Appendix interfaces and the notion of surface tension

Appendix interfaces and the notion of surface tension 9.3.1. Interface between two non-miscible fluids and surface tension 

The tension force is exerted on surface S, which, like a drum skin held by its rim, resists deflection. Further, b is the normal to curve C which is tangent to surface S and oriented outwards relative to surface S. The direction of the normal it to the surface is linked, by convention, to the direction of travel along citrve C, given by 

The spherical surface element is in equilibrium if the deflection of the interface is associated with a rise in ptessute through the interface that is inversely proportional to the radius of curvature. The pressure is greater on the concave side than on the convex side of the interface. For a non-spherical surface, relation [9.32] can be generalized by bringing in the radii of curvature along the principal directions of the surface  

The literature often provides an energy-based inteipretation of surface tension, as / has the dimension of an energy per unit surface (N m = J m ). While this interpretation is meaningful as a rule, because the increase in the interfacial area is achieved under expenditure of energy, the interpretation in terms of eneigy is sometimes ambiguous, as discussed hereinafter with the example of Jurin s law. 

Equilibrium in the contact line between three phases, Jurin s law 

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