268
8 Porous Nanostructured Materials
In Eq. 8.1, f i means the surface area fraction of the component i (defined as f i
= A i /A i where f 1 + f 2 = 1 is obvious for a two-component substrate) and i
is the corresponding wetting angle of the liquid on this component. If one of the
components is a gas, the contact angle has to be taken as 180°, and the thus obtained
Cassie–Baxter angle ( CB ) is obtained as [65]
cos CB = f 1 cos 1 − f 2 = f 1 (cos 1 + 1) − 1.
(8.2)
As CB increases, the sliding angle (i.e., the tilting angle of the substrate at which
the droplet rolls down along the hydrophobic surface) decreases. The Cassie–Baxter
angle and the sliding angle are larger than 150° and lower than 5°, respectively, for
superhydrophobic surfaces.
The hydrophobic character of a surface with fractal-like hierarchical porosity is
related to a number of other properties. Among others, anti-icing surface properties
stem from the same surface morphology as the hydrophobic character. The selfcleaning nature of a surface can also be due to the high porosity when dust particles
stay at the top part of the surface and their penetration between the dendrite- or
column-like surface elements is hindered; therefore, they can be washed away from
the surface without the application of either any tenside or a mechanical treatment.
The small corrosion rate of surfaces with morphology-related hydrophobic character is also related to the small wetted area. The reduced inclination to corrosion is
explained with the small effective contact area between the corrosive liquid and the
porous surface, as shown in Fig. 8.3c. The corrosion aspects of the superhydrophobicity have been discussed in detail in various topical reviews [66, 67]. However,
the above described wetting model implies that the wear or corrosion damage of
the surface may lead to an irreversible degradation when the fractal-like surface
morphology is destroyed.
The literature of the electrodeposition of superhydrophobic coatings has been
nicely reviewed recently by Tan, Palumbo and Erb [68]. Here, their categorization
is followed partly, except for suspension plating systems that were discussed in the
previous chapter. Beside the works to be listed below, it is worthwhile of noting
that several works cited in Sect. 8.1 mention the superhydrophobic nature of DHBTplated samples. The morphological origin of the superhydrophobicity is the same for
both sample groups.
8.2.2 Electroplated Metallic Coatings with Hydrophobic
Properties
As it can be seen from Fig. 8.3, the surface morphology of the roughness-related
superhydrophobic coatings is completely the opposite as that for smooth deposits.
This offers some guidelines for the working regime in connection with the Winand
diagram (see Fig. 2.19). On the one hand, highly dendritic coatings can be obtained
8 Porous Nanostructured Materials
In Eq. 8.1, f i means the surface area fraction of the component i (defined as f i
= A i /A i where f 1 + f 2 = 1 is obvious for a two-component substrate) and i
is the corresponding wetting angle of the liquid on this component. If one of the
components is a gas, the contact angle has to be taken as 180°, and the thus obtained
Cassie–Baxter angle ( CB ) is obtained as [65]
cos CB = f 1 cos 1 − f 2 = f 1 (cos 1 + 1) − 1.
(8.2)
As CB increases, the sliding angle (i.e., the tilting angle of the substrate at which
the droplet rolls down along the hydrophobic surface) decreases. The Cassie–Baxter
angle and the sliding angle are larger than 150° and lower than 5°, respectively, for
superhydrophobic surfaces.
The hydrophobic character of a surface with fractal-like hierarchical porosity is
related to a number of other properties. Among others, anti-icing surface properties
stem from the same surface morphology as the hydrophobic character. The selfcleaning nature of a surface can also be due to the high porosity when dust particles
stay at the top part of the surface and their penetration between the dendrite- or
column-like surface elements is hindered; therefore, they can be washed away from
the surface without the application of either any tenside or a mechanical treatment.
The small corrosion rate of surfaces with morphology-related hydrophobic character is also related to the small wetted area. The reduced inclination to corrosion is
explained with the small effective contact area between the corrosive liquid and the
porous surface, as shown in Fig. 8.3c. The corrosion aspects of the superhydrophobicity have been discussed in detail in various topical reviews [66, 67]. However,
the above described wetting model implies that the wear or corrosion damage of
the surface may lead to an irreversible degradation when the fractal-like surface
morphology is destroyed.
The literature of the electrodeposition of superhydrophobic coatings has been
nicely reviewed recently by Tan, Palumbo and Erb [68]. Here, their categorization
is followed partly, except for suspension plating systems that were discussed in the
previous chapter. Beside the works to be listed below, it is worthwhile of noting
that several works cited in Sect. 8.1 mention the superhydrophobic nature of DHBTplated samples. The morphological origin of the superhydrophobicity is the same for
both sample groups.
8.2.2 Electroplated Metallic Coatings with Hydrophobic
Properties
As it can be seen from Fig. 8.3, the surface morphology of the roughness-related
superhydrophobic coatings is completely the opposite as that for smooth deposits.
This offers some guidelines for the working regime in connection with the Winand
diagram (see Fig. 2.19). On the one hand, highly dendritic coatings can be obtained