Smooth surfaces liquid wetting

In the wetting and dewetting kinetics studies described earlier, the solid substrate was a flat and smooth surface. However, the sohd deformation due to the action of the vertical component of the hquid surface tension may be expected to act in any geometry. For example, viscoelastic braking is involved in the sliding of a liquid drop on a tilted rubber track [32],... 

If, when a liquid drop is placed on a smooth surface, the forces of adhesion between the solid and the liquid are greater than the forces of cohesion of the liquid, then the liquid will spread and will perfectly wet the surface spontaneously. If the forces reach an intermediate balance determined by the interfacial energies ylv, ysj and ysv, then the liquid drop will form a definite contact angle (0) with the solid surface (Figure 4.12). 

Components of interfacial tension (energy) for the equilibrium of a liquid drop on a smooth surface in contact with air (or the vapor) phase. The liquid (in most instances) will not wet the surface but remains as a drop having a definite angle of contact between the liquid and solid phase. 

Consider a vertical flat plate exposed to a condensable vapor. If the temperature of the plate is below the saturation temperature of the vapor, condensate will form on the surface and under the action of gravity will flow down the plate. If the liquid wets the surface, a smooth film is formed, and the process is called film condensation. If the liquid does not wet the surface, droplets are formed which fall down the surface in some random fashion. This process is called dropwise condensation. In the film-condensation process the surface is blanketed by the film, which grows in thickness as it moves down the plate. A temperature gradient exists in the film, and the film represents a thermal resistance to heat transfer. In dropwise condensation a large portion of the area... 

Wetting Equilibrium Limited wettability of plane, smooth surfaces by non-swelling liquids can be analyzed using the Young-Dupre Equation ( 7 ... 

Dropwise condensation is obtainable only when the cooling surface is not wetted by the liquid. In the condensation of steam it is often induced by contamination of the vapor with droplets of oil. It is more easily maintained on a smooth surface than on a rough surface. 

Figure 7,34. Illustration of the effect that surface roughness has on the wetting of a solid by a liquid drop. The plot shows schematically the evolution of the cosine of the contact angle on a rough surface (%) versus the same quantity on a smooth surface with the same surface composition (0c). The theoretical line refers to equation (7.27)...

A simple test of polymer adhesion is to take a table tennis bat and cover it with a smooth layer of rubber, as shown in Fig. 6.4. A ping-pong ball then adheres to the smooth surface sufficiently to support its own weight. The rubber surface is so smooth that it looks shiny and liquid-like, seeming to wet the ball as it makes molecular contact, but in reality there is no fluid present. When the rubber is immersed in water, alcohol or other liquid, the ball drops off."... 

It is thus evident that, within a first approximation. Equation 1.30 is valid for both wetting and nonwetting liquids, with a meniscus either present or absent. It is, however, worth pointing out that for macroscopic particles, this is valid only in the case of molecularly smooth surfaces. In this case, the equations for the molecular adhesive forces and for the capillary contraction force both contain the same macroscopic value of R. The situation is different for rough surfaces. Namely, the value of R in the expression for the molecular forces may be determined by the radii of microheterogeneities between which the contact is formed, while the value of R in the expression for the capillary adhesion force may be determined by the macroscopic radii of the particles. Consequently, particles with a microscopically... 

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