Micro-roughness

A silicon surface, no mater how well it is prepared, is not perfectly flat at the atomic scale, but has surface defects such as surface vacancies, steps, kinks sites, and dopant atoms. The dissolution of the surface is thus not uniform but modulated at the atomic scale with higher rates at the defects and depressed sites. The micro roughness of the surface will increase with the amount of dissolution due to the sensitivity of the reactions to surface curvature associated with the micro depressed sites. These sites, due to the higher dissolution rates, will evolve into pits and eventually into pores. Depending on the condition, a certain amount of dissolution is required before the initiation of pores on all types of materials. 

Only a perfectly smooth surface exhibits directed (specular) reflection, i.e., where the angle of reflectance equals the angle of incidence. This, however, is not the case under real-life conditions, where the surface micro-roughness cannot be avoided, and consequently, the UV can be slightly diffused over a finite range of reflection angles. 

The impression of a matte film surface is created by the diffuse reflection of light from a micro-rough surface. Such a structure is obtained by... 

The deposition velocities to grass of particles with diameters between 1 and 5 jum were also found to be proportional to w (Chamberlain, 1967), but for these particles impaction has much less effect. Particles smaller than about 5 jum diameter are unlikely to bounce off surface in normal ambient conditions. The presence of micro-roughness elements on surfaces increases vg for these particles, and Chamberlain (1967) found more deposition to real leaves than to smooth sticky artificial leaves when the particle size was less than 5 jum. 

Using liquid mercury, the Cambridge workers assumed that the area of the electrocatalyst was what it seemed to be no invisible roughness. The capacity (around 20 pP cm 2) was then taken by Bowden and Rideal to be the real capacity of the double layer. (By real, they meant unaffected by invisible micro-roughness. )... 

Flow of the adhesive to fill the surface micro-roughness is important for all bonding, but adhesive penetration into the substrate is not a significant issue in most bonding applications. 

Reliably seal micro roughness of the component surface. 

The platinum electrode can be improved by an active electrolytic process forming a platinum black surface. The reduced polarization impedance is due to an increased effective metal surface area (fractal surface). The electrode is prepared in an electrolyte containing (e.g., 3% platinum chloride), with the platinum as the cathode. Platinum black is deposited on the surface, and also here there are optimum values for current density and quantity of electricity a current density of about 10 mA/cm and a quantity of electricity (charge) of about 30,000 mA s/cm is recommended (Schwan, 1963). Best results are obtained if the platinum surface is sandblasted before platinum black deposit. However, the surface may be fragile, and a protein layer formed with tissue contact may easily smooth the micro-rough surface and increase polarization impedance. Platinum black electrodes are best stored in distilled water and short-circuited (Schwan, 1963). 

Real joints do not of course consist of simple, separate, elastic materials with a clear mathematical geometry. Metal adherend surfaces are micro-rough, possessing oxide layers, while concrete surfaces are macro-rough comprising aggregates and cement paste, and both surfaces readily adsorb air-bome contamination. The thickness and modulus of primer layers, if employed, is often unknown, and the thickness and properties of the adhesive layer are difficult to regulate and to determine. 

On aluminium surfaces it is possible to create a suitable micro-rough structure using an electrochemical anodization process [11-13]. In this process the electrochemical formation of the aluminium oxide layer is combined with its partial dissolution by an acid electrolyte solution. 

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. 

Many technical textiles are finished with a heavy polymeric coating of, e.g., poly(vinyl chloride) (PVC), polyurethane, or silicone, which effectively masks the textile fabric with a smooth surface. One exemplary application of coated textiles, where easy or self-attained cleanability is highly required, is the construction of textile roofs in modem architecture. The present solution to decrease dirt take-up is to apply hydrophobic topcoats, i.e. layers of lacquer of approx. 5 jm thickness, to the coated fabrics. However, the performance of these conventional topcoats with regard to cleanability is not sufficient. The approaches to the creation of superhydrophobic surfaces by means of micro-rough surfaces as discussed in Section 6.1 refer to the texture of textile fabrics as well as the actual fiber surfaces, and are not necessarily applicable to coated textiles. 

The adhesion of paint films is influenced primarily by the chemical composition of the binder. Films may adhere by a mix of all possible mechanisms, for example. Dispersion forces, Acid-base interactions. Hydrogen bonding, covalent bond formation and other specific chemical interactions, such as chelation. In addition to these molecular interactions across an interface, the micro-roughness of the substrate can contribute by increasing the surface area and points of contact (see Mechanical theory of adhesion). In addition, if the substrate is itself another paint film (or a plastic), chain ends of the overcoating polymer can diffuse into the substrate, provided that the binder types are mutually compatible (see Theories of adhesion). 

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