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droplet was determined in a chemical homogeneous and ideal flat
surface by Young's model (Fig. 5.2). In this model, contact angles
of a liquid droplet on both sides are equal and depend only on the
surface energy in solid-liquid (SL), solid-air (SG), and liquid-air
(LG) interfaces, respectively [7].
Figure 5.2
Young's model.
It has been proved that by modifying the surface chemical
composition of a flat surface, and thus reducing the surface energy,
the contact angle of water on such flat surfaces can typically be
elevated to the order of 100 to 120
°
[8-11]. A value as high as
160 to 170
could be reached if the surfaces are rough [12,13] or
microtextured [10,11]. To precisely predict the contact angle on a
rough surface, the effect of roughness on the solid-liquid interface
should be taken in account. From the Young's equation, two distinct
hypotheses were classically proposed to explain this effect: (i) the
Wenzel's model and (ii) the Cassie-Baxter's model [14-16].
The Wenzel's model assumes that the liquid completely fills the
grooves of the rough surface in contact (Fig. 5.3a). The situation is
described by Wenzel's equation. According to the Wenzel's equation
it is clear that, roughness will cause the contact angle to increase
only if the contact angle is higher than 90
°
. Oppositely, if the contact
angle of a liquid on a smooth surface is less than 90
°
, the contact
angle on a rough surface will be smaller (hydrophilic). Thus, the
effect of roughness can be divided into two regimes: (i) for
°
q
< 90
°
,
.
Based on the Wenzel's model, Cassie and Baxter further
developed and revised the Young's equation. They supposed that the
solid rough surface should be regarded as a mixed-phase interface
between solid and gas. Due to an increase in the height-to-area ratio,
air gets trapped between the liquid and the solid and thus, reducing
the adhesion forces (Fig. 5.3b). In this case, the relationship between
q
<
q
and (ii) for
q
> 90
°
,
q
>
q
W
W
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