Page 106 - Handbook of Surface Improvement and Modification

7.1 Methods and mechanisms of surface tension change 101

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snow sticking surfaces, and so on. The
wettability of films subjected to different
temperatures varied from hydrophobicity
to superhydrophobicity due to the increase
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of surface roughness. If films were stored
o
at subzero temperature (-15 C), their water
contact angle decreased relative to the cor-
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responding roughness. Returning films to
room temperature for 30 min. resulted in
the water contact angle returning to the
value prior to the cold storage, which indi-
cated the reversible switching of surface
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wettability (Figure 7.5). This process can
Figure 7.5. Reversible wettability transition of the PE be repeated several times. The magnitude
films (prepared at different temperatures: square, -15; of variation of contact angle depends on the
o
circle, 25; triangle, 180 C) under different storing
o
temperatures: (a) PE film storing at -15 C; (b) PE film temperature of its preparation (the lower
kept at room temperature. [Adapted, by permission, the temperature the larger the variation. 11
from Hou, W; Zhang, L; Long, Y, J. Colloid Interface The rough structure on superhydrophobic
Sci., 362, 629-32, 2011.]
surfaces led to the decrease of water con-
tact angle at subzero temperature, which
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further increased the ice-solid contact area and the ice adhesion.
Template synthesis of raspberry-like polystyrene/silica composite microspheres was
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used to develop material with wettability surface gradient. The microspheres were
obtained by sol-gel processing of tetraethoxysilane on carboxyl functionalized polystyrene
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template particles. The suspension was coated on a glass substrate and subjected to the
thermal gradient field which resulted with surface having wettability gradient from hydro-

Figure 7.6. (a) Synthesis of composite microspheres and (b) fabrication of wettability gradient surfaces.
[Adapted, by permission, from Fan, X; Zheng, L; Cheng, J; Xu, S; Wen, X; Cai, Z; Pi, P; Yang, Z, Surf. Coat.
Technol., 213, 90-7, 2012.]