Wettability Hysteresis

H, particularly when contact angles are in the range of intermediate wettability. Consequently, each mechanism should be expected to play a role in GDM capillary behavior. For instance, the attribution of capillary pressure hysteresis entirely to hysteresis of the contact angle in the sense of Eq. (3)59 can be correct only if GDMs are composed of bundles of tubes of cylindrical cross section.60... 

These surface modifications were performed in "pure" micro-wave (2.45 GHz, "single-mode") or in combined microwave/ radio frequency (2.45 GHz/13.56 MHz, "dual-frequency") plasma. Important systematic changes of the surface composition, wettability, and adhesion of thin metal films were observed for different substrate bias values, and for the different gases. The modified surface-chemical structure is correlated with contact angle hysteresis of water drops this helps to identify which surface characteristics are connected with the wettability heterogeneity and with adhesive bonding properties, and how they are influenced by plasma-surface interactions. 

To sum up, the effects on static contact angles of the departures from ideality of solid surfaces are qualitatively well understood and some of these effects are used in practice to improve or reduce wettability. Moreover, for simple geometries, a semi-quantitative agreement is obtained between experimental results and theoretical predictions. For surfaces with random roughness, predictions of wetting hysteresis present a great difficulty because the relevant size of defects is not yet well-established. 

R.E. Johnson, R.H. Dettre, Wettability and Contact Angles, in Colloid and Surface Science, E. Matijevlc, Ed., Wiley 2 (1969) 85. (Excellent review basic features, hysteresis, measurements, illustrations.)... 

The second factor is related to the surface chemical structures on active carbons, in particular, with their ability to oxidize easily and form the oxide adsorption centers such as hydroxyl, carbonyl or carboxyl groups. As a result the energetics and wettability of the surface changes that influences essentially on the adsorption isotherm and hysteresis. The difference between adsorption-desorption isotherms for oxidized and reduced adsorbents is clearly demonstrated in Fig. 6.2. 

Examination of Table 9.13 shows that further study of the parameters influencing K, is needed. This includes the effects of the wettability and the significance of the expected hysteresis in K,(.v). 

The effect of roughness on the wettability of an idealized sinusoidal surface has been studied with a digital computer. The equations of Wenzel and of Cassie and Baxter are discussed in relation to the model. The heights of the energy barriers between metastable states of a drop are seen to be of utmost importance in determining the magnitude of contact angle hysteresis. 

The wetting behavior of polymers is reviewed beginning with the thermodynamic conditions for contact angle equilibrium. The critical surface tension of polymers is discussed followed by some of the current theories of wettability, notably the theory of fractional polarity and theories of contact angle hysteresis. The nonequilibrium spontaneous and forced spreading of polymer liquids is reviewed from two points of view, the surface chemical perspective and the hydrodynamic perspective. There is a wide di.sperity between these two viewpoints that needs to be resolved inorder to establish the predictive relations that govern spreading behavior. 

Dettre, R.H., Johnson, R.E.J., 1964. Contact angle hysteresis n. Contact angle measurements on rough surfaces Contact SI32. The dream of sta3ing clean lotus and biomi-metic surfaces. In Fowkes, F.M. (Ed.), Angle, Wettability, and Adhesion. American Chemical Society, Washington, DC, pp. 136—144. 

The contact angle is 0ca < 90° when the solid is hydrophilic (so-called high energy solid) and the water wets such a solid well it is usually < 30° and approaches zero when the water completely spreads over the solid. For hydrophobic solids (so-called low energy solids) 0ca > 90° and may have values up to 150° for so-called superhydrophobic surfaces such as specially prepared (non-wettable) fluorohydrocarbons. There exists some hysteresis (amounting typically up to 10°) between the contact... 

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