Surface tension, γ
A molecule in the bulk of a liquid experiences
an equally attractive force in all
directions, but at the surface there is more
attractive force in the direction of the bulk
than away from it. This reduced attraction
at the liquid-gas boundary gives rise to the
measurable property of surface tension.
At the liquid-liquid phase boundary, it is
referred to as interfacial tension. Surface
tension can be measured with optical,
force, volumetric or bubble tensiometers,
while interfacial tension can be measured
with force and optical tensiometers.
Contact angle, θ
Surface tension alone does not explain
the shape of a drop along a solid. Interaction
between the liquid and solid can be
attractive or repulsive. When attracted
by the solid, the liquid spreads, forming
short drops with low contact angles. If
repelled, the drops are taller, rounder and
have high contact angles. The lotus effect
is a special case of water repelling, or
hydrophobic, behavior. The microstructuring
of a leaf makes the surface superhydrophic,
causing a rain droplet to roll
off by itself.
Surface free energy
Surface free energy is surface tension for
solids. Like in liquids, surface molecules in
solids have more attractive forces towards
the bulk than away from it. This leaves the
surface molecules inclined to attract neighbouring
substances, so the higher the
surface free energy the easier it is to wet a
substrate. In contrast to liquids, the surface
free energy of solids cannot be measured
with tensiometry but via contact angle
measurements with a range of liquids.
