Surface Forces and the Equilibrium of Liquids on Solid Substrates

Surface Forces and the Equilibrium of Liquids on Solid Substrates 

The disjoining pressure acts in the vicinity of the three-phase contact line, and its action becomes dominant, e.g., as a liquid profile approaches a solid substrate, or with colloidal particles or drops. In the study of wetting and spreading processes, its importance seems less common than in colloid science, in spite of the same nature of the forces and the same level of necessity. 

We shall start with a discussion about a well-known and much used Young s equation in spreading and wetting dynamics. We advise reading the original paper by Young, as it is a masterpiece of scientific literature. Yet we hope to convince the reader of the ill-founded thermodynamic support of the (historical) standard form of such relationship. We shall argue and prove that the thermodynamically 

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Surface Forces and the Equilibrium of Liquids on Solid Substrates Young s equation (Equation 1.1) should now be rewritten as... 

In this section, we shall consider the properties and stability of liquid films on solid substrates under partial or complete wetting conditions. We shall account for the disjoining pressure action alternatively with the action of surface forces. As discussed in Chapter 1, the adsorption of liquid on solid substrates is a manifestation of the action of surface forces. But before we start, let us recall that partially or completely wetted solid surfaces, at equilibrium, are always covered by a liquid film that is at equilibrium with the vapor pressure, p, of the surrounding air. The free energy of such a solid covered substrate is lower than the free energy of the corresponding bare solid substrate. Hence, in all cases here... 

The classical analysis of Young and Laplace of static wetting problems rests on the characterization of each interface by a macroscopic surface tension. At the intersection of three bulk phases, the three phase contact line is at rest only if the capillary forces represented by these surface tensions balance. When the three phases are a solid substrate S, a wetting liquid L and a vapor V, the mechanical equilibrium condition parallel to the solid gives the Young-Dupr6 equation for the contact angle Oq... 

Lor partial or complete wetting at non-equilibrium adsorption of vapor and flow from a droplet or a meniscus on the solid substrate occurs. As a result at equilibrium the solid surface is covered with a thin liquid layer of equilibrium thickness /te. The presence of this layer changes the interfacial tension of the bare solid surface, ysv. to PsvAe> which is the surface tension of the solid substrate covered with a thin liquid layer of the equilibrium thickness h. Lorma-tion of this thin layer is caused by surface forces. 

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