Wettability of surfaces

VIGNETTE VI SURFACE CHEMISTRY AND MATERIALS SCIENCE Wettability of Surfaces and Fabrication of Microstructured Materials... 

FIG. 6.1 Control of wettability of surfaces through chemistry (a) schematic illustration of formation of hydrophobic lines on a hydrophilic surface with self-assembled monolayers (SAMs) and micromachining (b) top view of the shapes and confinement of water drops on an engineered surface. (Reproduced with permission of Abbott et al. 1992.)... 

MD simulations and mode-coupling calculations [59,68,69] have shown that the magnitude of the hydrodynamic slippage can be correlated to the wettability of surfaces, which is characterized by a contact angle 9 [59] ... 

The general relationships described above are not specific to a certain system. However, given the need for optimal separation of valuable particles from the associated gangue, the colloid and surface chemistries that are involved may be quite complex. As in the flotation process, selective agglomeration by immiscible liquids depends strongly on the relative wettability of surfaces, and the same fundamentals of surface chemistry apply to the conditioning of particles to yield the required affinity for the wetting liquid. 

Our results on the wettability of surfaces covered with highly fluorine-substituted alkyl groups stimulated several research laboratories to apply these surface properties to polymeric coating materials for textile fibers and fabrics as a means of imparting to them nonstaining, oil-, and water-resistant properties. Such products are becoming very prominent today and the contact angle is used for product control and trade specifications [1]. 

Hydrophobicity refers to the poor wettability of surfaces by water and is quantified using contact angle measurements. The experimental setup consists of a microsyringe to instill fine droplet of the liquid on the polymer specimen surface, fiber optic illumination, telescopic focusing, and computer... 

The wettability of surfaces depends on their surface energy. High-energy surfaces or polar surfaces have a high wettability. On non-polar surfaces with low-energy characteristics, dirt can stick only with difficulty. ETFE-foils have a non-polar surface. For example, the ETFE-foU ET 6235 shows a surface tension of approximately 25 mN/m (Hodann, 2007). Therefore, the smooth and non-polar surfaces of ETFE-foils show an anti-adhesive effect. 

Abbot NL, Fofkers JP, Whitesides GM (1992) Manipulation of the wettability of surfaces on the 0.1 - to 1-micrometer scale through micromachining and molecular self-assembly. Sciaice 257 1380-1382... 

In summary, our results have demonstrated that a colloidal monolayer can be used as a flexible template to create ordered nano-structured arrays. Combining it with other techniques, a series of ordered nano-structured arrays with centimeter size could be easily fabricated on many different substrates. These nano-structured arrays are of surface roughness on the nano- and micro-scales, or similar to the microstructure of lotus leaves. Our results have demonstrated that such micro/nano-structured arrays, not only of insulators and semiconductors but also of metals, display morphology-dependent wettability. Significant enhancement of both hydrophobicity and hydrophilicity can be achieved by fabricating special surface micro/nano-structure on any material. This means that we can also realize tunable and controllable wettability of surface of any material by designing the proper surface structure. From this study, it can thus be expected that the nano-devices based on our nano-structured arrays would be waterproof and self-cleaning, in addition to their special device functions. 

In many biological and technological processes, adsorption from solution is of crucial importance. Most of these processes take place in aqueous media. Examples are the adsorption of toxic or other adverse components on active carbon in the production of potable water, adsorption of contaminants from the gastrointestinal tract on medical carbon, adsorption and desorption of herbicides and pesticides on plant and soil materials, and adsorption of surfactants to influence the wettability of surfaces. Applications of adsorption are given further attention in Section 14.5. 

Baytekin, H.T., Wrrth, T, Gross, T, Treu, D., Sahre, M., Theisen, J., Schmidt, M., Unger, W.E.S. (2008) Determination of wettability of surface-modified hot-embossed polycarbonate wafers used in microfluidic device fabrication via XPS and ToF-SIMS. Surf Interface Anal., 40, 358-363. 

The chemical nature of the substrate, combined with the liquid surface tension, defines the liquid CA on a perfectly clean, flat surface. To control the wettability of surfaces, often (hydrophobic) polymers with specific low surface tension end-groups are used. The surface tension of typical substituent end-groups decreases in the following order CH2(36dyn/cm) > CH3(30dyn/cm) > CF2(23dyn/cm) >... 

Wettability of Surfaces on the 0.1- to 1-Micrometer Scale through Micromachining and Molecular Self-Assembly, 257, 1380-1382. 

Processes for sorting of different solids are gravity separation, magnetic/electric separation, radiation sorting, and separation based on the wettability of surfaces. 

Another useful concept for assessing the wettability of surfaces is that introduced by Zisman and collaborators [121], namely the critical surface tension of wetting, that was discussed in detail in Chapter 11. These authors found that, for a given surface and a series of related liquids such as n-alkanes, siloxanes or dialkyl ethers, cos 0 is a reasonably linear function of la- The surface tension at the point where the line cuts the cos 0=1 axis is known as the critical surface tension of wetting, y. It is the surface tension of a liquid that would just spread to give complete wetting. 

Wettability and Non-wettability of Surfaces An equally broad range of applications have membranes and filters made of cellular ceramic materials. One of the features of ceramics, indispensable in such applications, is the ability to suck liquids through tubular pores. This requires wetting of the solid surface by a liquid. Wettability and non-wettability can be assessed by considering the equilibrium conditions in the solid-liquid-gas system ... 

High density polyethylenes have higher cohesive strength than low density polyethylenes but poorer wettability of surfaces when melted. These differences in properties are related to die linear structure and high crystallinity of the high density polyethylene and the branched chain structure of the low density polyethylene. 

Wang N, Zhao Y, Jiang L (2008) Low-cost, thermoresponsive wettability of surfaces poly(A-isopropylacrylamide)/polystyrene composite films prepared by electrospinning. Macromol Rapid Commun 29(6) 485-489. doi 10.1002/mtirc.200700785... 

It is frequent that low wettability surfaces influence badly the production line of the goods. It is desirable to introduce surface treatment processes to low wettability materials such as thermoplastics. For instance, short wavelength UV irradiation with low pressure mercury lamps and ambient pressure plasma irradiation are popular for the purpose. Even shot-term irradiation can improve the wettability of surface very much and the bond strength increases, as shown in O Fig. 41.9. 

---