Page 75 - 4498

wetting, this angle is related to the surface tension of the liquid (σ ), the
L
surface tension of the solid (σ ), and the interphase tension at the solid-
s
liquid boundary (σ ) by the Young equation: cos θ = (σ – σ )/σ . The
L
SL
SL
s
hydrophilic and hydrophobic of surfaces relative to various liquids are
determined by the magnitude of θ. On a hydrophilic surface, a liquid
spreads; that is, there is incomplete (0° < θ < 90°) or complete (θ → 0°)
wetting. Spreading does not occur (θ > 90°) on a hydrophobic surface

(Figure 3.6.1b). For example the drops of water do not wet surface of leaf
(fig.3.6.2)
The contact angle depends on the relationship between the forces
binding the molecules of the liquid to the molecules or atoms of the body

being wetted (adhesion) and the forces
binding the molecules of the liquid to
each other (cohesion). The reversible

work of adhesion and work of
cohesion, respectively, are calculated
from the equations W = σ (I + cos
L
Ad
θ) and W = 2σ . Since θ is always >
Co
L
0° when W < W , wetting will
Ad
Co
improve with an increase in the ratio
W /W . The term S = W /W is
Ad
Co
Ad
Co
Figure 3.6.2 called the spreading coefficient. The
delay often observed in establishing
equilibrium contact angles is called wetting hysteresis. A distinction is
made between kinetic (dynamic) and static hysteresis. Possible causes of
hysteresis include roughness of the surface, structural features of the
surface layer, and relaxation processes in the liquid phase. Selective

wetting occurs when a solid comes into contact simultaneously with two
immiscible liquids. Surfactants are efficient regulators of wetting, being
able to either improve or impede the process.
Wetting has great significance in nature, technology, and daily life.

Good wetting is necessary in dyeing and washing, in processing
photographic materials, and in applying paints and varnishes. Wetting also
figures in the impregnation of fibre materials and in gluing, soldering, and

amalgamation. An attempt to reduce wetting to a minimum is made in
producing hydrophobic coatings and waterproofing materials. In some
cases, for example, in flotation and emulsification using solid emulsifying

agents, the contact angle must be kept within a set range of values.
Wetting plays a vital role in metallurgical processes and in the dispersion

75