Superhydrophobic surfaces contact angle

In addition to high contact angle, superhydrophobic surfaces should have low contact angle hysteresis, which serves as a measure of energy dissipation during the flow of a dtop along the solid. 

Superhydrophobicity, polyurethane, contact angle hysteresis, surface reorganization... 

Bachmann, J., McHale, G., 2009. Superhydrophobic surfaces a model approach to predict contact angle and surface energy of soil particles. Eur.J. Soil Sci. 60, 420. 

In order to achieve stationary and highly spherical microdroplets, the possibility to use superhydrophobic nanostructured surfaces has also been explored to make lasing22 and Raman lasing microdroplets23, where the high contact angle makes it possible to make long-term measurements on nearly spherical microdroplets at rest. 

Figure 7.9 Water drop on a superhydrophobic surface showing a high apparent contact angle app- The combined effect of hydrophobicity and roughness on the right length scale, causes the Lotus effect.

Equation (645) shows that contact angle is a thermodynamic quantity, which can be related to the work of adhesion and interfacial free energy terms. When 6 values are small, the work of adhesion is high and considerable energy must be spent to separate the solid from the liquid. If 0 = 0°, then W L = 2yv if 0 = 90°, then W L = yLV, and if 0 = 180°, then W1L = 0, which means that no work needs to be done to separate a completely spherical mercury drop from a solid surface (or a water drop from a superhydrophobic polymer surface), and indeed these drops roll down very easily even with a 1° inclination angle of the flat substrate. 

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... 

The wettability of solid surfaces is a veiy important properly of surface chemistiy, which is controlled by both the chemical composition and the geometrical microsttuc-ture of surface [21-23], When a liquid droplet contacts a solid surface, it will sptead or remain as droplet with the formation of angle between the liquid and solid phases. Contact angle (CA) measurements are widely used to characterize the wettability of solid surface. Surface with a water CA greater than 150° is usually called superhydrophobic surface. On the other hand, when the CA is lower than 5°, it is called superhy-drophilic surface. Fabrication of these surfaces has attracted considerable interest for both fundamental research and practical studies [23-25]. 

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