Adhesive forces, of water

One result of adhesive forces is the curved surface of a liquid, called a meniscus. As Figure 11-18 shows, water in a glass tube forms a concave meniscus that increases the number of water molecules in contact with the walls of the tube. This is because adhesive forces of water to glass are stronger than the cohesive forces among water molecules. 

Soil wettability is another important factor besides contamination, which governs water retention and transport processes [97]. Strong interaction of mineral phases with CDs suggests that the properties of the soils may also be seriously affected by CDs, which may have an influence on soil remediation processes. The effect of RM-/9-CD on surface and pore properties of common clay minerals such as bentonite, illite and kaohnite had been studied and it was found that the wettability of the soil decreased at low CD concentration and increased again at the highest load. The increased concentration of CDs reduced the effective radius of the soil bed which improved the average force of interaction among soil particles via water layer, despite the simultaneous decrease of adhesion forces of water to the soil. 

Cohesive and adhesive forces involving water are dynamic. It is not one set of water molecules, for example, that holds a droplet of water to the side of a glass. Rather, the billions and billions of molecules in the droplet all take turns binding with the glass surface. Keep this in mind as you read this section and examine its illustrations, which, though informative, are merely freeze-frame depictions. 

Adhesive forces between water and glass cause water molecules to creep up the sides of the glass, forming a meniscus. 

The meniscus of a liquid is the curved surface it forms in a narrow tube (Fig. 5.18). The meniscus of water in a glass capillary is curved upward at the edges (forming a concave shape) because the adhesive forces between water molecules and the oxygen atoms and —OH groups that are present on a typical glass surface are stronger than the cohesive... 

The adhesion forces of silica and alumina particles in the DI water and slurry solution were measured by AFM and are shown in Fig. 16.27 [66]. The smallest adhesion force, 0.38 nN, was observed between the copper surface and alumina particles in a citric acid solution at pH 6. The largest adhesion force of alumina particles, 5.83 nN, was measured in DI water. 

The largest adhesion force of alumina particles in DI water was attributed to a stronger electrostatic attraction between alumina particles and copper surface in DI water owing to their opposite signs of zeta potentials. The smallest adhesion force of alumina particles in the citric acid slurry was attributed to the... 

FIGURE 16.27 The adhesion forces of the particles on copper in DI water and citric... 

Capillary action When water is placed in a narrow container such as a graduated cylinder, you can see that the surface of the water is not straight. The surface forms a concave meniscus that is, the surface dips in the center. Figure 13-15 models what is happening to the water at the molecular level. There are two types of forces at work cohesion and adhesion. Cohesion describes the force of attraction between identical molecules. Adhesion describes the force of attraction between molecules that are different. Because the adhesive forces between water molecules and the sihcon dioxide in glass are greater than the cohesive forces between water molecules, the water rises along the inner walls of the cylinder. 

Figure 11.19 Wax is a hydrocarbon that cannot form hydrogen bonds. Therefore, coating the inside of tube with wax will dramatically decrease the adhesive forces between water and the tube and change the shape of the water meniscus to an inverted U-shape. Neither wax nor glass can form metallic bonds with mercury so the shape of the mercury meniscus will be qualitatively the same, an inverted U-shape.

The combination of changes in surface chemistry and surface roughness can enhance stimuli-responsive wettability. Here, we have presented a simple one-step method to prepare apolybenzoxazine-silica nanoparticle film, displaying reversible wettability via a solvent treatment. The difference in the water CAs of the same composite film, before and after solvent treatment was achieved to be 19° with silica content of 120 phr. Compared with a traditional superhydrophobic surface, the as-prepared film can be modified to show not only hydrophobicity but also a high adhesion force with water. Our results are considered to be significant in terms of their importance to academic research and industrial applications. 

When a liquid layer is formed in a gap between particles and surface as a result of capillary phenomena (see Fig. IV.6, items b and c), the force of particle adhesion will be equal to the capillary forces. In connection with the fact that the force of adhesion and the dimensions of the adherent particles are subject to measurement, McFarlane and Tabor [90] have recommended the use of Eqs. (IV.40) and (IV.41) to determine the surface tension of water or other liquid, provided that the values of and r are known. We have listed below certain experimental results obtained by these investigators on the determination of surface tension by measurement of the adhesive force of glass spheres in the presence of capillary-retained liquid on glass surfaces ... 

The final value of adhesive force of glass fibers in distilled water does not depend on the applied pressure (over a range of 14 to 1400 dyn), even though the time to establish the equilibrium adhesive force does depend on the load. With a load of 14 dyn, this equilibrium is reached in 24 h with a load of 230 dyn, it is reached in 5-6 h. 

Adhesion in Relation to Temperature of Liquid Medium. As in air (see Section 18), in distilled water we find that the adhesive force of particles increases as the temperature is raised. For example, in the case of adhesion of spherical glass particles with a diameter of 40 /xm to a glass surface, the adhesion number with a detaching force of 3.0 10 dyn was found to increase as the water temperature was raised from 5 to 95 C. On surfaces painted with a chlorinated PVC enamel, this increase was more pronounced [77]. 

The effect of surface painting on adhesive force in water can be shown in the example of adhesion of cylindrical particles with diameters of 20 and 40 /xm and... 

Thus we see that the presence of a paint or varnish coating on a metal surface may change the actual magnitude of forces of particle adhesion, but the general relationships characterizing the adhesion remain the same as on the unpainted surface. Among these relationships, we should mention the log-normal distribution of particles with respect to adhesive force, the dependence of adhesion on particle size, and the lower adhesive forces in water in comparison with air. 

A method of determination of the Gibbs free surface energy and the radial dependence of adhesion forces of adsorbents in respect to the interfacial water was developed on the basis of measurements of the dependence of H NMR signal intensity of unfrozen water on temperature at T< 273K (Figures 1.93 and 1.94). The free surface energy values are computed for a variety of oxide, carbon, carbon-mineral, polymeric, and composite materials (Turov and Leboda 1999,2000, Gun ko et al. 2005d). 

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