## Surface Tension

A surface is a notion of geometry. For surfaces whose radii of curvature approach molecular dimensions these concepts become ambiguous. A real interface is a three-dimensional non-uniform region interposed between two bulk phases. Gibbs and others employed a geometrical surface they imagined as a dividing surface interposed between the two phases.

Rigorous definitions of the interfaces employ metric tensor fields and a system of orthogonal curvilinear coordinates that reduce the metric tensor to its diagonal form at any point. We locate our surface between the two phases in relation to the mean positions of the molecules in question after we statistically average these molecules over their disordered thermal motions or in terms of the distance of closest approach of the molecules of one phase to those of the condensed phase. If we place our dividing surface appropriately, we may choose adsorption equal to zero. The surface amount or Gibbs adsorption may be positive or negative, and we define this adsorption to be the excess of the amount of a component molecule in our system compared with that present in a reference system having the same volume as our system in which the bulk concentrations in the two phases remain uniform up to the Gibbs dividing surface.

Adsorption of a component of a multiphase multi-component system such as hemolymph occurs if concentrations of the components in the interfacial layers differ from those in the adjacent bulk phases. The Gibbs dividing surface is a geometrical surface chosen parallel to the interface. If the hemolymph contacts a wall, the boundary surface becomes this dividing surface. Two surfaces can meet along a linear interface. We can also have foam as an array of films and channels and regard these in our models as surfaces and lines. Line tensions of the channels play roles in forming foams (Ref: Capillary Hydrodynamics).

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