Thin film coatings surface hydrophobicity

Eastman and Zhn [52] performed a study of SisN4 tips, subsequently coated with gold and finally with paraffin thin films, adhering to mica. The study was purely qualitative, because they did not have accnrate valnes for either their lever stiffnesses or their tip radii. By enclosing their AEM in a sealed container and controlling the humidity they found that hydrophobic tips show a lower adhesion with a surface expected to have a thin water film, as also observed by Jarvis and Pethica [51]. Surprisingly they did not see any dependence of adhesion on hnmidity for any of their tips. 

The surface forces apparatus (SEA) can measure the interaction forces between two surfaces through a liquid [10,11]. The SEA consists of two curved, molecularly smooth mica surfaces made from sheets with a thickness of a few micrometers. These sheets are glued to quartz cylindrical lenses ( 10-mm radius of curvature) and mounted with then-axes perpendicular to each other. The distance is measured by a Fabry-Perot optical technique using multiple beam interference fringes. The distance resolution is 1-2 A and the force sensitivity is about 10 nN. With the SEA many fundamental interactions between surfaces in aqueous solutions and nonaqueous liquids have been identified and quantified. These include the van der Waals and electrostatic double-layer forces, oscillatory forces, repulsive hydration forces, attractive hydrophobic forces, steric interactions involving polymeric systems, and capillary and adhesion forces. Although cleaved mica is the most commonly used substrate material in the SEA, it can also be coated with thin films of materials with different chemical and physical properties [12]. 

Cross-linking of thin-film PE can also be induced by excimer UV lamp irradiation, for coating purposes, to change a hydrophilic surface into a hydrophobic one. With this method a PE film can be bonded to another material without adhesive. 

Figure 3. Comparison of three different thin films as regards their surface topographies and wetting behaviors. The AFM section shows the roughness parameters Umax and rms. The SEM section shows the surface topography with a magnification of 75 000. CA-m stands for contact angle measurement and shows water droplets on the surfaces after hydrophobic coating. The values in hlack are the CA values related to the picture, and the values in light gray descrihe the CA values for non-coated surfaces.

Here, we employed polymethacrylates to provide the roughened and oxidized surface of aluminium sheets with superhydrophobic properties. Polymethacrylates can be easily synthesized and their properties varied by copolymerization of methacrylate monomers that have different side chains. The correlation between the structural composition of polymethacrylates and their wetting behavior is well known from model studies carried out on thin films on smooth surfaces [19, 20], but there is no information about the wetting behavior of polymethacrylate hlms on micro-rough surfaces. We have synthesized poly(tert-butyl methacrylate) and poly(methyl methacrylate) containing different hydrophobic and hydrophilic sequences. In dependence on the polymer composition the wetting behavior was studied on polymer-coated smooth silicon wafers and rough aluminium surfaces. 

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