals [1881], when
he formulated his equation of state of real gases. He found that the behavior
of these gases can be accurately described by introducing into the equation
of state for ideal gases
pv = RBT ()
• a minimum volume b, which can be interpreted as the volume occupied by
the close-packed molecules, and
Dieter Langbein: Capillary Surfaces, STMP 178,